Here are some simple ways of programming a 16F84 (or 16C84) in-circuit. I was going to publish this stuff in Everyday Practical Electronics as the hardware described here can be used to upgrade their simple programmer. Unfortunately, because they will be describing their own enhanced PIC programmer suite called the PIC Toolkit (to appear in the July 1998 issue) EPE weren't very interested. Therefore, I've stopped working on the article but here's what I'd put together before I packed up.

Programming "in-circuit" means the PIC is programmed while installed in the project board (or "target board" ) under development. The target board supplies the PIC with power and has a connector that allows the programmer access to RB6, RB7, /MCLR and +5V/VDD as shown below.

The 1k resistor and diode ensure the PIC runs normally when the programmer is not attached; they also enable /MCLR to be controlled by the programmer - either grounded or pulled up to around +12V to enter programming mode. If you simply want to program a 16F84 you don't even need these components and all the target board must do in this case is supply the PIC with +5V. Otherwise, apart from RB6 and RB7, all other PIC pins should be connected as required by the project. The pins RB6 and RB7 are reserved for use by the programmer hardware but could be used by the target too under some circumstances.

The programming hardware is based on the quick-and-dirty approach but the serial port versions are more conservative than most in that they don't try to obtain the programming voltage (VPP) from the serial port itself. The PIC can be connected directly to the PC parallel or serial ports (via a couple of resistors) or via simple interfaces for the parallel and serial port I've called PARPIC and SERPIC respectively.

The PC is connected to the programmer via four logical signals called CLK, OUT, RESET and IN. Only the CLK, OUT and GND connections are required but the software will always control RESET and IN as well. The hardware defines exactly which PC pins are used in the programming process. VPP is applied by closing the switch shown above however if a suitable voltage is available on the target board the switch may be installed there instead (see target schematic).

A direct connection from the serial or parallel port needs virtually no hardware. All you need to do is run the software and close/open the reset switch shown and connect/disconnect VPP when asked. If a VPP supply is not available on the target board an external source can be connected to the programmer (perhaps a couple of 9V batteries in series if the zener diode shown is fitted). This scheme works pretty well but the only way to verify the program has been downloaded correctly is to run it.

Adding a couple of transistors allows the PC to take control of /MCLR and read the PIC contents via the parallel port. This makes things a little more convenient and enables the PIC contents to be verified.

A slightly different configuration makes it possible to take control of /MCLR and read the PIC contents using the serial port. This version also introduces inverting buffers and steals VDD/+5V from the target so that the signals on RB6/RB7 are guaranteed to be between 0 and VDD. (A direct connection to the serial port relies on the PIC protection diodes to keep RB6/RB7 within the limits -0.6V and VDD+0.6V.)

The circuit above can be used to test out the simple in-circuit programmers. I've put together a few programs to exercise the board including a novel single digit clock (written in assembly language) and simple digital die (written in HI-TECH C).

The software to go with the programmers is based on PP V-0.5. So far there is only very brief documentation but there isn't much to say in any case.

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Copyright © David Tait 1998.