I bought 2 Olimex ATMega128 header boards from sparkfun few weeks ago. Now I tried to program it, unfortunately PonyProg didn’t find board (i used AVR STK Serial Port Dongle Programmer on the normal COM port,no usb converters). When i connected 6V to board (4xAA batteries, varta, from pack,not used) voltage on ICSP plug (2 pin) was about 3.5-3.7 volts. Voltage of batteries without any load was 6.2 volts, just right on the input leg of 78L05 (build in board voltage regulator) it’s 5.8 volts, and on the output leg of regulator voltage is 3.5-3.7 volts. That situation was on both boards. Accidentally, i short circuited output leg and ground leg on voltage regulator on one of the boards, smelled a bit, but now it reads about 4.2 volts on output leg.
But as was on olimex scheme - there should be 5 volts, that’s why,imo, i can’t connect to chip. Why this is so?
The 78L05 needs at least 8V (preferrably 10V) on it’s input. D1 is in series with the input, and that will drop about 0.7V. To work properly, use a 9V or 12V supply.
All linear regulators (including the 7805) have a minimum input voltage above the output voltage. It is usually 2-3 volts for a regular one, and can be as low as 0.7 volts for a low-dropout (LDO) one. This is due to the voltage drop across the pass transistor.