I'm trying to make a pic programmer. Something like
(which is also the software I'm using to program the pic)
I understand the VPP part that supplies a regulated 12.7V to the VPP pin that is controlled through by a transistor from a parallel port pin(TxD) and how the 5V for the VDD is derived from it(through D2). I don't understand what the diodes D3-D6 are doing and what the resistors R1 and R2 are doing. Any ideas?
After reading the programming specs for the 16F688 and 18F4X it seems pretty easy to program but I'm getting confused on the details. Do I have to limit the current going into the pins or can, say, I just hook up the pins directly? (there is something about latching though that has to do with those diodes D3-D6 but I don't know what this really is)
I already built my regulated PS that gives me +12V(11.8V for some reason),
5.2V, and +- Variable from 24V down to 1.25V. I was going to use that to power the MCU and use the parallel port to send the data but I'm not sure if I can hook it up "directly" or not. One problem I think I will have is that the voltage on my parallel port pints is only about 3.25V instead of 5V, not sure if this is enough for the clock and data.Could I just use 4 transistors, one on each pin I need and the Base being a pin from the parallel port?
something like
PP pin X | \\ / \\ R1 / | ------- / \\ V ---/ \\---+-- MCU Pin Y | \\ / \\ R2 / | GND
Where V is 12 volts if MCU Pin Y is MCLR and 5V else.
Would something like this work?
Main thing I'm woried about is screwing up my pics(I only have 4) so I don't mind experimenting but I'm not exactly what I need to be careful about. I know that the pins are Low Z on output and Hi Z on input but is that enough to get by?
Thanks, Jon