The chief advantage of this programmer is its ability to protect both the target and the PC from odd voltages when one side
is powered off. The buffers are open-drain, and each is pulled up to the Vcc of the receiving party. The MC74LCX07 has an
operating Vcc rating of 2.0 to 5.5V. In addition, this programmer is the only buffered programmer that allows the PC and the
target to operate at different power supply voltages. Great for working with 3.3V microprocessors on a 5V PC. The avrdude config
entry is:
although you can use the BSD programmer configuration, since it includes Vcc on pin 2.
Things to look out for are the values of series resistors R1 and R4. If the pullup resistor on the reset line of the microcontroller
is of a low enough value, the reset line may not go low enough when pulled low by the programmer. The same is true for the MISO
line on the PC side. R4 is pulling down against a pullup, and the voltage at the port pin must be low enough to make a logic "0" when
pulled by the programmer.
This one was very difficult for me to build, due to the SO14 package, but I think it is my programmer of choice for AVR's. It is as
fast as the other parallel programmers, safer than any, and so far, 100% reliable. Perhaps I'll do a circuit board and rebuild this
one (and not try to cram it into a headshell).
Schematic Diagram
Parts
C1 0.1uF Ceramic capacitor
IC1 MC74LCX07 Open-drain buffer
R1-R4 330 Ohm 5% 1/8W resistor
R5-R8 1k Ohm 5% 1/8W resistor
R9-R13 10k Ohm 5% 1/8W resistor
X1 DB25P solder cup connector
Not shown
Perfboard
DB25 headshell
6 or 10 conductor ribbon cable
6 or 10 conductor ribbon connector
Notes
The target microcontroller must provide it's own power, and that of the AVR ISP output pull-ups.
The PC provides pull-up power for the AVR ISP MISO line.
