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In the commercial ones you'll find an ASIC that will do the entire job. If it's an older version you may get some discrete parts (and also some limitation). If you do some research on this direction, you should start with the so-called pin drivers. These can drive a pin with an externally programmed (Hi and Lo) voltage, required for the function of that specific pin at the moment of programming/verifying. This way you can use 3,3v, 5v or even 13v to drive a pin. Beside that you'll need lots of I/Os to configure each pin's direction and voltage. In top of that, from the PC side, you'll have to make a database of the devices covered by the programmer, specifying the pin settings and the programming protocol.
i did some research on pin drivers, They are having internal mosfet and come in 8 pin ICs that can drive a pin high or low or floating. So it means if a hobbyist want to make a universal programmer then he will need pin driver for each of the pins he wish to program. For ex if we need 40 pin MCUs programmer then we need 40 Pin Drivers+Pin Reading ICs. Then the pin drivers will be controlled by a controller and nothing else to do just putting high/low or a particular voltage as sofware says.
This means all the logic in put in the software and hardware is very dumb,
Great information Zoli, That helps in understanding commercial uni progs.
here is found datasheet of intersel pin drivers http://www.intersil.com/data/fn/fn7279.pdf
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