Hello Attila,
Thanks for the Beta update, sorry about the delay in responding, I didn't get much time for this test project. I am now able to read back the flash from an ATmega328P (Arduino UNO chip). It works up to 2 MHz most of the time, I typically use the 100kHz setting, seems solid. It also seems to work at both 3.3 Volts and 5.0 Volts VCC on the test board.
I also ran this test on an ATmega2560 (Arduino MEGA chip). Unfortunately, it did not work. I have the Mode set to 4. Many times it would not even read the signature or fuses, even at slow speeds. But it would work often enough to confirm that it is communicating with the chip. This board only has 5 Volt VCC operation, I'm not sure if that would be an issue, what do you think?
I will try some programming tests next, I just wanted to confirm I could read things back before I started erasing the chips. If the problem is related to buffer size, perhaps programming will not encounter this issue and will still work.
Thanks