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


We have legacy system of four boards: one containing chip acting as SPI master, three others containing chips acting as SPI slaves.

My goal is to eliminate two of the slave boards; however, the master board won’t work unless all three slave boards reply to its ping.

Therefore my intent is to put a small board together with SPI slave chip that will fake replies to master pinging. Typically I’d have used two chips, each receiving one slave select line meant for each of the legacy slave boards being eliminated. Is there any way I could combine these two slave lines together and only use one chip acting as SPI slave?

Thank you.

Best Answer

Let say that CS is active low, so combining together multiple CS gives you AND logic. If any of 3 CS is low then the output shall be low, if all 3 CS are high (ne device is selected) the output CS shall be also high.


simulate this circuit – Schematic created using CircuitLab