How to control 2 devices using the SPI bus and one Chip Select

Question

Just wanted to check my understanding of the SPI0 bus on the pi zero w.

I have a DAC (MCP4921) driving the gate of a MOSFET as a current source for a load in the drain. I also have a ADC (MCP3008) reading the voltage across a 1 Ohm resister connected between the source and ground. In the python code, I would set the value for the DAC to produce a certain gate voltage, and therefore a drain current, and read the current value from the ADC.

Ignoring the need to level shift signals between the +5V chips and +3.3V chips (ie using 2 74HCT125N chips), am I correct in my thinking that I can use one CS signal to control both devices, and connect the input of the DAC to the MOSI pin for SPI0 and the output of the ADC to the MISO pin? Basically, I don't need to use the SPI1 bus, and I can do all the work on the SPI0 bus.

Thanks!

Mark

Answer 1

Multiple devices can be connected to a SPI bus (MISO, MOSI, SCLK).

Each device on the bus requires a chip select signal connected to a GPIO. Normally all the chip select signals are high which indicates no device is selected.

To talk to a device you set its chip select low, write to the device via MOSI, and read the results back via MISO. You then set its chip select high to release the device.

You need to have separate chip select lines (GPIO) for the DAC and the ADC.