Olin Lathrop, Embed Inc
Last updated 17 December 2021
This page gives some background on in-circuit serial programming of Microchip PIC microcontrollers and suggestions on the electrical implementation for best results. Most of the information here is generic and applies regardless of what programmer is used. Specifics of the Embed Inc programmers are mentioned when this is relevant.
"Programming" a PIC in this context refers to storing the program onto a PIC, not generating or writing the program. This process starts with a HEX file, which specifies exactly how the non-volatile memory bits of a PIC are to be set. The process of programming is copying this information from the HEX file into the PIC.
There is no way to plug a PIC into a standard computer (PC). A separate piece of hardware, called a programmer is required to connect to an I/O port of a PC on one side and to the PIC on the other side.
There are many different PIC programmers available. Most of these use the USB, with older types using a serial port (COM port), or even the parallel port (printer port). The type of programmer, how it connects to the PC, and the various advantages and disadvantages of each are not within the scope of this document.
On the PIC side there are two possibilities, socket and in-circuit. A socket programmer provides a way to connect just a bare PIC to the programmer. Since all the connections are built into the programmer, their details are irrelevant to the end user.
In-circuit programmers connect to the PIC while it is in the target circuit. Due to variations in the interconnect scheme and the target circuit surrounding the PIC, there is no PIC programmer that works with all possible target circuits or interconnects. The purpose of this document is to help the circuit designer understand the constraints imposed on the circuit by in-circuit programming, and to give some guidance on how to design circuits most likely to work with a variety of in-circuit PIC programmers. The parameters of the Embed PIC programmers are mentioned and used as examples.
PICs are programmed using 5 signals. The data is transferred using a two wire synchronous serial scheme, with the clock always controlled by the programmer. The ICSP signals are:
Negative power input to the PIC and the zero volts reference for the remaining signals. Voltages of the other signals are implicitly with respect to GND.
Positive power input to the PIC. Some programmers require this to be provided by the circuit (circuit must be at least partially powered up), some programmers expect to drive this line themselves and require the circuit to be off, while others can be configured either way (like the Microchip ICD series). The Embed programmers all expect to drive the Vdd line themselves and require the target circuit to be off during programming.
Programming mode voltage. This must be connected to the MCLR pin, or the Vpp pin of the optional ICSP port available on some large-pincount PICs. To put the PIC into programming mode, this line must be in a specified range that varies from PIC to PIC. For 5 V PICs, this is always some amount above Vdd, and can be as high as 13.5 V. The 3.3 V only PICs like the 18FJ, 24H, and 33F series use a special signature to enter programming mode and Vpp is a digital signal that is either at ground or Vdd. There is no one Vpp voltage that is within the valid Vpp range of all PICs. The minimum required Vpp level for some PICs is above the maximum allowed of other PICs.
Clock line of the serial data interface. This line swings from GND to Vdd and is always driven by the programmer. Data is transferred on the falling edge.
Serial data line. The serial interface is bi-directional, so this line can be driven by either the programmer or the PIC depending on the current operation. In either case this line swings from GND to Vdd. A bit is transferred on the falling edge of PGC.
For many products containing PICs, it is desirable to load a blank PIC on the board during assembly, then program it in-circuit as part of the final production test procedure. This usually reduces cost and allows for a much shorter lead time from new code available until it can be deployed in newly produced units. While the advantages of last minute in-circuit programming are usually compelling, it is not free, and ICSP capability must be considered up front when the circuit is designed. It can be difficult and costly to retrofit a circuit for ICSP after other design decisions have already been made. On the other hand, the added cost of ICSP is usually minimal when it is planned on from the start.
Here are issues to consider when designing a circuit for ICSP:
This may sound obvious, but it requires some thought. For low volume products, the cost of an additional connector may be minor, but what connector is appropriate? For high volume designs, the per-unit cost must be kept to a minimum, and some complexity can be pushed onto the test fixture. Here are some suggestions:
This is a 6 pin "phone" type connector. It's not a great connector but has the primary advantage of being directly compatible with the Microchip ICD and other programmers. Some third party programmers (including some of the Embed programmers) have ICD outputs for easy compatibility. This type of connector is a good choice for hobby projects or prototypes where the ICSP connections will be used for debugging or frequent programming during development. For example, this is the connector used on the ReadyBoard-01 and ReadyBoard-02 for the main processor.
Due to the way the standard ICD cable is wired, the pinout of the target connector must be flipped from the pinout of the same RJ-12 connector built into the ICD. The ICD cable pinout is described in detail in a later section of this document.
Another problem with this connector is that the PGC and PGD lines are adjacent on the flat cable, and therefore susceptible to crosstalk. There is more on this also later in this document.
These type of connectors are cheap, widely available, reliable, and require less board area than an RJ-12 jack. We recommend the keyed version so that the cable can only be plugged in one way. It is not quite as easy to plug and unplug the cable from this connector as for an RJ-12 jack, so this is a good choice for infrequent programming as apposed to active debugging. For example, this is the connector used on the ReadyBoards for the power supply control processor.
A 6-pin version of this type of connector can also be used. An extra GND pin added between the PGC and PGD lines greatly reduces crosstalk between these lines, which can be a major problem. The programming output lines of the Embed PIC programmers are available via such a 6 pin header or bare pads (among other options) to facilitate making cables with reduced PGD/PGC crosstalk.
This is the ultimate in low per-unit cost, but requires a custom fixture to hold the unit during programming. This can be a good choice for high volume designs.
The programming algorithms for the 5 V PICs require that Vpp (MCLR pin) be raised above Vdd during programming. The requirements range from a few volts above Vdd up to about 13 V. This pretty much makes it impossible to have MCLR driven from a regular digital output as these wouldn't tolerate such voltages. Since MCLR is a CMOS input and therefore high impedence, it's usually sufficient to drive it from a digital output thru a resistor. Somewhere in the 10 kΩ to 100 kΩ range is usually a good value. The low end is limited by the current the digital output can tolerate thru its protection diode when 13 V is applied to the other end of the resistor. The high end is limited by the voltage offset caused by the leakage current of the MCLR pin times the resistance, and the additional noise susceptibility on a high impedence node in general.
Another consideration on the resistor size is the output impedence of the programmer's Vpp driver. This is deliberately a minimum value for some programmers to avoid damaging the target circuit, while others may have only passive drive in one direction. For example, the USBProg is voltage-regulated when driving high and 20-30 Ω when driving low. The ProProg also has low impedence when driving high and about 100 Ω when driving low. If in doubt, we recommend 20 kΩ series resistance from MCLR to the rest of the circuit on the board. There should be a direct connection from the programmer Vpp line to the MCLR pin.
Another issue is that some PICs can be configured so that MCLR has an internal pullup. Enabling this feature makes designing for ICSP more difficult since a simple series resistor to the rest of the circuit would not work. The resistor and the internal pullup form a voltage divider so the MCLR voltage seen by the PIC when the external circuit is driving it low will be some minimum value. Usually the external resistor would need to be too low to be useful for ICSP to guarantee MCLR is below the maximum threshold for a logic-low when driven by the on-board circuit. A separate transistor may be required, or the system designed so that the internal pullup is not needed. This is one of the areas where ICSP will impact the design. Nothing is free.
Note that on some PICs, like the 10F20x, the internal pullup is always enabled when the MCLR pin is configured in the MCLR role (as apposed to configured as a normal digital input). There is no universal answer, but this must be carefully considered in the circuit design. The 10F series is already the most challanging to design for ICSP because 5 of only 6 pins will be connected to the programmer.
Many programmers require control of Vdd during programming. There are three main reasons for this:
In general, the more full featured and robust programmers expect to control Vdd for the reasons indicated above. The Embed programmers are all in this catagory.
When the programmer will drive Vdd during programming, the target circuit must be off, or at least the power supply to the PIC must be off. Furthermore, the circuit must tolerate the PIC Vdd pin driven up to the PIC's maximum Vdd spec. This can be 5.5 V, and may be several volts higher than the circuit normally operates at or is designed to run at.
Even if the circuit can withstand 5.5 V power, it must not draw more current than the programmer can supply. Specifications for programmers vary widely. The EasyProg can only supply 20 mA safely, the USBProg and LProg 100 mA, and the ProProg 250 mA with the supplied wall wart and 500 mA with a fixed input supply.
Beware that some linear regulators, like common 7805 for example, can be damaged by raising their outputs above their inputs. A diode from the regulator output to its input my be required.
In some cases it will be necessary to split the power supply of the circuit so that the PIC is powered from a separate segment. Each segment may be driven thru a diode from a master supply. This prevents power on one segment from driving the others, but also presents another problem of the voltage drops accross the diodes. Sometimes it is sufficient to make the master supply 600-700 mV higher to compensate for the diode drops. Sometimes the master supply has a voltage feedback path accessible to the circuit. In that case the feedback can be taken from the output of one of the diodes. The voltage of that power segment will be well regulated, with the voltage of the other segments being within a few 10s of mV because the diode drops will match reasonably well. These are only some ideas. There is no universal answer and each case must be carefully considered with all the issues in mind.
These are digital signals and will always be driven within the range of GND to Vdd at the time, although Vdd may be higher than normal when PGC and PGD are driven. The drive impedence of the programmer on these lines must also be considered, since this forms a voltage divider with any in-circuit impedence tied to these pins. The EasyProg has 2 kΩ drive impedence on both lines, the USBProg 150 Ω, the LProg 200 Ω, and the ProProg has 1 kΩ drive impedence on PGD and 270 Ω on PGC.
On large PICs with many pins, it may be appropriate to dedicate the PGD and PGC pins for ICSP. In that case these pins should be configured as outputs and driven low or high during normal operation. They should not be left as floating inputs. In other cases it may be sufficient to put a resistor between these PIC pins and the rest of the circuit. This was discussed in detail for Vpp (above).
The PIC PGM pin is used to enable a certain type of low voltage programming on some PICs. Even though this is not one of the programming lines and is not connected to the programmer, it should be held low during programming. According to the documentation the PGM input should not matter during high voltage programming, but we have seen cases where it does anyway. A 100 kΩ resistor to ground is a simple fix in most cases.
While this is really another circuit constraint, this issue is so unintuitive, little known, poorly documented, but serious that it deserves its own section.
The standard Microchip cable unfortunately puts PGD and PGC on adjacent lines. Since this is a flat cable, this leads to crosstalk between the two. For writing to the target, the programmer drives both lines. In that case a little low pass filtering can be applied by the programmer to soften the edges and reduce the coupled amplitude on one line from an edge on the other.
However, there is a case that must be addressed by the target circuit. The PGD line is bidirectional, meaning it is sometimes driven by the target PIC. In that case PGD is just a normal digital output on the PIC. These are designed to drive from one state to the other as quickly as possible. Such an edge produced on PGD by the target PIC can couple onto the PGC line when using the standard cable supplied with an ICD, or any other cable where PGC and PGD are adjacent. The target PIC then sees a PGC (clock) pulse that the programmer didn't produce, and the serial communication gets out of sync. The net result is that programming appears to be flaky or not work at all.
This entire effect can happen within the time for the PGD edge to propagate from the target PIC, thru the cable to the programmer, and back thru the cable to the target circuit. This means that this problem can not be solved at the programmer end of the cable. No amount of clever circuitry at the programmer can make this issue go away. It must be dealt with at the target circuit.
This issue is particularly important for dsPICs since they are faster and therefore have stronger digital output drivers and faster edges that couple better between signals. Although somewhat less severe, we have observed this issue when programming 18F PICs. We don't recommend assuming this won't happen on 16F or other PICs. We're not sure that it doesn't, and ignoring it would essentially be relying on a maximum digital output edge slope. This could easily change between production lots, over temperature, or as new fab processes are brought on line.
We recommend filtering the PGD output from the target PIC by adding 100 Ω in series followed by 47 pF to ground. This limits the slope of edges and attenuates the high frequecy components that can couple from PGD to PGC. We also recommend adding the same capacitance to ground to the PGC line close to where it enters the target board by the programming connector. This reduces the impedence of the PGC line at high frequencies, which reduces its susceptibility to crosstalk.
We had previously recommended 22 pF for both capacitors instead of 47 pF, but have meanwhile found cases where 22 pF was insufficient. We feel that 47 pF provides sufficient margin but is still below the level where it could interfere with the normal operation of the lines. We have used 100 Ω and 47 pF filters on PGD and PGC on dozens of target boards and have not observed any problems due to crosstalk.
While adding capacitance on the programming lines could slow edges and prevent fast programming, even 100 pF should not cause any problems. For example, the USBProg has 150 Ω output impedence on both lines. 150 Ω times even 100 pF is a time constant of 15 ns, making the 90% settling time about 45 ns. This is small compared to the minimum 500 ns time from data to clock edges used by that programmer. Other Embed programmers have higher impedences and therefore longer time constants, but also longer data setup times to compensate. All the Embed programmers will work with an additional 100 pF load on the PGC and PGD lines.
We define the pins of the connectors on an ICD cable as shown in this drawing:
The standard ICD cable is wired so that the pins are flipped between the ends. In other words, pin 1 on one end is connected to pin 6 on the other end, pin 2 to pin 5, etc. The pinout of each end is:
|Signal||ICD2 end pin||Target end pin|