Atmega ISP programming headers – muxing suggestions

atmegaisp

I'm currently working on a project that involves multiple Atmega devices. I'm planning to leave ISP headers on the board, but I don't want to leave 1 set of ISP headers for each chip (I'll have 4-5 of such chips). But instead, I'm thinking of cleverly placing jumpers and headers as to "select" the device being programmed. This way I get to use only 1 set of ISP headers.

My question is which ISP line can I select for muxing. ISP lines consist of the following

VDD
GND
SCK
MISO
MOSI
Reset

My guess would be if I disconnect SCK line for the 4 devices that I'm not programming, but connect this line user a jumper for the device that I am programming, then I end up with programming this 1 target device. Is this a feasible idea?

Best Answer

Ideally you would MUX the whole lot, but i understand if this is not feasible.

I think you are right to use the SCK line, but make sure that the disconnected lines are not floating. Crosstalk on a floating line could cause problems.

The Atmega chips program when reset is LOW. When in Reset, the chip is listening for programming instructions on the SPI port.

Here is how I would tackle it:

  • Pull all reset lines low together (so the non-target micros dont drive the MOSI line)
  • Add pull up/down to each SCK line at the chip (check polarity)
  • MUX the SCK line
  • Leave all other lines connected
  • Use 100Ohm resistors in all lines

The risk here is that the chip in reset will drive the MOSI line, but i dont think it will. Check this first.

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