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Constructional Project MICROMITE EXPLORE-40 A wealth of software has been written for the Micromite; The Back Shed online forum is a great place to find much of it. This compact Explore-40 board is a Micromite in the same form factor as the popular Pico boards, allowing a Micromite to be used with hardware designed for the Pico. PROJECT BY TIM BLYTHMAN T HE RASPBERRY PI PICO has taken a well-deserved place as one of the most popular microcontroller boards. It is cheap, easy to use and can be programmed in C, BASIC, Micro­P ython and even with the Arduino IDE. Our Pico BackPack project in the March 2023 issue capitalised on those features, providing stereo audio and a microSD card interface with the 3.5in LCD panel that we had previously used with the Micromite V3 BackPack. These new features can now be accessed from Micromite BASIC, since the Explore-40 board allows a Micromite processor to be plugged into the Pico BackPack. Thanks in part to the ongoing work of The Back Shed forum members, software is available to use these new features. The Micromite Explore-40 is not just a Micromite/PIC32 breakout board. It has been designed to include niceties like an inbuilt USB-serial converter, plus some LEDs and pushbuttons. The Explore-40 The Explore-40 is typical of minimal Micromite implementations that include the Microbridge USB-­serial converter. The circuit resembles earlier Micromite boards like the Explore-28 from the September 2020 issue. Since this board is patterned after the Raspberry Pi Pico and thus a bit larger than the Explore-28, we have Micromite Explore-40 Features & Specifications » Allows a PIC32 Micromite processor to be plugged into a Pico socket » All 28-pin Micromite I/O pins are available » Onboard Microbridge serial interface/programmer » USB-C socket for power and data » Micromite BASIC software examples for all Pico BackPack features » Supports LCD touch panel with backlight control » Supports IR receiver » Stereo audio output » microSD card interface » Realtime clock interface » Add-on 3.5mm board provides 3.5mm stereo audio socket with Pico BackPack » Power and status LEDs » Reset and Mode pushbuttons » In-circuit serial programming (ICSP) header for the PIC32 Micromite chip 6 taken the opportunity to add some extra features. Circuit details Fig.1 shows the circuit of the Explore-40. IC1 is a PIC32MX170F256B in a relatively large 28-pin SOIC package. This is the familiar 28-pin part we have used for many Micromite projects. Its I/O pins are connected to pins on the pair of 20-way headers that match the pinout of the Pico. We’ll explain our choices for this specific mapping a bit later. As the Pico has more pins than the 28-pin PIC32, there are some empty positions on those 20-way headers. IC2 is a PIC16F1455 programmed with the Microbridge firmware. The Microbridge was originally published as a separate board in the May 2018 issue. It has since been incorporated into many Micromite designs. It can function as a USB-serial converter, allowing communication between a computer and the Micromite chip. The Microbridge can also act as a programmer, allowing new firmware (such as a new version of Micromite BASIC) to be easily installed on the Micromite chip. As such, it connects to the data lines on USB-C connector CON1, as well as the serial and programming pins of IC1. IC2 also drives LED1, which indicates its mode (USB-serial or programming) and shows serial traffic. Onboard pushbutton S2 selects IC2’s mode. Practical Electronics | August | 2025 The Micromite Explore-40 CON1, the USB-C socket, has connections to power via the VBUS pins. The CC1 and CC2 pins are connected to ground via 5.1kW resistors, signalling to the USB source (eg, a computer) that it should supply 5V on the VBUS pins. The VBUS voltage goes via schottky diode D1 to REG1, an MCP1700 3.3V low-dropout regulator. The diode also connects to pins 40 and 39 of the Pico headers, emulating that handy feature of the Pico boards. It means that an alternative source of 5V power can be fed into pin 39 (possibly via another diode) without any risk of back-­feeding the USB power supply. REG1 and its capacitors provide a 3.3V rail that powers IC1, IC2 and power indicator LED2 (the latter via a 1kW resistor). The 3.3V output is also available on pins 35 and 36 of the headers, as it is on the Pico boards. IC1’s pin 1 (the MCLR reset input) has also been taken to pin 30 (RUN on the Pico). Pins 3, 8, 13, 18, 23, 28, 33 and 38 of the headers are connected to ground, like the Pico, and we have connected as many of the PIC32’s I/O pins as we can to the remaining I/O pins on the headers. Because of this compatibility, we’re sure readers will find the Explore-40 handy in other situations where a Pico might be used. IC1’s MCLR reset pin, 3.3V, ground and the two ICSP programming pins are also available at the CON2 ICSP header, allowing the chip to be programmed by an external programmer. IC1 can also be reset by pressing S1, which pulls MCLR to ground. A 10kW resistor pulls this up otherwise. A reset button is one feature that the real Pico lacks! Micromite Explore-40 Kit from Silicon Chip (SC6991, ~£21) A complete kit is available for the Micromite Explore-40 with all the parts listed in the parts list on page 11 (not including the Audio Breakout Board or Pico BackPack) – see page 78 for details. Pin mapping The mapping of the 40-pin header has been mostly chosen to match the functions of the Pico BackPack to that of the Micromite. For example, the Micromite has fixed SPI and I2C pins, so the mapping matches the wiring of these two peripherals on the Pico BackPack. Similarly, the pins for interfacing with the LCD on the V3 BackPack have been arranged identically on the Explore-40. This allows identical Micromite OPTIONs to be used. The infrared/IR pin (Micromite pin Practical Electronics | August | 2025 Fig.1: the Explore-40 has much in common with the Micromite V2 BackPack and the Explore-28, although we’ve added a USB-C socket, power indicator LED and a reset button. The I/O pin mapping to the two 20-pin headers is designed to allow the Micromite processor to work with the Pico BackPack and retain some compatibility with software designed for the V3 BackPack. 7 Constructional Project 16) has also been connected to the IR receiver on the Pico BackPack. Pins 21 and 22 on the Micromite have been connected to pins 11 and 12 of the Pico header; these are used for audio on the Pico BackPack and are a convenient pair for this purpose. If not used for audio, they can be used as the Micromite’s COM1 serial port. The serial console pins have been allocated to pins 1 and 2, allowing the console to be connected to the Pico Digital Video Terminal. That doesn’t leave many pins spare to be allocated. We have connected pins with analog functions where possible, although the Pico has fewer than the Micromite. We’ll detail the OPTIONs and pins that should be used with the Pico BackPack later, when we explain the software features in more detail. A small add-on While putting together this design, A 3.5mm jack socket breakout board for the Pico BackPack Building this board is simple, as you can see from our photos. As long as you connect the R, G and L pins to the matching pins on CON3 of the Pico BackPack, the board can be installed in a few different ways. It can be mounted on either side of the board, giving four main configurations. We think the method shown in our photos is the simplest, gives a compact result and does not put the audio socket awkwardly close to the microSD card socket. The Audio Breakout extends slightly beyond the Pico BackPack and is intended to sit just inside a UB3 Jiffy box so that the socket can be accessed through a small hole in the side. We suggest fitting the audio socket to the PCB first. That will allow you to easily check that your chosen positioning does not foul any other components. The photo shows the assembly of the listed parts that can then be fitted to the Pico BackPack. Once fitted, you can simply plug in headphones or an aux cord to hear audio from CON3 on the Pico BackPack. This is the recommended placement of the 3.5mm jack socket breakout board on the Pico BackPack, sitting above some passive components in the audio section. Although it’s designed to work with the Pico BackPack, you can also use it for breadboarding or prototyping. Fig.2 (below): when assembling the breakout board, ensure the socket is pushed firmly against the PCB. We used straight headers, but you could use rightangled headers. 8 we realised adding a 3.5mm audio output jack socket to the Pico BackPack would be a nice touch. We initially omitted this from the Pico BackPack because the board is quite tight for space. To solve this, we’ve designed a very small daughterboard that can be connected to the Pico BackPack, breaking out the CON3 audio connector into a 3.5mm stereo socket. It is shown in Fig.2. You don’t need to use the Explore-40 to use the daughterboard; it can also be used with a Pico or Pico W. You can see it in our photos, mounted above the Pico BackPack PCB. We have a panel showing how to build this board and add it to the Pico BackPack. Programming the chips IC1 can easily be programmed via IC2 once you have built the board, but IC2 is best programmed before it is soldered to the board, especially as there is no ICSP header for it. It is possible to use a Micromite to program a Microbridge; there are notes on how to do that included with the Microbridge firmware at https://pemag. au/Shop/6/4269 Still, it is easier to program IC2 with something like a PICkit or SNAP if you have an appropriate SMD adaptor, so we recommend doing that if possible. If you buy a kit from us, both ICs will be programmed already; there is also the option to buy programmed chips separately. Construction The Explore-40 uses mainly SMD parts, including SOIC ICs, M2012 (0805 imperial) passive components measuring 2.0 × 1.2mm, and a somewhat fine-pitch USB-C socket. It is not super difficult, but neither is it extremely easy; it would be ideal to have some SMD soldering experience before assembling it. There are also components on both sides of the PCB. You will need the usual SMD tools and consumables. A fine- or medium-­ tipped soldering iron, solder, flux paste, tweezers and good ventilation are essential. Some solder-wicking braid and a means of securing the PCB are also advised. Blu-Tack will do the job if you don’t have a PCB vice. You should also have a suitable solvent for cleaning up flux, such as one recommended by your flux supPractical Electronics | August | 2025 The Micromite Explore-40 plier. Alternatively, isopropyl alcohol or methylated spirits will be effective for most fluxes. Fig.3 shows the PCB overlays, which you should refer to during assembly. Start by soldering CON1, the USB-C socket, since it has the closest pin pitch. It will also be difficult to get to once other components are installed. Apply flux to the pads and slot the socket into its holes on the top of the PCB. Clean the iron's tip and add a small amount of fresh solder. The end-most leads are a bit wider, so tack one of those in place, then check that the other leads are aligned to their pads and that the part is flat against the PCB. Adjust it until you are satisfied. The locating posts should help here. You can then solder the mounting pins from the reverse of the PCB. It might help to add some flux to the bottom and top of those pins to help the solder take. Try not to add too much solder to the mounting pins, as it might get in the way later. Next, solder the remaining pins of CON1 on the top of the PCB. Use the braid and extra flux to remove any bridges that have formed. Place the braid on the solder, apply the iron and gently move both away together once the solder has been taken up. Fit the two ICs next, being sure to get the correct orientation. IC2 faces the opposite direction to IC1 and is on the opposite side of the PCB. Add flux to the PCB, rest the ICs in place and tack one lead before soldering the others. Adding flux to the pins before soldering will help it flow. Check for bridges after soldering and remove any with more flux paste and the solder wick. Regulator REG1 mounts on the same side as IC2. It’s easy enough to solder but small enough to lose sight of easily. Add some flux and place it as shown. Tack one lead, then check the alignment of the others before soldering. The diode mounts on the opposite side of the board from the USB-C socket. Ensure that the PCB's cathode mark matches the diode orientation and avoid bridging its pads to the socket's mounting pins. Now solder the remaining parts on the underside of the PCB methodically. The resistors will have small codes printed on top (per the parts list) but the capacitors will be unmarked. You Practical Electronics | August | 2025 Fig.3: we’ve placed components on both sides of the PCB to best use the available space. The CON1 USB-C socket and the two microcontrollers have the tightest pin pitches, so they should be fitted first. Avoid using too much solder for CON1 through-hole mounting pins in case it bridges to D1 or the nearby resistors. This diagram is shown at 150% of actual size for clarity. may be able to tell them apart by their thickness if you manage to get them mixed up. In each case, add flux to the pads, rest the part in place, tack one lead, then check and solder the other. Next come the two LEDs on the top side of the board. We recommend using red for LED1 (MODE) and green for LED2 (POWER), although you could choose your own scheme. You can test the colour and polarity of the LEDs with a multimeter set to diode mode. The cathode will be the end connected to the black multimeter lead when the LED lights up. Fit the LEDs with the cathodes towards the COM2 silkscreen marking (the overlay also shows a K near each cathode). Solder the last 1kW resistor and 100nF capacitor. Clean both sides of the PCB thoroughly with your chosen flux solvent and allow the PCB to dry. It’s then a good time to inspect the soldering for any bridges or dry joints you might have missed. If you find any, fix them before proceeding. Fit the two tactile switches next. They have much larger pads, making them easier to solder than the other parts. Your board should look like the photos now. If something is not right, check for 5V at the USB pin, at upper right, and around 4.7V (due to the diode) at the SYS pin below it. Check the USB-C socket and 5.1kW resistors if the USB voltage is absent. An absence of voltage at the SYS pin suggests the diode is reversed or not connected, while a lack of 3.3V could point to a problem with the regulator or a short circuit on the 3.3V rail. If you need to fit the CON2 ICSP header to program IC1, do that now. Be aware that you may not be able to leave CON2 attached afterwards since it might be too tall to fit between the Pico BackPack PCB and the LCD Testing There are still some parts to fit, but now is a good time to do some initial tests. Connecting USB power to CON1 should cause LED2 to light up. The 3.3V pin should measure between 3.2V and 3.4V relative to ground. Pressing S2 should cause LED1 to light up, assuming IC2 is programmed correctly. The Explore-40 is a compact board (51 × 21mm) that allows the Micromite to substitute for a Raspberry Pi Pico in some circumstances. IC1 and the two LEDs are the polarised components on the top of the PCB. We recommend using red for LED1 and green for LED2. 9 Constructional Project fitting the Explore-40 to a Pico BackPack with an LCD panel above. If you just plan to use it on a breadboard, for example, you just need to be sure that the pins align with the sockets in the breadboard. Fitting it to a Pico BackPack The underside of the Explore-40 shown at actual size; note the orientations of IC2 and D1. REG1 is also polarised, but its correct orientation should be obvious. panel. You can use IC2 to program IC1, after all. If you connect the Explore-40 to a computer and open a serial terminal program such as TeraTerm, you should be able to communicate with the Micromite firmware. The default baud rate is 38,400. You can press S1 and check that the Micromite’s boot message is printed via the terminal. The Explore-40 is now complete enough to plan how you will fit it to the Pico BackPack. The most significant difference is that the Explore-40 has components on its underside, so it will not mount flush like a Pico could. The following assumes that you are For our prototype, we used low-­ profile header sockets and removed the plastic shroud from the pin headers to allow the board to be swapped (eg, for a Pico) if needed. However, we found that quite fiddly to achieve. As you can see in the photo below, there is very little clearance above the Explore-40, even though we removed the SD card socket from the LCD panel above. Still, that is an option to consider since there is no connection to the SD socket on the LCD panel from the Pico BackPack. If you want to do that, use a pair of flush nippers to gently cut and detach each pin from the SD card socket, then use a soldering iron to remove the remnants of each pin. Follow with some solder-wicking braid and flux paste to remove any solder residue. If you are happy to permanently solder the Explore-40 to the Pico BackPack, the height of the plastic spacers on standard pin headers will prevent the underside components from touching the PCB below. To do this, sandwich the headers between the Pico BackPack and Explore-40 PCBs, then tack a few pins in place before soldering the remainder and trimming the excess lengths away. If you are doing something different, we recommend test-fitting the parts first to be sure they will fit and not cause any fouling with the LCD panel above. It’s also possible to fit the Explore-40 to the underside of the Pico BackPack PCB, although that will make it difficult to access the buttons or see the LEDs. If you want to do that, we suggest using socket headers fitted to the top of the Explore-40 that will mate with pin headers mounted on the underside of the Pico BackPack. Software support Combining the Explore-40 with the Pico BackPack (and 3.5in LCD panel) brings two new features that were not present on the V3 Micromite LCD BackPack. These are the microSD card and audio output. First we’ll recap the features that are shared with the Micromite V3 BackPack and how they are configured and used. This will be a quick way to check that the Explore-40 is working as expected. These features should all behave identically to a Micromite V3 BackPack. Note, though, that the Explore-40 and Pico BackPack lack the RAM or FLASH IC and temperature sensors that the V3 BackPack includes. The Micromite firmware does not have a built-in driver for the 3.5in LCDs, but there is a loadable driver developed by Peter Mather. We have customised this to suit the configuration of the Explore-40 and Pico BackPack hardware; it is the “3.5IN DRIVER. BAS” file in the software downloads package. The code is much the same as that found in the Display Drivers folder of the Micromite firmware download. We have just changed the line in the MM.STARTUP subroutine to suit our pin allocation. The “3” at the end indicates a landscape configuration, with the microSD card socket near the top of the screen. Load this file onto the Micromite (for example, using the AUTOSAVE command), then perform a LIBRARY SAVE and restart the Micromite by pressing S1 or entering the CPU RESTART command. You should see the screen clear and you can run the GUI TEST LCDPANEL command to confirm it is working. To configure, calibrate and test the touch panel, use these commands: OPTION TOUCH 7,15 GUI CALIBRATE GUI TEST TOUCH We used low-profile header sockets to mount our prototype Explore-40, but if you solder it directly to the BackPack PCB using standard header pins, you will gain clearance since the Explore-40 will sit lower. With some care, the unused SD card socket on the underside of the 3.5in LCD panels can be removed, giving extra clearance below. Use solder-wicking braid to clean off any excess solder left behind. 10 If the required components and jumpers are fitted to the Pico BackPack, the backlight is also driven from IC1’s pin 26, just like the V3 BackPack. This can be controlled using PWM channel 2A. The following will set Practical Electronics | August | 2025 The Micromite Explore-40 the duty cycle and backlight brightness to 50%: PWM 2,250,50 IR receiver & realtime clock The IR receiver on the Pico BackPack is routed to the dedicated Micromite IR pin, pin 16, so the IR receiver can be used by simply setting up the IR interrupt with the IR command. The command would be something like: IR DevCode, KeyCode, IR_Int A basic interrupt subroutine to test this could be: SUB IR_Int PRINT “DEVICE:” DevCode “KEY:” KeyCode END SUB The RTC commands support the realtime clock chip: RTC GETTIME RTC SETTIME year, month, day, hour, minute, second You can then retrieve the current time and date from the TIME$ and DATE$ variables. MicroSD card support The Micromite lacks a native driver for interacting with SD cards. Peter Mather has again done some excellent work in creating a CSUB driver to do that. However, there are a few provisos to using this software. Since the Micromite does not have an interface for file handling (unlike the Micromite Plus), everything is done via calls to the CSUB. The driver is quite simple and cannot do things like create or append to files. So, if you wish to write to a file, the recommendation is to create a large file on the card, which the driver can then overwrite. Even with these restrictions, the driver takes up about onesixth of the flash memory available for programs. More background information on this and suitable code can be found at https://pemag. au/link/abxr We have configured pin 4 as the CS (chip select) pin for the microSD card socket. This is the same pin that is wired to the SD card socket on the LCD panel for the Micromite V3 BackPack. So you could try this on a V3 BackPack, although we haven’t tested it. Practical Electronics | August | 2025 Parts List – Micromite Explore-40 1 51 × 21mm double-sided PCB coded 07106241 1 16-pin USB-C data and power socket (CON1) [GCT USB4105] 1 5-way pin header, 2.54mm pitch (CON2; optional, for ICSP) 2 20-way pin headers, 2.54mm pitch 2 SMD 2-pin tactile switches (S1, S2) Semiconductors 1 SS14 40V 1A schottky diode, DO-214AC/SMA (D1) 1 PIC32MX170F256B-50I/SO 32-bit microcontroller programmed with the Micromite firmware, wide SOIC-28 (IC1) 1 PIC16F1455-I/SL 8-bit microcontroller programmed with the Microbridge firmware, SOIC-14 (IC2) 1 MCP1700-3.3 3.3V low-dropout voltage regulator, SOT-23 (REG1) 1 red M3216/1206/SMA SMD LED (LED1) 1 green M3216/1206/SMA SMD LED (LED2) Capacitors (all SMD M2012/0805, X7R) 1 22μF 10V X5R/X7R 2 1μF 16V 3 100nF 50V Resistors (all SMD M2012/0805, ⅛W) 1 10kW (code 1002 or 103) 2 5.1kW (code 5101 or 512) 5 1kW (code 1001 or 102) Optional extras 1 Pico BackPack (without Raspberry Pi Pico) plus 3.5in LCD (March 2022) 1 3.5mm jack socket breakout board (see panel and parts below) Audio Breakout Board 1 double-sided PCB coded 07101222, 20 × 15mm 1 stereo 3.5mm PCB-mounting jack socket (CON3A) [PJ-307 or similar] 1 3-way pin header (CON3) The Explore-40 module is a drop-in replacement for a Pico on the Pico BackPack (described separately). 11 Constructional Project The driver file is named “SDCARD_ SPI1.BAS”. It is installed similarly to the LCD panel driver, using the AUTOSAVE and LIBRARY SAVE commands. We’ve also created a HEX file that contains these two libraries loaded into a working copy of Micromite BASIC version 5.05.05, named “MM BASIC SD ILI9488.HEX”. You can load this with the onboard Microbridge or a PICkit programmer. Audio support The audio driver is another CFUNCTION that is controlled via calls with various parameters. This is based on a similar driver we created for the Advanced GPS Computer in the June & July 2022 issues. This uses a pulse-width modulation (PWM) output to synthesise an analog voltage signal, with the PWM switching frequency being filtered out by a low-pass filter attached to that pin. The analog voltage is varied using a timer interrupt to update the PWM duty cycle for each sample to be played. The big difference is that this driver is capable of stereo output, although it is limited to eight bits of resolution and an 8kHz sampling rate. Given that the Micromite has enough flash memory to play only seven seconds of audio, or enough RAM for about six seconds, we think it is a fair compromise. The AUDIO folder in the software downloads contains several files, including the CFUNCTION driver, some encoded audio samples and BASIC code to demonstrate how to use the driver. The samples are created as CFUNCTIONs, although they do not contain executable code. They consist of a 32-bit header that indicates how many bytes are in the sample, followed by that many bytes. Stereo samples are stored with the left channel data first. A mono sample played in stereo mode will play twice as fast since two bytes are used every sample period. The driver is installed by loading the “CFUN_LIBS.BAS” file onto the Micromite, then using the LIBRARY SAVE command. Since the CFUNCTION returns a value, we need to do something with that value, like print it. Use this to start the driver: PRINT AUDIO(0) A sample is used by loading its BASIC file, then performing a LIBRARY SAVE. Tell the driver where the sample is located like this: PRINT AUDIO(1, PEEK(CFUNADDR SAMPLE_NAME)) Then start playback with: PRINT AUDIO(2) ‘mono PRINT AUDIO(12) ‘stereo The sound will play in the background and stop automatically. Using values 6 (mono) or 13 (stereo) as parameters will cause the playback to loop endlessly. Playback can be forced to stop with: PRINT AUDIO(3) You can also wait for playback to finish with: The 3.5mm jack socket breakout board is a neat fit under the LCD panel, even when mounted on header pins. Like the Explore-40, you should trim any excess pin length with flush nippers or sidecutters. 12 DO WHILE AUDIO(4)<>0:LOOP The “BASIC_SUBS.BAS” file has some more sample code and variables that can be used to make it easier to see what each parameter does. The file named “AUDIO MMBASIC. HEX” contains the libraries, samples and BASIC code, alongside a working copy of Micromite BASIC version 5.05.05. Notes The 28-pin Micromite has somewhat limited peripherals, so there are some limitations. For example, the timer that provides the interrupt to fetch new audio samples is the same one used for the IR decoder. So we don’t think it is possible to use the IR and audio features at the same time, although it should be possible to switch between them. The audio output uses two of the remappable PWM channels, so the PWM feature on pins 4 and 5 cannot be used at the same time as the audio. Pin 4 is mapped to the microSD card socket, so we expect it will be used for that feature instead. In any case, the CFUNCTION libraries take up quite a bit of program memory, as do audio samples, if kept in flash memory. Note also that the PIC32MX170F256B microcontroller can be programmed in the low(er)-level C language using the MPLAB X IDE. For example, we have also seen an MPLAB X project that can play stereo audio from an SD card, so this could be another way to program this hardware combination. Summary If you are a Micromite fan and yearning for the features of the Pico BackPack, the Explore-40 is the perfect way to bridge that gap. It adds microSD card support and stereo audio features that were not included on the earlier Micromite BackPack designs. There are some limitations to what the Micromite can achieve, but it is still a handy platform for learning the BASIC language. The Explore-40 also adds nice touches, like the modern USB-C socket and reset button. These features can be handy regardless of whether the Explore-40 is used by itself, on a breadboard or as part of PE a BackPack. Practical Electronics | August | 2025