Electrical – SPI bus – how to route two Slave Select lines to the same slave

I've run SPI (2MHz clock) about 5m from 1 box to another box and I didn't hesitate in designing the clock and data to be balanced output. The (custom) cable between the two also used twisted pair and screen on both data and clock .

I also sent isolated power to the remote box via dc-to-dc convertors. I didn't have enough time to get this one wrong so possibly my solution was an overkill but hey, it worked. My reasoning behind this decision is that I didn't want "current consumption" spikes going down the screens of the twisted pair. The screenes were not connected to ground at the PC sending end. Treat digital signals like precious analogue signals to get the best performance - always have your screen terminated at the receiving end and if you must (for whatever reason) terminate grudgingly (as well) at the send end.

It was for transmitting 128 channels of low speed analogue signals to a break out box from a PC. I suspect, that if I'd needed to I could operate this at 20MHz clock.

First, you have identified a really annoying part of SPI on the AVR: Connecting SPI slaves will interfere with the proper operation of ICSP.

Second: The SPI bus has MISO, MOSI, and a chip select pin. You can break out the MISO, MOSI and GND pins, and perhaps VCC, plus a different CS pin for each daughter board you want to connect. As long as the CS is not active for the daughter boards, and the daughter board chips do not try to use the bus in master mode, you should be OK.

Third: You can use a buffer chip of some sort (perhaps 74HC125 or similar) with a tri-state function to isolate the ICSP feature from the SPI-talking-to-slaves feature; set the buffer chip up to require a particular pin high to let signals through, and use software to pull that pin high only when your code wants to talk to the slaves, and keep it low after a code reset.