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PIC In-Circuit Programming Add-On By KEITH ANDERSON Does your PIC programmer have provision for in-circuit programming? Some such as Microchip’s PICSTART Plus don’t but you can add this useful function to your programmer by building a simple adaptor! O FTEN, THE EASIEST way to program a PIC is to remove it from its circuit and plug it into an appropriate programmer. However, when developing new projects, this can become a real chore and so professional developers use a range of tools that allow programming and even debugging without removing the micro from the application circuit. Microchip refers to this method of programming as In-Circuit Serial Programming (ICSP). WARNING! This adaptor was designed and tested for use with Microchip’s PICSTART Plus programmer, which allows all PICs to be inserted with pin 1 aligned to pin 1 of the programming socket. Some low-cost programmers lack this flexibility and require the smaller PICs to be inserted with pin 1 in some location other than the socket’s pin 1. This unit will not work with this type of programmer! 96  Silicon Chip While not all low-cost programmers support in-circuit programming, some can be modified to do so. Hobby programmers are usually supplied as kits, so it is often possible to solder wires to appropriate places within the circuit for connection to the application circuit’s ICSP header. However, this method is not general to all programmers. The method used here is to plug an adaptor into the socket on the programmer. The adaptor picks off the programming signals and makes then available for connection to the application circuit, just like a “real” ICSP programmer. The programmer still “thinks” it is programming a PIC in the socket, when in fact it is programming the PIC in the application circuit. connections, which are: DATA, CLK, MCLR/VPP, VDD, and GND. The suggested connector layout (Fig.1) uses a 6-pin dual-in-line header (Farnell 302-1427), with a spare pin available for other uses. The circuit example in Fig.2 shows how the connector might be hooked into your PIC-based designs. Note that a switch must be inserted in series with the positive supply (VDD) to the PIC, so that either the application circuit or programmer can provide power. By controlling power to the PIC micro, the programmer is able to generate the necessary supply sequencing during the programming cycle. A 3-pin jumper could be used in place of the switch to save money and minimise use of board space. The switch (or jumper) must be downstream from all filter/decoupling capacitors and positioned as close as possible to the PIC’s VDD pin. ICSP adaptor The circuit for the adaptor is shown in Fig.3. The large 28-way header Adding programming support If your PIC-based project is to support in-circuit programming, then you must include a suitable connector on the PC board to accept the ICSP signals from the programmer. A minimum of five connections is required to carry the ICSP and power/ground Fig.1: if you want to program your PIC in-circuit, you must include an ICSP header on your board. Here are the recommended pinouts for the header. siliconchip.com.au Fig.2: this simplified circuit shows how to include the ICSP header in your projects. If the RB6 & RB7 port bits are used as inputs or to drive low-impedance outputs, some form of isolation will be required, otherwise the CLK and DATA signals from the programmer will be overloaded. In some cases, this can be as simple as two 1kW resistors. If possible, don’t use RB6 & RB7 for any other purpose in your design – that way, you won’t need to add isolation circuitry. Fig.3: the circuit for the adaptor. Note that the ICSP signals for each family (8, 18 & 28/40-pin) originate from different pins on the programming socket. (comprised of J1 & J2) plugs into the programmer’s socket, with the ICSP signals made available on one of three 6-pin headers (J3-J5) for connection to the application circuit, depending on the type of PIC in use. Somewhat fortuitously, Microchip assign the pins required for This view shows how the 28-way head­er the ICSP functions consistently, so pins protrude through the PC board. that most of the range of PICs can be grouped into just three families: 8-pin, 18-pin and 28-pin (to the very simple; just make sure that you programmer, 40-pin PICs look like insert the 6-pin headers (J3-J5) into 28-pin PICs). the PC board the right way around. A 6-pin header is provided on the The arrow on the header must line up adaptor for each family type. Con- with the square pad (pin 1) on the PC nection to the application circuit is board in each case. made with a short length of 6-way IDC A cunning trick is needed to install cable, terminated on each end with a the two 20-way SIL header strips (J1 & 6-way IDC socket (Farnell 302-2109). J2) when using a single-side PC board. Unlike some ICSP adaptors, the use of Insert each header “upside down” in a specific header (rather than jumpers its holes, so that the long ends of the or switches) for each family of PICs pins protrude from the underside of provides a simple visual indication the PC board (see photo). of correct device selection! A little extra pin length can be obtained by pushing each pin through Assembly the plastic until the top is flush A PC board (coded 07112051) is with the top of the plastic. Obviavailable for this design. Assembly is ously, this must be done before the siliconchip.com.au Fig.4: overlay diagram and full-size PC-board pattern for the adaptor. Ideally, the adaptor should be produced in double-sided, platedthrough PC board technology, but it can also be assembled on a singlesided board with a little trickery. pins are soldered to the PC board. The ICSP cable must not be more than 300mm long, although 190mm is recommended for best results. The prototype was tested with a PICSTART Plus programmer and a couple of repSC resentative circuits. December 2005  97