I have an ATmega8 with a Nokia LCD 5510 connected to it. Everything works great on a breadboard and my next step is to move it to a more permanent board. My problem is that I can't program the ATmega8 while the LCD is also connected. I assume it's because they use the same SPI pins and that the LCD somehow interferes with the programming. How can I do this different so that I can flash the ATmega via ISP without unplugging the LCD?
Electronic – SPI device prevents ISP programming
atmegaisplcdspi
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Best Answer
You have to prevent any other device from driving the SPI lines during programming.
To avoid driver contention, a series resistor should be placed on each of the three dedicated lines if there is a possibility that external circuitry could be driving these lines.
Take a look at Application note AVR910.
So it would be best if the reset lines of the ATMEGA and the display are functionally connected to prevent the display from doing anything while the programmer holds reset.