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8-pin 14-pin 20-pin PIC PROGRAMMING HELPER It’s incredible what you can achieve with an 8-pin microcontroller. However, programming and debugging these chips can be a challenge due to the need to use the programming and reset pins for other purposes. This little board makes working with these (and some larger) PICs much easier! W e include 8-pin PIC microcontrollers in many of our projects because they are very handy for doing certain jobs, and cheap to boot. Apart from a handful of 6-pin parts, which are only available in SMD packages, they are some of the smallest microcontrollers around. For example, we used a PIC12F1572 8-pin micro in our LED Christmas Ornaments project (November 2020; siliconchip.com.au/Article/14636). In that case, despite only having eight pins with two dedicated to power, it was able to control twelve LEDs and light them up in patterns. We have also used parts like the PIC12F617 in projects such as the Car Radio Dimmer Adapter (August 2019; siliconchip.com.au/Article/11773), the MiniHeart heartbeat simulator (January 2021; siliconchip.com.au/ Article/14706) and the Refined Fullwave Universal Motor Speed Controller (April 2021; siliconchip.com.au/ Article/14814). If you only need five or six I/O pins, then devices like these are handy and compact, while still being computationally very capable. John Clarke even used one to replace a hard-to-get rotary switch with a potentiometer in the Digital Effects Pedal from April 2021 (siliconchip.com.au/Series/361) But consider that once you subtract the power pins, you’re left with at most six I/Os, and you usually need three of 64 Silicon Chip these (MCLR, PG[E]D and PG[E]C) for programming and debugging. Unless your application only needs three I/ Os, you will inevitably end up sharing some of these pins’ functions. These shared connections can cause significant hassles. This became apparent as we worked on an upcoming project that pushes a PIC12F1572 to its limits, using five I/O pins and running the processor at its highest operating frequency. Some background Microchip PIC microcontrollers have long used a five-wire programming interface. The voltages and protocol have varied over the years, but these five wires have always performed broadly the same roles. The PICkit 2 and PICkit 3 programmers both sport six-way headers; the later PICkit 4 and Snap programmers have eight-way headers. This is because these programmers now support Microchip parts that do not belong to the PIC family, such as AVR and SAM devices which came into Microchip’s stable with their 2016 purchase of Atmel. While the exact pin mapping of these five wires varies between PIC families and pin counts, the small number of pins on the 8-pin parts means that there are not many permutations. By Tim Blythman Australia’s electronics magazine The purpose of the Helper device we have developed is to switch the function of some pins on your micro between programming/debugging and application-specific I/Os during development. This will make your life much easier. While we can’t promise that this Helper will work with all 8-pin PICs, it should work with most. The main exception we’re aware of is PIC10F parts (some of which come in 8-pin packages, but only six are connected). Table.1 shows the five connections used for PIC programming, their order on the programming header and what pins they connect to on an 8-pin PIC. Note that the ground pin is located in the centre of the group, reducing the chance of damage if the header is reversed. One way to re-use pins 4, 6 & 7 on an 8-pin PIC is to mount it in a socket on the board, then when you need to program it, unplug it and insert it into a programming socket. After programming, it can be re-inserted into the original socket on the board. But this can quickly become tedious as the chip is repeatedly moved between the programming socket and the test circuit. It also means you can’t perform in-circuit debugging (ICD). The alternative is so-called ICSP (in-circuit serial programming), which allows the chip to stay in place and be programmed ‘in circuit’. But siliconchip.com.au Fig.1: most of the circuitry is for switching the pin connections for PIC chip IC1 between the ICSP header (CON3) and the TGT PCB pads, which plug into a development board. S2 is used to energise the relays. The board can be split between CON1 and CON2 to allow some distance between the circuits if necessary. this might not be possible when pins 4, 6 or 7 need to be used for the project at hand, depending on how they are used. Pins 6 & 7 are usually fully featured; in the case of the PIC12F1572, they can be used as analog inputs to the ADC (analog-to-digital converter), comparator or as PWM outputs. In most cases, MCLR can also be used as an input, if desired. In our recent design using the PIC12F1572, we used pins 6 & 7 as analog inputs to sense the rotation of potentiometers, so both are connected to a low-impedance analog voltage source. This prevents successful in-circuit programming. Also note that some programming modes apply up to 13V to the MCLR pin (pin 4). If this is being used as an input, anything else connected to it must handle this during in-circuit programming. A solution to this is that some PIC parts are available with a so-called debug header variant. This is a specialised part with extra pins to separate the programming and debug functions from the other pin functions. A board fitted with jumpers often allows the header to emulate different parts. But these parts are much more expensive than their off-the-shelf counterparts, as might be expected for something that sees very limited production. And they are not available to suit all PIC parts. An example is the AC244053, which can emulate the PIC16F1454, PIC16F1455 or PIC16F1459. This specialised chip is a 28-pin SOIC (SMD) device, necessary to provide all 20 pins of the PIC16F1459 plus the separate debugging/programming pins. You can purchase it from the Digi-Key website for around $75: www.digikey.com.au/products/en? keywords=AC244053 Our solution Header pin Pin on PIC Label 1 4 MCLR Master clear and reset. It can also be used to apply Vpp (above 5V) to enable programming mode on the attached chip. 2 1 Vcc Power, which could be provided by the programmer or the connected circuit. 3 8 GND Circuit ground 4 7 PGD Programming data signal; driven by the programmer during writes and driven by the chip during reads. 5 6 PGC Programming clock signal, usually driven by the programmer. For a slightly cheaper and more generic solution, we’ve designed a tool that works with most 8-pin PIC microcontrollers. We use a set of relays to switch between the programmer and the target PCB, ensuring only one is connected at a time. This removes conflicts, ensuring that the pins are dedicated to only one role at a time. So you can easily switch between programming the chip and testing its functions. Note, though, that it might or might not allow you to use in-circuit debugging; it depends on whether your code will still work with the debugging pins disconnected from their other roles. While debugging a semi-functional circuit is annoying, we have done so in the past and successfully fixed difficult bugs in our code. You might need to temporarily modify the code to ignore the state of the dual-use pins; that’s still better than not being able to use in-circuit debugging at all! Fig.1 shows the circuit diagram of the Helper. In a similar vein to the debug header, the Helper has a set of Australia’s electronics magazine June 2021  65 Table.1: PICkit programming header & 8-pin PIC pin mapping siliconchip.com.au Role pins that slot into a DIL socket on the target PCB, where the programmed chip will go when development is complete. This header is marked TGT PCB, and its pins run to the headers marked CON1 and CON2. We’ll explain what these are for shortly. Pins 1, 4, 6 and 7 of the TGT PCB header are wired to the normally-closed contacts of 5V DPDT miniature telecom relays RLY1 and RLY2. The common contacts of RLY1 and RLY2 are wired back to IC1, which is where a real 8-pin PIC will be installed during development. Pins 2, 3, 5 and 8 of the TGT PCB header are also connected to the corresponding pins of IC1. This socket and header combination is our ‘emulated’ chip. When RLY1 and RLY2 are not energised, the target circuit will behave as though it has a PIC chip directly plugged in. The normally-open contacts of RLY1 and RLY2 are wired back to ICSP header CON3 (along with the ground connection, pin 8, from IC1). When the relays are energised, IC1 is connected to the ICSP header, allowing it to be programmed. Mini-USB socket CON4 and screw terminal CON5 allow 5V to be provided, via S2, to the coils of RLY1 and RLY2 so that the switchover can be effected by holding down pushbutton S2. D1 is the back-EMF suppression diode for the relay coils. So far, we have described the critical parts of the Helper that provide trouble-free programming. But since we’ve gone to the trouble of designing a PCB, we thought we’d add a few more features. CON1 and CON2 are wired straight through, and the PCB can be scored between these connectors, allowing it to be broken apart and the two parts wired together (eg, using a ribbon cable). The need to have the two parts physically distant is handy, but we found a degree of mechanical separation was also very useful. The TGT PCB header is a fair but not firm fit into a standard IC socket, so having the flexible wire connection allows some movement of the main PCB without affecting the seating of the emulated IC. You could also use the pads of CON1 or CON2 to wire directly to your development system’s PCB if it isn’t an 8-pin DIP part. For example, enamelled copper wire could be soldered directly to the pads of a SOIC (or smaller) IC footprint. Both CON1 and CON2 have their pins arranged to match the standard numbering used on 8-pin chips for simplicity. If bridged, JP1 and JP2 connect the relay power circuit (CON4 and CON5) to the target circuit. We joined these to allow the relay to be powered by our These photos show the construction we used for our first project using the Helper. Both CON4 and CON5 are unused, as we can provide power from our modified Snap programmer. 66 Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.2: many larger (14-pin and 20-pin) PICs have the same configuration on their top 8 pins as an 8-pin PIC. By expanding the number of lines, we can make a board that will work with those chips too. Just make sure to check your PIC’s pinout before connecting it up; if it is one of the recent ‘enhanced’ 14-pin or 20-pin chips, chances are it will work. programmer, so we don’t need to supply power via CON4 or CON5. Note that this requires JP4 (see below) to be closed too. JP3 is connected across S2’s contacts, so it provides a slightly more permanent way of setting the relays to the programming position. You could also use this to connect an external toggle switch if you prefer something with a non-momentary action. JP4-JP7 can bridge the relay contacts of the Vcc, MCLR, PGC or PGD lines, respectively. We jumpered out Vcc in our rig to prevent the IC from losing power as the relay contacts change over. Still, you might prefer to leave it open to allow any other circuitry to fully reset after a programming sequence. S1 simply connects the MCLR pin to GND, resetting the microcontroller under normal conditions. It shouldn’t be pressed while S2 is active (and the programmer is driving the MCLR line), but it’s often handy to reset the microcontroller while testing. We’ve also provided a position (marked C1) for a bypass capacitor for IC1. Since the target circuit will usually have provision for this, it is not normally necessary. We didn’t populate it in our prototype. siliconchip.com.au Depending on the load incurred by your circuit, this capacitor could be used to maintain power to IC1 while the relay contacts change over. The capacitance needed for this to work depends heavily on the circuit current draw and switching time. The specified relays are rated to switch in about 4ms, so for example, if your circuit (including the microcontroller) typically draws 20mA, you would need 47μF to keep the supply voltage from dropping below 4V during that 4ms period, or 22μF to prevent it from falling below 3V. You can scale the capacitor value proportionally for heavier or lighter loads. Handling larger chips Despite having more I/Os, larger chips such as those with 14 or 20 pins can still suffer from the same problems as 8-pin chips. We tend to use all the pins for something, regardless of how many there are, which means that we often have to be careful what we connect to the programming pins. The good news is that many 14-pin and 20-pin PICs use the same pinout as the 8-pin types, just with more pins added below. So all we have to do to make the Helper usable with these devices is to expand the PCB slightly, Australia’s electronics magazine adding extra pins on both the socket for IC1 and the target chip header, as shown in Fig.2. CON1 and CON2 change to DIL headers to accommodate the extra pins, allowing a ribbon cable with standard IDC inline sockets to join the two boards if split apart. Note that 18-pin parts like the PIC16F88 that we’ve used for many years (but no longer recommend for new designs) has a different pinout from the newer ‘enhanced’ range of PICs, so it and similar chips will not work with this project. Most chips with more than 20 pins use a different pinout too, and many are also wider, so we didn’t think the compromises necessary to make this board support them were worthwhile. Construction The 8-pin PIC Programming Helper is built on a double-sided PCB coded 24106211 which measures 37 x 72mm, with a narrowed section at one end. Refer to the PCB overlay diagram, Fig.3, during construction. The 14/20-pin version (which also supports 8-pin PICs) uses a PCB coded 24106212 which is 37 x 105.5mm (Fig.4). The assembly procedure for the two boards is essentially the same. June 2021  67 Fig.3: construction of this 8-pin version of the Helper is straightforward, but we recommend fitting a socket for IC1 so that you can change it out for different parts in the future. Also note that the pins fitted to the TGT pads should be installed underneath for the correct orientation when plugged in. Fig.4: building this version that suits 8, 14 & 20-pin PICs is almost the same as the 8-pin only version. It just uses larger sockets for IC1 and the TGT connections, and larger headers for CON1 and CON2 (if fitted). 68 Silicon Chip If you intend to separate the PCB between CON1 and CON2, do this now so that no components are damaged. Carefully score both sides of the PCB to break the copper connections. This will reduce the chance of tearing them off the PCB. Then, while firmly holding the larger half of the PCB in a vice or pliers, flex the smaller (CON1) half of the PCB with pliers along the line. Once the PCB separates, you can tidy up the rough edges with a file. Take care to do this in a ventilated area (such as outside) to minimise inhalation of the resulting glass fibre dust. The first part to fit is the mini-USB socket (CON4), as it’s the only surfacemounted part. Some flux paste is handy, but since only the two outer power pins need to be connected, you could get away without it. Apply flux to all the pads and slot the connector into the holes in the PCB. Solder the smaller pads to the PCB. We’ve extended the two mandatory (power) pads to make this easier. If you created any solder bridges, remove them using solder wicking braid and a bit more flux paste. Then solder the four larger pads to the PCB to mechanically secure the part. Some time and heat may help here due to the larger metal mass. Clean up any excess flux at this point. Fit diode D1 next, noting the location of the cathode band. Then install the relays. They will have a stripe on their body to indicate the pin 1 end, or perhaps have a pin 1 dot like an IC. This end goes nearest the diode, as shown in Figs.3 & 4. Solder two leads to secure the relays roughly in place and adjust them to be flat against the PCB one lead at a time. Finally, solder the remaining pins. We recommend using a socket for IC1 so that the PIC chip can be changed when necessary. Our photos show the socket fitted with a PIC12F1572 for our current project in progress. Use the technique described above to ensure that the socket is flat. The seven jumpers (JP1-JP7) are simply two-pin headers. These can be easier to handle if fitted with the jumper shunt first, as it provides plastic surfaces that won’t transfer heat as quickly. Solder these in the positions marked. CON5 is intended to take a two-way screw terminal, but you could solder wires directly to the pads instead. Australia’s electronics magazine CON3 should ideally be a rightangled header to suit your programmer; our photos show the Helper connected to a low-cost Snap programmer, but the PICkit series is also suitable. You can temporarily fit the header to your programmer to ensure it is correctly aligned while soldering. Next, fit buttons S1 and S2, pressing down firmly to snap them into place before soldering. For the TGT PCB pad, we simply soldered header pins to the underside of the PCB. We aligned them by slotting them into an 8-pin DIL IC socket during soldering. If you are using machined-pin IC sockets, you should solder machined pins to the Helper, or else they will not plug in properly. Test fit them before soldering to ensure that they will be held securely in the socket. The advantage of using standard square header pins is that your prototype board (that the TGT PCB will plug into) could be fitted with socket strips during the testing phase, making plugging and unplugging this board very easy. They will also fit standard dual-wipe sockets, although they are a tight fit. Regardless, as you can see from our photos, these pins are fitted to the underside of the PCB to maintain the correct pinout. If you have broken the PCB between CON1 and CON2, use a ribbon cable to bridge the gap. For the 8-pin version, simply wire pin 1 to pin 1 through to pin 8 to pin 8. You could also fit header pins to both ends and use jumper wires to join them. For the 14-pin/20-pin version, you’re better off fitting 2x10-pin headers to the boards at both ends, then using a length of 20-way ribbon cable fitted with IDC line sockets at either end. Make sure when you plug it in that the pin 1 stripe is at the pin 1 end of both headers. Testing Apply 5V power via CON4 or CON5 and press S2; you should hear the relays clicking. If not, then the relays or diode D1 might be reversed. You should get a similar result by shorting JP3. Another simple test is to use any circuit that has a socketed 8-pin PIC (or 14-pin or 20-pin if you built the larger version). Remove the PIC from the socket and place it in IC1’s socket, then fit the TGT PCB pins into the vacated siliconchip.com.au Usage See the separate panel at right for information about how we modified our Snap programmer to provide 3.3V or 5V power to the target circuit. With this modification, we’re able to use the much cheaper Snap in a wider variety of roles. Since this modification provides adequate USB power when set to 5V, it can easily power the relays, and we don’t need to supply any other power to the Helper; JP1 and JP2 just need to be shorted. Other programmers (such as the PICkit 2, PICkit 3 or PICkit 4) can be used with this arrangement, although these programmers might only be able to source a limited amount of current. Our experience is that they can supply a fair bit beyond what a PIC needs, but if it is not sufficient, power the relays on the Helper via CON4 after removing shunts from JP1 and JP2. In our setup, we’ve also fitted JP4 to provide 5V to the programming target. Our project is intended to be powered externally, but this means we don’t have to make a separate power connection to our breadboard prototype. Plug your development PIC into the onboard socket, then connect the TGT PCB header to your custom PCB’s IC socket to complete the ‘emulation’. At this stage, the breadboard project is in normal operating mode and can be powered up. To reprogram IC1, press and hold S2 (or toggle a switch attached to JP3), then start the programming process. Once it is complete, release S2 (or re-toggle the switch across JP3). If necessary, press S1 momentarily to reset your target PIC, and it will be back in the normal operating mode. Summary While we had a specific use case in mind when designing this project, it is generally useful while working with most 8, 14 or 20-pin PIC microcontrollers. The various jumpers provide the means to set up different combinations of connections, including powering it from various sources. We hope it will become a handy tool in your development and prototyping toolkit, as it has for us. SC siliconchip.com.au Modifying the Snap programmer to provide power The Snap programmer is great value, packing many of the same features as the PICkit 4 for around a third of the price. But two features it lacks are the ability to provide power to a target chip, and providing the higher Vpp voltage needed to use high-voltage programming mode. Luckily, the second aspect is becoming less important. Practically all newer PICs support low-voltage programming for most cases. Where the MCLR pin is not needed as an input, it’s likely that high-voltage programming is not required, except for a few older PICs. If we can use the Snap to provide power to its ICSP header, then it can come very close to supplanting the PICkit 4. In a stroke of luck (or was it by design?), there are a pair of test pads on the Snap which provide both 5V and 3.3V power. These are located adjacent to U5, a 3.3V MCP1727 linear regulator capable of delivering up to 1.5A. Our update is to solder a 3-pin socket header to these pins. They are spaced around 6mm apart, so this can be done reasonably elegantly with a 0.1in (2.54mm) pitch header simply using the outside pins. The photo below should make this fairly clear. Start with a 3-way female socket and trim the middle pin close to the plastic shroud. Solder one pin to the pad marked 5V0 and the second pin to the pad marked 3V3. To connect power to the ICSP header, we used half a jumper wire soldered to pin 2 of the ICSP header. You can plug this into the left-hand socket for 5V, the right-hand socket for 3.3V (which is necessary for most PIC32 parts) or the centre socket to provide no power. Just make sure that the shortened middle pin isn’t contacting anything on the board. With this simple addition, we are now using the Snap for practically all our development work. Note that it doesn’t have the current limiting that a PICkit 4 would provide. ► socket. If all the pins are connected correctly, then the circuit should work as designed. The added header has been tilted to prevent it from being too bulky, and to allow the flying lead to enter at a comfortable angle. Parts List - PIC Programming Helper 1 double-sided PCB coded 24106211 measuring 37 x 72mm, for 8-pin PICs only, OR 1 double-sided PCB coded 24106212 measuring 37 x 105.5mm, for 8, 14 or 20-pin PICs 2 compact 5V DIL telecom relays (10-pin DIP, eg, TQ2-5V or EA2-5NU) [Silicon Chip Online Shop Cat SC4159 or SC4158] 7 2-way male pin headers and jumper shunts (JP1-JP7) 2 4-way male pin headers OR 2 10-way male pin headers (to connect to TGT PCB; see text) 1 5-way male right-angle pin header (CON3, ICSP) 1 8-pin, 14-pin or 20-pin DIL socket (for IC1) 1 2-way mini screw terminal block (CON4) 1 mini Type-B USB socket (CON5) 1 1N4004 1A diode (D1) 2 tactile switches (S1, S2) Optional parts to split 8-pin version 1 10cm length of 8-way ribbon cable Optional parts to split 8/14/20-pin version 1 10cm length of 20-way ribbon cable 2 20-pin IDC line sockets 2 10x2 pin headers Australia’s electronics magazine June 2021  69