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

Question

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.

Answer 1

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}

Answer 2

SPI chip selects are typically active low, so and AND gate will probably do what you want. Attach the output of the AND gate to the chip select pin on the slave. Connect the two chip selects from the master to the inputs of the AND gate. One chip select to each input.

Make sure the propagation delay of the AND gate meets the timing requirements of your system.

Also make sure the voltage ratings on the AND gate match your signal levels.