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Micromite LCD BackPack V3 This BackPack is the most convenient and powerful yet. It has all the features of the V1 and V2 BackPacks, supports both 2.8-inch and 3.5-inch touchscreen displays and boasts five new by Tim Blythman optional features which provide convenient functions. These include extra memory, temperature, humidity and pressure sensors, a real-time clock, an infrared receiver and more! I n our recent article on 3.5-inch touchscreen displays (see the June 2020 issue), we looked at three different screens. But we were particularly impressed by one. It uses an ILI9488 controller with SPI interface and has the same connections as the popular 2.8-inch touchscreen display used by the original and V2 BackPacks. For that article, we supplied code to drive that new display from an Arduino and a standard Micromite. We also mentioned that we planned to write some CFUNCTIONs to speed it up, as the BASIC code is quite slow at refreshing the screen. Not only have we now done that, but we’ve also designed a new version of the BackPack to properly accommodate the larger, higher-resolution screen with twice as many pixels as the original (480×320 compared to 320×240). While this article gives sufficient information for you to fully understand what we’ve done, if you haven’t seen the V2 BackPack article in the May 2018 issue of PE, you might want to read that before coming back to this article. Essentially, the BackPack is a small PCB that hosts a PIC32 running the Micromite firmware. It also provides a simple power supply, a USB interface, a header and mounting screws for a colour touchscreen and an I/O pin header. The best part about it is that MMBasic has native touchscreen support. It’s such a great idea that we’ve used the BackPack in numerous other projects. 16 But the V3 BackPack is more than just a screen upgrade. While you can build the new V3 BackPack using the same components as the V2 BackPack, you can also add several extra components to add handy features without needing to connect extra modules, PCBs or wiring. You can fit it with an infrared receiver/decoder for remote control, a Flash memory IC or SRAM, a DHT22 temperature and humidity sensor, a DS18B20 temperature sensor or a DS3231 realtime clock IC. There’s also a header for connecting additional I2C devices, such as a BMP180/BMP280/BME280 temperature/pressure/humidity sensor, which can be mounted directly to the board if desired. Also, this BackPack lets you use the SD card socket that’s mounted on the back of the touchscreen board. All the functions that were in the original and V2 BackPack are retained in the V3 BackPack, including its 50MHz 32-bit processor loaded with a powerful BASIC interpreter, which can be programmed over a virtual USB serial port. Circuit description We’ll start by describing the core functions, which are carried over from the V2 BackPack. Refer to Fig.1, the circuit diagram. IC1 is the main processor which runs the MMBasic interpreter and handles other functions. It is a PIC32MX170F256B (or the 50MHz variant,) in a 28-pin dual inline package. It requires some bypass capacitors for normal operation: two 100nF MKT capacitors across its supply rails and a 10µF ceramic capacitor to filter its internal core supply. There’s also a 10kΩ resistor used as a pull-up on IC1’s RESET line, to prevent spurious resets. IC2 is a Microchip PIC16F1455 microcontroller which is both a USB/serial converter and a PIC32 programmer – the Microbridge article in the May 2018 issue of PE describes its functions in more detail. When running as a USB/serial converter, pin 5 on the PIC16F1455 receives data (ie, data from the Micromite to the PC USB interface) and pin 6 transmits data (from the PC USB interface to the Micromite). These signals also run to the edge pins for the console connection (CON1) in case you build this PCB but for some reason do not plug the Microbridge IC, IC2, into its socket. In this case, you can use an external USB/serial converter. The PIC32 programming interface from the Microbridge is on pins 7, 2 and 3 of IC2. These provide the reset function, program data and clock signals, which connect to pins 1, 4 and 5 respectively on the Micromite (IC1). The programming output on the Microbridge is only active when it is in programming mode, so the Microbridge Practical Electronics | August | 2020 Features • Compatible with Micromite LCD BackPack V1 and V2 • Suits 2.8-inch and 3.5-inch touchscreen LCD modules • Built-in Microbridge provides serial communications and programming interface • Mini USB socket for power and communication • Native support for 3.5-inch display using initialisation CFUNCTION • Manual or software (PWM) dimming for LCD backlight • 4-pin I2C communication header • Optional onboard infrared receiver • Optional onboard DHT22 temperature and humidity sensor or DS18B20 temp sensor • Optional onboard DS3231 real-time clock • Optional onboard Flash memory/RAM IC • Optional onboard BME180/BME280/BMP280 temperature/pressure/altitude sensor does not interfere with the Micromite when it is using pins 4 and 5 as general-purpose I/O pins. Switch S1 is used to select programming mode and LED1 indicates the mode (lit solid when in programming mode). CON2 is the main I/O connector for the Micromite and is designed so that it can plug into a solderless breadboard for prototyping. The connector also makes it easy to add a third PCB to the LCD BackPack ‘stack’ which can carry circuitry specific to your application (eg, amplifiers or relay drivers). This connector is wired identically to the original BackPack. The Micromite communicates with the LCD panel using an SPI interface where pin 3 on the Micromite feeds data to the LCD while pin 25 provides the clock signal. When the Micromite pulls pin 6 low, it is communicating with the LCD panel, and when pin 7 is pulled low, the Micromite is communicating with the touch controller on the display panel. The 28-pin Micromite has only one SPI port and so pins 3, 14 and 25 (SPI data and clock) are also made available on CON2 so that you can also use this SPI serial channel to communicate with external devices. Backlight control You have two choices for controlling the brightness of the LCD’s backlight. The first is to fit MOSFETs Q1 and Q2 to the PCB, along with their associated resistors (this area is marked with a box on the PCB). When you do this, PWM output 2A on the Micromite is Practical Electronics | August | 2020 used to control the backlight brightness from within your program. This is described in more detail later. Alternatively, as with the original BackPack, you can fit VR1, a 100Ω trimpot. This is in series with the power to the backlight LEDs, so it limits the current drawn by them and therefore sets the brightness. Note that you should install one set of components or the other (not both). You also have the option of fitting a link across VR1’s pads to permanently set the backlight to full brightness. The LCD panel has a 3.9Ω resistor in series with the backlight so you will not burn out the backlight if you set the PWM output to 100%, wind VR1 to zero ohms or link it out. The power supply is derived from either the 5V connector pin on CON1, or if JP1 is installed, from USB connector CON4. Powering the Micromite LCD BackPack from USB power is handy during program development, but for an embedded controller application you would typically remove the jumper from JP1 and supply 5V power via CON1. Note: do not power the BackPack from both CON1 and USB – you could cause damage to the USB interface on your computer. The 3.3V power supply for both the Micromite and the Microbridge is provided by REG1, which is a fixed-output regulator with a low dropout voltage suitable for use with USB power supplies. This supply is also made available on CON2 so you can use it for powering external circuits (to a maximum of 150mA). Improvements As mentioned above, one of the major improvements with the BackPack V3 is that you can use either a 2.8-inch 320×240 pixel touchscreen or a 3.5inch 480×320 pixel touchscreen. The board is sized to fit the larger screen. It still fits comfortably inside a UB3 jiffy box, the same box which we’ve used to house several Micromite BackPackbased projects over the years. We have also designed the board so that with both screen options, the onboard SD card socket is wired up to IC1. While the Micromite Plus software has read/write support for SD cards, it will not work on any throughhole PICs. The regular Micromite code, which works on our 28-pin DIP chip, does not natively support SD cards. However, it would be possible to write BASIC code (or perhaps a CFUNCTION) to access an SD card with the regular Micromite, so we decided to wire up the SD card socket that already exists on the touchscreen module. This extra header also helps to hold the touchscreen squarely onto the BackPack module without needing mounting hardware. The SD card is connected to the same SPI interface that’s used to drive the touchscreen, but it has a separate CS line, which is connected to pin 4 on the Micromite. If you don’t insert an SD card, it won’t have any effect on this pin so it can be used for other purposes. We decided if we were making these changes then we should add some other useful features at the same time. Added features The BackPack V3 has provision for many extra onboard components which provide various useful functions. None of these need to be fitted; if you leave them off, the board will work much the same as the V2 BackPack, except for the option of the larger screen and SD card access. These five optional extra components can be used to add extra features to your Micromite project without needing to add another board or module. 1. 3.3V Infrared receiver (IRD1) This mounts near the edge of the board, so that its leads can be bent to face outwards for convenient remote control of the unit. Its supply is filtered for reliable operation. Its output is connected to Micromite pin 16, which is the MMBasic IR input pin, making it trivial to receive remote control commands in BASIC. The IR receiver should ideally be a 3.3V type such as the Vishay TSOP2136 or TSOP2138. However, we tried 4.5V receivers (eg, Jaycar ZD1952) on a 3.3V supply and they normally worked fine. 17 2. Serial Flash memory or static RAM Use either an 8-pin DIP or SOIC package for IC3. If you aren’t using the SD card interface, you can fit a Flash or SRAM chip with a standard pin-out to the board and use this to store configuration data, logging data or temporary working data. These chips are easier to drive than SD cards; the BASIC code to access them is easy enough, and we provide a sample sketch to do this. The memory chip’s SPI interface is connected to the usual SPI pins on the Micromite, while the chip select line (CS, pin 1) goes to pin 4 of IC1, REG1 MCP1700-3302E +5V JP1 MINI USB TYPE B CON4 5V 12 13 4 8 9 1kW 10 MODE S1 D–/RA1 IC2 RC5/RX PIC16F1455 D+/RA0 MCLR/RA3 RC4/TX RC0/SCL/AN4 GND 6 DATA IN 12 +3.3V 7 3 AN3/RA4 14 CON5 ICSP 1 RESET 10kW 2 +3.3V CON6 10 10 14 MISO 14 16 IRPIN 16 21 21 5 PGEC 22 22 24 24 25 SCK 25 26 26 7 SD (3.5in) 8 Vcc WP CS HOLD 1 IC3 FLASH /RAM Vss +3.3V 2 SO SCK RB12/AN12 RB2/AN4 RB7/TDI L_RST L_CS 2 3 GND 1 VCC +5V VR1 100W RB14/AN10 1kW RB15/AN9 19 27 +3.3V 20 Q2 S IRLML2244 G D 10kW D CON2 Q1 2N7002 G +3.3V PWM BACKLIGHT CONTROL (OPTIONAL) S 100nF 4 1 +3.3V 4.7kW 4.7kW 2 3 +3.3V SCL 16 SDA 15 PGEC 3 4.7kW SDA RST 1 TS2 TS1 3 l 2 Vcc 10mF 1 IRPIN (IC1 PIN16) DATA DHT22 GND 1 3 2 2 3 1 4 SC MICROMITE Micromite LCDLCD Backpack V3 V3 BACKPACK Vcc DQ DS18B20 VBAT 32kHz 14 CON9 1 NC 100W K A IC4 DS3231 INT/SQW 5–12 GND 13 IRD1 LED1 2 Vcc SCL SDA (IC1 PIN5) SCL 4 18 3 10-47mF 2 Ó2019 6 L_D/C MANUAL BACKLIGHT RB13/AN11 MICROMITE I/O 6 LED (A) PINS ON IC1 RB11/PGEC2 8 CON8 IRD1 4 RB9/TDO +3.3V IC 5 23 RB10/PGED2 4 SCK 2 14 25 3 MISO 8 RB8/TCK GND 5 SI RA3/CLKO +5V +5V SCK 9 6 RA0/AN0 RB5/PGED3 MOSI T_CS 7 RA2/CLKI 4 2 MOSI 9 MISO 10 RB1/AN3/PGEC1 9 4 PGED 1 CON7 SD_CS 5 T_IRQ 11 RA1/AN1 18 3 4 SCK PGEC 100nF 3 MISO 5 7 RB0/AN2/PGED1 18 SDA 2 MOSI 12 4 17 1 RA4/SOSCO PGED 17 SCL +3.3V 13 4 3 GND 2x 10kW SD (2.8in) 14 RB4/SOSCI 3 MOSI RESET CON3 AVDD 15 PGEC3/RB6 RB3/AN5 IC1 PIC32MX170F1 MCLR 256B-50I/SP 3 1 2 0V K SD_CS VDD 11 LCD TOUCHSCREEN 28 13 DATA OUT PWM2/RA5 l 100nF 100nF 5 RC2/SDO/AN6 AN7/RC3 A LED1 11 VUSB3V3 RC1/SDA 10mF +3.3V Rx +V 5V +3.3V Tx 1 1 2 3 X 4 GND 10mF +3.3V OUT IN POWER AND +3.3V CONSOLE CON1 100nF 3. 4-pin I2C header This connects to the I2C bus and 3.3V power supply (CON8). Two 4.7kΩ pullup resistors are provided on the SCL and SDA lines, although these can be left out if pull-ups are provided externally. The pinout of CON8 matches the commonly available BMP180/BMP280 temperature and atmospheric pressure sensor modules, as well as the BME280 temperature/pressure/humidity module. So any of these can be soldered directly to the BackPack at CON8. Alternatively, a four-way header can be fitted and leads run to one of the many same as for the SD card. That is why you can’t use both at the same time. If fitting this IC, you have the option to fit either or both of the pull-up resistors on pin 3 (write protect/WP) and pin 7 (HOLD). These may be required to read and write data on the chip. We’ve also provided for a 100nF supply bypass capacitor; always a good idea. Ensure your IC’s pin-out for this board matches that shown and that it can run off a 3.3V supply. This is by far the most common pin-out for 8-pin serial memory chips and they will virtually all operate from 3.3V, but it’s best to check. 2 RTC BATT 1 Q1: 2N7002 2 3 Q2: IRLML2244 D G D G S S REG1 MCP1700 IN OUT GND Fig.1: the Micromite LCD BackPack V3 circuit comprises the complete V2 BackPack circuit, (TS1 & TS2 ARE ALTERNATIVES) which is based on 32-bit microcontroller IC1, plus numerous optional components. This includes an infrared receiver (IRD1), a Flash memory or RAM chip (IC3), a real-time clock chip (IC4), a temperature/humidity or temperature sensor (TS1/TS2) and an I2C header (CON8). GND Practical Electronics | August | 2020 commonly available Arduino compatible I2C modules, such as character LCD screens and other sorts of sensors. 4. DS3231 real-time clock IC This (IC4) also uses the I2C serial bus. It too has a 100nF bypass capacitor and a header (CON9) to connect a back-up battery so that the time and date are maintained even when the board has no external power. Micromite BASIC has built-in commands for I2C-based realtime clocks, so this is another feature that can be used easily. The I2C pull-up resistors need to be installed if a DS3231 chip is installed, unless they are already present on another connected module. 5. Environment sensor This is a DHT22 one-wire temperature and humidity sensor (TS1) or a DS18B20 one-wire digital temperature sensor (TS2). These connect to pin 5 of IC1, and there is provision for the required 4.7kΩ pull-up resistor too. Data from the DHT22 can be read by a CFUNCTION which is available for download with the Micromite firmware, while there is a built-in BASIC function to read the temperature from a DS18B20. If fitting a DHT22, it’s best to lay it over on its side over the top of the DS18B20 footprint to allow a display to fit above. Software support As noted above, we have written CFUNCTIONs to provide support for the 3.5-inch display; 2.8-inch and smaller displays based on the ILI9341 are natively supported by the Micromite. The CFUNCTIONs for the 3.5-inch displays ‘hook into’ the existing graphics commands, so once the display has been initialised, the drawing commands are the same. If you have already written some MMBasic software, you only need to add a few lines at the start to support the higher-resolution 3.5inch display. The other great thing about our CFUNCTIONs is that they do not take complete control of the SPI bus, allowing other SPI devices to be used. Unfortunately, these CFUNCTIONs interfere with the touch controller’s BASIC functions, so we have had to write a separate set of CFUNCTIONs to handle the touch panel. Most of the above-mentioned optional components are already supported by MMBasic, so we didn’t need to write much code to allow you to use all the new features of the V3 BackPack. The only thing that is not natively supported is the Flash or SRAM memory IC, for which we’ve written demonstration code, as explained earlier. Practical Electronics | August | 2020 High-value ceramic capacitors Previous Micromites have required between one and three capacitors which were either specified as SMD ‘chip’ ceramics (10µF) or through-hole tantalum capacitors (47µF). This is because of the strict ESR requirements for some of the parts; 10µF tantalum capacitors often had too high an ESR to work reliably. Some people didn’t like having to solder the chip capacitors, and tantalum capacitors are more expensive and can be less reliable. Since then, through-hole 10µF ceramic capacitors have become available, and they use our preferred dielectric too (X7R). So we have specified those in the parts list. The other two options are still valid and can be used instead, if you prefer. Construction We’ll start by assembling the basic V3 BackPack (effectively, a V2 BackPack), and then describe what parts to add if you want to use any of the optional features. See Fig.2, the PCB overlay diagram, to see which parts go where. Begin by fitting the surface-mount components. This includes the miniUSB socket, CON4, and possibly some of the capacitors as well as MOSFETs Q1 and Q2 for PWM backlight control. The pads for the mini-USB socket have been extended to make them easier to solder. Line the small posts in the underside of the socket up with the holes in the PCB; this should make everything else correspond. If so, solder one of the large mechanical pads in place to keep the socket in position and flush against the PCB. Now apply flux to the pads for the electrical connections. You should be able to touch the iron to the pad extensions, allowing the solder to run up to the pins on the socket. Ensure the four pins are well attached and not bridged. If there are any bridges, carefully remove with solder wick. The pin with the shorter pad is not used in this application and does not need to be soldered. Solder the remaining mechanical pads to complete the attachment of the socket. Double check your work, as it will be difficult to access the pins later with the other components installed. If you use SMD capacitors, they will all be the same type, but the two transistors are not. Check these are not mixed up before soldering them in place. For the other SMD components, which are all quite small, an easy way to fit these is to apply solder to one of the pads then hold the component on top with tweezers. Apply the iron again to allow the solder to melt onto the component lead. This avoids having to handle three things at the same time. If necessary, adjust the location of the parts so that the pins are fully lined up The V3 BackPack can also be populated with other sensors and ICs to extend what it can do without requiring external circuitry. These extra components include temperature and humidity sensors, an infrared receiver or a Flash IC for non-volatile data storage. Here’s how the 3.5-inch display fits over the BackPack V3 PCB. It can also accommodate the 2.8-inch display if you wish, but it’s designed to suit the larger display. 19 Fig.2: the slightly larger V3 BackPack PCB can accommodate a 2.8-inch (320×240 pixel) or 3.5-inch (480×320 pixel) LCD touchscreen, using the inner or outer set of four mounting holes respectively. whether you are using a 2.8-inch or 3.5-inch display – do fit both if you wish to experiment. It’s a good idea to temporarily fit the headers onto the display you are using during soldering as this will keep the headers aligned squarely and correctly. CON3 can be fitted at the same time, to simplify lining up the display with the BackPack. All that is left is to install the semiconductors. LED1 is mounted with its cathode (flat side) towards the USB socket. Ensure REG1’s outline matches the footprint on the PCB and solder it down close to the PCB. Both screens share the CON3 I/O and power connector, while CON6 makes electrical connections to the SD card socket on the smaller display, and CON7 on the larger display. CON2, the I/O header, is identical to that of the two earlier BackPack designs. with the pads, and when you are happy, apply some solder to the remaining pins. Finally, go back and retouch the first pin to relieve any stress in the solder. Through-hole parts The remaining components can be added in the usual order. Fit the 10kΩ resistor between IC1 and IC2, and the 1kΩ resistor near LED1. If you are using PWM backlight control, the two resistors below Q1 and Q2 must be fitted. (Remember to check resistor values with a DMM.) Alternatively, you can fit potentiometer VR1 for manual backlight control, or a wire link as shown in our photo (below right) if you prefer to have the backlight fully on all the time. If your potentiometer is more than 9mm tall, it may foul the display PCB and can be laid over in the space set aside if necessary. Solder the capacitors next. If you are using tantalum capacitors, then the larger 47µF capacitor goes next to IC1, while the two 10µF capacitors sit either side of REG1. Tantalum capacitors are polarised, so take care that the positive leads go to the pads with a ‘+’ sign. If you are using ceramic capacitors instead, their polarity does not matter and you can use a 10µF ceramic in place of the 47µF tantalum, ie, all three highvalue capacitors will be the same type. There are three MKT or multi-layer ceramic through-hole capacitors to fit. Solder them in place and trim their leads. Fit the two IC sockets next, if you are using them. These are a good idea 20 in case you need to replace one of the chips. The notches on both face to the left, towards the USB socket. Note that if you do use sockets, IC2 will touch the underside of the SD card socket on the 3.5-inch display. This shouldn’t cause any problems, but it can be avoided by separating the boards with 12mm tapped spacers. The tactile switch sits near the lefthand edge of the board. Ensure it is pushed down firmly against the PCB before soldering its pins. It may take some force, but should pop into place. JP1 can also be fitted, below the USB socket. Unless you are powering the BackPack from an external 5V power supply, the jumper shunt will need to be fitted to source power from the USB socket. The other headers should be fitted now. You will probably only need to install one of CON6 or CON7, depending on Fitting optional components The parts list mentions what components you need to populate each optional add-on section. These are all through-hole parts, except the Flash IC (IC3), which can be a through-hole or surfacemount type, and the DS32321 IC (IC4), which is only available in a surface-mount package. If fitting IRD1, you also need to mount the adjacent 100Ω resistor and 10µF capacitor used to filter and bypass its supply. It’s a good idea to mount IRD1 with long enough leads that you can bend its lens to face in the same direction as the screen. It can be soldered on either side of the PCB, as long as its lead connections are not reversed compared to what is shown in Fig.2. To fit IC4, the DS3231 IC, apply a small amount of flux to the pads and solder one pin in place. Check that its pin 1 dot is oriented as shown in Fig.2. Once you are happy that the part is flat and lined up with the other pins, carefully solder the rest. Ensure that no solder bridges have formed; if necessary, clean them up using flux paste and solder braid (wick). This is the basic version of the V3 BackPack. With these parts fitted, this provides equivalent functions to the V2 BackPack, except for the ability to use the larger 3.5-inch touchscreen. The two four-way headers at left allow either a 2.8-inch or 3.5-inch touchscreen to be fitted to this board. Practical Electronics | August | 2020 Parts list – MicroMite BackPack V3 (to provide the same functions as the V2 BackPack) Optional parts for infrared reception 1 three-pin 3.3V‡ infrared receiver (IRD1) [eg TSOP2136] 1 10µF 16V X7R multi-layer ceramic or tantalum capacitor (3216/1206 SMD or dipped leaded*) 1 100Ω 1/4W, 5% resistor ‡see text 1 double-sided PCB, coded 07106191, 99 x 54.5mm, available from the PE PCB Service 1 mini USB type-B socket, SMD (CON4) [Altronics P1308] 1 SPST momentary tactile pushbutton (S1) 1 28-pin narrow (0.3in) DIL socket for IC1 1 14-pin DIL socket for IC2 (optional) 1 4-way header (CON1) (Micromite UART breakout; optional) 1 18-way straight header (CON2) 1 14-way female header (CON3) 1 5-way right-angle header (CON5) (for ICSP; optional) 1 4-way female header (CON6 or CON7) 1 2-way header and jumper shunt (JP1) 8 M3 x 6mm panhead machine screws (for mounting LCD) 4 M3 x 12mm tapped spacers (for mounting LCD) 1 2.8-inch or 3.5-inch LCD touch panel [eg, SILICON CHIP ONLINE SHOP Cat SC3410] Optional parts for external RAM or Flash memory 1 SPI RAM or Flash IC, DIP-8 or SOIC-8 (IC3) [eg, 23LC1024 RAM or AT25SF041 Flash; pinout as in Fig.1] 1 100nF 50V MKT polyester or multi-layer ceramic capacitor 2 10kΩ 1/4W, 5% resistors Semiconductors 1 MCP1700-3302E/TO, TO-92 (REG1) 1 PIC32MX170F256B-50I/SP programmed with MMBasic firmware, narrow DIP-28 (IC1) – from micromite.org 1 PIC16F1455-I/P programmed with the Microbridge firmware, DIP-14 (IC2) – from micromite.org 1 3mm red LED (LED1) Optional parts for temperature/humidity sensor 1 DHT22 digital temperature and humidity sensor (TS1) OR 1 DS18B20 digital temperature sensor, TO-92 (TS2) 1 4.7kΩ 1/4W, 5% resistor Capacitors 3 10µF 16V X7R multi-layer ceramic capacitors (3216/1206 SMD or dipped leaded*) OR 2 10µF 16V tantalum AND 1 47µF 16V tantalum 3 100nF 50V MKT polyester or multi-layer ceramic Resistors (all 1/4W, 5%) 1 10kΩ 1 1kΩ Optional parts for PWM backlight control 1 2N7002 N-channel MOSFET, SOT-23 (Q1) 1 IRLML2244TRPBF P-channel MOSFET, SOT-23 (Q2) 1 10kΩ 1/4W, 5% resistor 1 1kΩ 1/4W, 5% resistor Optional parts for manual backlight control 1 100Ω 1/2W mini horizontal trimpot You will also need to fit the adjacent 100nF capacitor and both I2C pull-up resistors (4.7kΩ). It’s also a good idea to connect a battery (2.3-5.5V) via CON9. A CR2032 lithium battery is commonly used with the DS3231 and will last many years. You can either solder its leads to the pads for CON9 or fit a pin header and connect the battery using patch leads or similar. If you don’t connect a battery, IC4 will lose its time whenever power to the board is cut. There isn’t much room for a battery on the PCB (no mounting location is provided) so you’ll have to figure out how to mount it (eg, double-sided tape) and wire it back to CON9. If mounting it somewhere on this PCB, make sure it’s properly insulated so it can’t short to any of the tracks or components. Either the DHT22 (TS1) or DS18B20 (TS2) temperature sensor can be fitted, Practical Electronics | August | 2020 Optional parts for real-time clock 1 DS3231 RTC IC, SOIC-16 (IC4) 1 100nF 50V MKT polyester or multi-layer ceramic capacitor 2 4.7kΩ 1/4W, 5% resistors 1 2-pin header for CON9 (optional) 1 2.3-5.5V battery [eg, CR2032 lithium button cell; Jaycar Cat SB1762] Optional parts for external I2C interface 1 4-pin header (CON8) 2 4.7kΩ 1/4W, 5% resistors ^ ^ These resistors shared with RTC above. Optional parts for temperature/pressure/altitude sensor 1 GY-68 BMP180 temperature/pressure sensor module OR 1 GY-BMP280 temperature/pressure sensor module OR 1 GY-BME280 temperature/pressure/humidity sensor 1 4-pin header (CON8) 2 4.7kΩ 1/4W, 5% resistors ^ * eg, Mouser Cat 810-FA26X7R1E106KRU6 or Digi-Key Cat 445-173437-1-ND PCB, reduced kit and PCB with presoldered SMD parts The BackPack V3 PCB is available from the PE PCB service, A reduced kit of parts and PCB with the SMD parts presoldered are available – visit micromite.org for full details. but not both. They connect to the same pin on the Micromite (pin 5) but use different communication protocols. They share a single 4.7kΩ pull-up resistor (inside the box labelled TS1), which needs to be fitted if either TS1 or TS2 is installed. TS1 is tall, so it can be fitted laid over towards IC4 – the vented side of the case faces away from the PCB. If IC4 has already been fitted, there should still be room to lay TS1 on its side, but you will need to initially mount it slightly above the board so that it will sit flat on top of IC4 when bent over. If fitting an SMD Flash or RAM chip for IC3, orient it with pin 1 towards the bottom edge of the board, as shown in Fig.2. You can solder it using a similar technique as described for IC4 above. The through-hole version is easier to solder, and is oriented with its pin 1 dot or notch towards the left, as shown. In either case, you will also need to fit the adjacent 100nF bypass capacitor and the two 10kΩ pull-up resistors. Note that some Flash ICs have internal pull-ups; in this case, you can omit those resistors. Check your device’s data sheet to find out. To connect an external I2C module, including a BMP180 (GY-68 module), BMP280 (GY-BMP280 module) or BME280 (GY-BME280 module), fit pin header CON8 and the two 4.7kΩ resistors above it. As mentioned earlier, you can solder the module directly to CON8; match up its pinout, as printed on the module, with that shown in Fig.2 or on the PCB. Note that some modules include pullup resistors for the SDA and SCL lines. In this case, either don’t fit the resistors on the BackPack, or remove them from the module. There should be just one set of pull-up resistors in the circuit. 21 Programming the chips Both chips are available pre-programmed from our Micromite partner, micromite.org, but you only really need IC2 to be pre-programmed since it is capable of loading the software onto IC1, using pic32prog (see below). However, having IC1 pre-programmed will save you some effort. While it is possible to program IC2 using a BASIC program on IC1 and a 9V battery, we only recommend this if you have no other way, and this has a bit of a ‘chicken and egg’ problem, in that it only works if IC1 has already been programmed. (See http://geoffg. net/microbridge.html for more information on this technique.) You can program IC1 after fitting it, either using the ICSP header (CON5) and a PICkit or similar programmer, or by using IC2 in its Microbridge role. More information on using the Microbridge and its pic32prog software can be found in the article from May 2018 issue of PE. We’ll proceed assuming that you have pre-programmed chips, so fit them now. If you have used sockets, gently bend the leads of the ICs inwards to fit the sockets, otherwise, solder the chips directly to the PCB, taking great care that they are orientated correctly. Both ICs should have pin 1 facing towards the USB socket. It’s a good idea to solder two diagonally opposite corners and ensure the IC is flat and level before soldering the remainder. The V3 BackPack is now usable and can be tested. Plug the BackPack into a computer and it should show up as a new USB-serial device. Communication occurs at 38,400 baud on a freshly programmed Micromite, if you want to check this out now, using your favourite serial terminal program. Drivers Under Windows 10 and Linux, a driver should be automatically installed. If it is not, then the driver can be found at: www.microchip.com/ wwwproducts/en/MCP2200 – while this is a different device, it uses the same USB identification (VID and PID) codes as the Microbridge firmware. (Incidentally, the MCP2200 is nothing more than a PIC18F14K50 that has been programmed to act as a USB-serial bridge, which is why this driver works). When properly installed, the Micromite BackPack should appear as a new virtual COM port on your computer. Configuring the display The backlight controls work unchanged compared to the V2 BackPack (assuming you have fitted Q1, Q2 and their associated resistors). The backlight intensity is set on a scale of 0 to 100 with the PWM function thus: PWM 2,250,BACKLIGHT This command works because pin 26 is the first output of PWM channel 2. Alternatively, the backlight can be turned on or off by using the SETPIN and PIN commands to set the output of pin 26 high or low. If you are using a 2.8-inch display, then the same instructions as given in the article from May 2018 (on the V2 BackPack) apply. The following commands initialise and calibrate the display: OPTION LCDPANEL ILI9341,L,2,23,6 GUI TEST LCDPANEL OPTION TOUCH 7,15 GUI CALIBRATE GUI TEST TOUCH The 3.5-inch panel works slightly differently, as it depends on a CFUNCTION to work and is not quite as ‘transparent’ as the inbuilt display driver. See the panel titled Driving the 3.5inch touchscreen for details on how to set up and use the larger screen. If you have fitted any of the optional components, see the opposite panel, Using the optional components, which describes the software required to use them. Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au Breaking news from While we were preparing this article, Geoff Graham told us that Peter Mather had made a post on his forum, ‘The Back Shed’, describing a driver that he had created for the ILI9488 display controller. The Back Shed is a great place to get information on the various Micromites and other topics; see: www.thebackshed.com/forum/ His code for the display controller can be found at: www.thebackshed.com/forum/forum_posts.asp?TID=11419 It is implemented as a CSUB which is run by the Micromite at startup. The initialisation process is different to our CFUNCTION, but after that, you use the same native graphics commands as with our code. The code shown on the forum is for a different Micromite board, so the initialisation line needs to be changed to suit the pinouts used on the BackPack. Copy and paste his code labelled ‘MM2’ into a blank program, then change the second line from: ILI9488 16,2,9,1 to: ILI9488 2,23,6,1 These parameters determine the display CD pin, RST pin, CS pin and orientation. This changes the pin values to suit the BackPack. The orientation is a value from 1 to 4, as explained in the main text of this article. Upload the program to the Micromite and run the command: LIBRARY SAVE 22 To store the CSUB as a library instead of BASIC code, restart the processor with the command: WATCHDOG 1 The driver will then be loaded. At this stage, the Micromite is at the same state as if the OPTION LCDPANEL command had been run for the 2.8-inch screen, and normal touch panel initialisation can continue, like this: GUI TEST LCDPANEL OPTION TOUCH 7,15 GUI CALIBRATE GUI TEST TOUCH Readers who are comfortable with the usual way of setting up touch panels on the Micromite, such as the ILI9341, may prefer this method as it works similarly. However, note that you will lose the ability to use the SPI peripheral for other purposes, as is the case with the 2.8-inch display. Peter also noted the glitch with the MISO pin on these displays which we found (and worked around) while trying them out in our June 2020 3.5-inch displays article and then on the V3 BackPack board. Finally, future releases of the Micromite V2 firmware will include a copy of Peter Mather’s ILI9488 CSUB driver. Practical Electronics | August | 2020 Using the optional components Using the infrared receiver (IRD1) MMBasic only supports an infrared receiver on pin 16 of the 28pin PIC32, so that is where we have connected it. You therefore lose this pin as a general purpose I/O when you fit IRD1. MMBasic can trigger a software interrupt when a valid command is received and then call a user-defined subroutine; set up as follows: IR DevCode, KeyCode, IR_Int DevCode and KeyCode specify the variable names which will contain the device and key codes respectively when the user routine (‘IR_ Int’ in this case) is called. So you could define the function like this: SUB IR_Int PRINT “DEVICE:” DevCode ”KEY:” KeyCode END SUB Using the real-time clock MMBasic has built-in routines to use an RTC module connected to the hardware I2C pins, as is the case here. Set the Micromite’s internal clock from the DS3231 IC thus: RTC GETTIME Setting the time on the DS3231 is done with a single command specifying the current date and time: RTC SETTIME year, month, day, hour, minute, second If you are using any other I2C devices, you can connect them via CON8. If, as is often the case, the module(s) have their own pull-up resistors, either remove them or omit the onboard I2C pull-up resistors. It may work with both in place, but this is not recommended Temperature and humidity sensors The temperature from a DS18B20 (TS2) can be read with a single MMBasic command: TEMPERATURE = TEMPR(5) Functions for communicating with a DHT22 were built into early versions of MMBasic, but have been removed in later versions; instead, a CSUB is supplied to do the same job. The required code and documentation can be found in the ‘Humid.pdf’ file in the ‘Embedded C Modules’ subfolder of the Micromite firmware download, available from http://geoffg.net/micromite.html#Downloads After the CSUB has been copied into the BASIC program, the temperature and humidity can be read by a single command like this: HUMID 5, TEMPERATURE, HUMIDITY The first parameter (5) tells this function which Micromite pin the DHT22 sensor is connected to. The results are saved in the TEMPERATURE and HUMIDITY variables. Due to the way the DHT22 works, the results are actually from the previous time this command was issued, with the current call starting the next conversion in the background. Therefore, you will need to ignore the values of TEMPERATURE and HUMIDITY the first time you call this command. Hence, it’s a good idea to issue it during your initialisation routine. Practical Electronics | August | 2020 Using a RAM chip We test-fitted our board with a 23LC1024 RAM IC (IC3). It’s similar to the 23LCV1024 used in the 433MHz Wireless Range Extender project (see PE, May 2020). There is no WP (write-protect) function on the RAM IC, but it does have a HOLD pin which needs to be held high, so the 10kΩ pull-up resistors are still required. We’ve written a sample program to demonstrate using such a chip, which is named ‘23LC1024 RAM IC.bas’. It simply writes data to the chip, based on the contents of the TIMER variable, then reads those values back and prints them out on the Micromite terminal. The CS pin of IC3 is hardwired to the Micromite’s pin 4, and this is set as a constant at the start of the sample program. The SETRAMMODE subroutine provides page, byte and sequential options. Using the sequential option means that the entire RAM contents can be read or written in one pass. A read or write starts with a STARTRAMREAD/STARTRAMWRITE command, which pulls CS low and sends a command sequence containing the supplied start address. After that, subsequent calls to RAMREAD or RAMWRITE read or write a single byte before incrementing the address pointer. The sequence ends with a call to ENDRAMREAD/ENDRAMWRITE which brings CS high, releasing the SPI bus. Using external Flash memory For testing out the Flash interface, we tried an AT25SF041 4Mbit (0.5MB) Flash IC (again, as IC3). On this chip, the WP and HOLD pins are internally pulled high, so the 10kΩ resistors are not needed, although they were fitted to our prototype; it doesn’t hurt to have both internal and external pull-ups. Writing to the device is a bit more complicated than for a RAM chip, but reading uses the same command and format as the RAM IC. Flash memory cannot usually be written byte by byte. An entire ‘page’, 4KB in this case, must be erased (set to all 1s), then data can be written byte by byte (or ‘programmed’ according to the data sheet terminology). Writes occur in blocks of up to 256 bytes. The data to be written is actually stored into a RAM buffer; it isn’t written to Flash until the CS line goes high, at the end of the process. There are a few more details than what’s described here; so the device data sheet is a good place to check out the subtleties of the process. One wrinkle, for example, is that the writes will wrap around at addresses that are multiples of 256 bytes. There is also a software flag (WEL; write-enable latch) that must be set before any changes (erase or write) can occur to the Flash memory contents. Thus, a typical write sequence would consist of setting the WEL flag, erasing a page, setting the WEL flag again and then writing the actual data. The sample program is called ‘AT25SF041 FLASH IC.bas’. Unlike the RAM demo, which loops continuously, this program reads the Flash once, writes data to the Flash once, then rereads it, displaying the results on the terminal. This is to avoid wearing out the Flash. The Flash chip we used has a minimum endurance of 100,000 cycles, which means that it would take 27 hours at one write per second (to the same part of the Flash memory) to potentially cause a failure. Using a BMP180, BMP280 or BME280 sensor module We published an article in the December 2018 issue of PE explaining how to use a BMP180 or BMP280 module with a Micromite. You can download the sample BASIC code for free from the August 2020 page of the PE website. The BMP180/BMP280 provide temperature and pressure/altitude data, but the BME280 also includes humidity. You can find MMBasic source code to read data from a BME280 sensor at the (very useful) BackShed forum: www.thebackshed.com/forum/forum_posts.asp?TID=8362 23 Driving the 3.5-inch touchscreen When using the 2.8-inch touchscreen, you set it up once using the OPTION command (as described in the main text) and from then on, the Micromite automatically configures it each time the chip is powered up. But because MMBasic doesn’t natively support the 3.5-inch touchscreen, setting it up is a bit different. You need to run some code at the start of your program, every time the chip is powered up, to configure this display. This code initialises the display and also sets up the ‘hooks’ into Micromite BASIC’s graphics commands so that you can draw to this screen using the same commands as for the 2.8-inch display. One big difference of this implementation is that it does not block use of the SPI pins to other interfaces. In fact, the user program must start the SPI peripheral just as for any other interface. This is also the reason why the in-built touch commands won’t work, as they too require exclusive use of the SPI interface. Although the control pins for the LCD and touch controllers (such as CS, DC and RESET) are hardwired into the CFUNCTION to match the hardware that is on the BackPack, they need to be set up by the user program. The advantage here is that control can be taken back if your program wants to use these pins for other purposes. The CFUNCTION assumes that all this setting-up has been done, and will fail if it has not. This is so that the CFUNCTION has minimal overhead and is thus quite fast. This is handy, as the 3.5-inch displays have twice as many pixels to manage as the 2.8-inch displays. The following code needs to appear before the display functions can be used with the 3.5-inch display. You can also find this code in our example programs: DIM INTEGER ROTATION=1,BUCKET, ILI9488_SPI_ADD ILI9488_SPI_ADD=PEEK(CFUNADDR ILI9488_SPI) SPI OPEN 20000000,0,8 SETPIN 2,DOUT SETPIN 23,DOUT SETPIN 6,DOUT BUCKET = ILI9488_SPI(ILI9488_SPI_ADD, ROTATION) The first line defines three integer variables. ROTATION sets the display orientation. Set it to a value between one and four. Mode one is portrait, two is landscape, three is upside-down portrait and four is upside-down landscape. BUCKET (the ‘bit-bucket’) is used as a place to store the return value of the CFUNCTION. BASIC insists on us storing the return value of a function when calling it, so even though we don’t need to use that return value, we need somewhere to store it. ILI9488_SPI_ADD is used to hold the Flash memory address (shortened to ‘ADD’) of the CFUNCTION. This needs to be passed to the CFUNCTION during the initialisation stage, as it needs this to set up the hooks into the native graphics functions. The address of the CFUNCTION is retrieved by using the PEEK function on the second line. We have called the CFUNCTION ‘ILI9488_SPI’, so if you change this, you will need to change that second line too. The next four lines set up the micro’s SPI peripheral and set up the I/O pins used to control the screen’s CS, DC and RESET lines. Finally, the display is initialised by our CFUNCTION according to the ROTATION setting. After this, you will normally clear the screen using a command like this: CLS(RGB(BLACK)) Our demonstration program, ‘ILI9488_SPI_minimal working. bas’, can be downloaded from the August 2020 page of the PE website. 24 This sets up the display as described above and then draws some patterns on the screen using the inbuilt graphics functions. Using the touch interface As mentioned in the text, MMBasic’s built-in touch panel support doesn’t play well with our new driver. We suspect that this is because the display driver is not initialised when the touch controller attempts to start up at Micromite boot time. So we have written a separate CFUNCTION to provide the touch functions. The ‘ILI9488 with touch calibration.bas’ demonstration program (in the same download package from the PE website) shows how to read raw touch data and also calculate touch locations on the screen. In addition to initialising the display controller, as noted above, the following lines are required to use the touch controller: DIM INTEGER TOUCH_X0,TOUCH_Y0, TOUCH_X1,TOUCH_Y1 TOUCH_X0=110 TOUCH_Y0=1993 TOUCH_X1=2001 TOUCH_Y1=76 SETPIN 7,DOUT These four variables provide touch panel calibration. Our calibration sketch generates a new set of calibration values for a specific touch panel, which can be pasted back into your program. The ROTATION variable also needs to be set, as described earlier, since the calibrated touch coordinates depend on the display rotation that is being used. The last line sets up the Micromite pin used to drive the touch controller’s CS (chip select) line. To retrieve the x-axis component of the current touch position, use the following CFUNCTION call: X=XPT2046(0,ROTATION,TOUCH_X0,TOUCH_Y0, TOUCH_X1,TOUCH_Y1,MM.HRES,MM.VRES) This CFUNCTION requires no initialisation, although it assumes that the SPI interface has already been set up, as this is required to use the display anyway. This CFUNCTION reduces the speed of the SPI bus below the 2.5MHz limit of the touch controller IC for the duration of the CFUNCTION, and returns it to its previous value afterwards. To read the y-axis, the value of one is passed as the first parameter instead: Y=XPT2046(1,ROTATION,TOUCH_X0,TOUCH_Y0, TOUCH_X1,TOUCH_Y1,MM.HRES,MM.VRES) To retrieve the raw ADC values (which are necessary for the calibration), values of two, three or four are passed as the CFUNCTION’s first parameter. The z-axis value (with the first parameter as four) corresponds to the pressure on the touch panel, and is used by our function to check whether a valid touch is occurring; for example: RAWX=XPT2046(2) RAWY=XPT2046(3) RAWZ=XPT2046(4) By using the z-axis value, the IRQ pin on the touch controller is not needed for the 3.5-inch displays, although it is left connected on our board, for use with the 2.8-inch displays. Practical Electronics | August | 2020