GPICD Users Manual - Hardware Interface

GPICD - The GNU PIC Programmer and In-Circuit Debugger

2. The Hardware Interface

This chapter describes a simple and cheap ICSP/ICD hardware interface. Alternatively, you may want to use another interface, such as the one described by Microchip in the application note AN589.
The GPICD hardware interface is connected to the parallel port of the computer. It is powered by the target system with a unique 5V voltage source. The hardware part of GPICD is very easy to make: it uses a few discrete components and can be packaged in a small plastic box, connected to the target board using a flat cable. The picture below illustrates this configuration...

The GPICD hardware interface is mainly composed of:

A DC-DC converter generating the 12V+ programming voltage from the 5V target power supply.
Some signal buffers (transistors) driving a clock and a bidirectional data line, implementing a synchronous serial link for communicating with the PIC ICSP (In-Circuit Serial Programming) engine and/or the target debug monitor.
Some transistors for switching the RESET pin of the target PIC in Run (5V), Reset (0V) or Programming (12V+) modes.

Above is the hardware interface schematics.
The hardware interface schematics and PCB layout are also available for download in Eagle format.
Please note that I never realized the PCB: I only have a hand-wired board (which is not so difficult to do, given the low number of components). Here is a picture of my hand-made piece of art...

Photo: hand-wired version of the GPICD hardware

Five pins of J2 are enough to connect to the target microcontroller. The following table give the description of the signal available on this connector, along with the PIC pin it must be wired to.

*Pin*	*Description*	PIC16F87[0236]	PIC16F87[147]	PIC16F8[34]
1	MCLR - Driven to 0V (reset), 5V (run) or 13V (program)	1	1	4
2	Vdd - 5V Power provided by the target	20	32	14
3	Vss - Ground	8,19	12,31	5
4	PGD - ICSP/ICD data line	28	40	13
5	PGC - ICSP/ICD clock	27	39	12

The MCLR pin must be pulled up with a 100k resistor, in order for the target to continue working properly when the GPICD interface is disconnected.

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