I have got a selfmade robot with ATmega 32A. Everything worked very good without any problems. But I have had not enough RAM on it for the RTOS. I decided to replace ATmega 32A with ATmega 1284P. ATmega 32A has only 2kB RAM instead of 16kB on ATmega 1284P. In the datasheet Ive found that the pins of both chips are the same. I dont need to make another board. After replacing I am not able to connect with the chip. The signature does not match selected device! The bytes were read as 0x00 0x00 0x00. The fuses and erase function are also disabled. The ISP frequency is already 6,48 kHz. I have tried with 125 kHz and so on, it doesnt work at all. I am using 20 MHz crystal. I have also tried to run the chip with the 16 MHz crystal and with external signal to the XTAL1 pin, from other chip.
I am using AVR Studio 4.18 with AVR ISP mkii.
Does anybody know the reason?