SPI question - Do I need slave select when I only have one slave?

As per the thread title. I only have one slave. Do I still need slave select, or can I just tie it to ground on the slave?

As I understand it, toggling slave select doesn't signify start or end of a command, it only tells which slave to take orders, so it seems redundant for projects with only one slave.

At low clock speeds I can save a small amount of CPU time by removing the slave select. I'm also concerned about leaving slave select high when in sleep mode, as this could increase current consumption.

The current through SS pin is normally negligible as long as you're keeping it between the supply voltage and ground like you're supposed to (ie, you're not during off power to the slave) - that's not a concern.

Typically devices will be fine with SS being tied low all the time - but read over the datasheet to make sure it doesn't say anything about having to bring SS high for something.

DrAzzy:
The current through SS pin is normally negligible as long as you're keeping it between the supply voltage and ground like you're supposed to (ie, you're not during off power to the slave) - that's not a concern.

Sorry, I'm not quite understanding. Do you mean that it's negligible as long as the slave select is not left high while the slave is switched off? In that case maybe the slave would draw power through the slave select.

I intend not to bother with isolating the slave power during MCU sleep, as the MCU and slave combined will draw around 0.3 uA (MCU in sleep mode and slave not receiving or processing any commands) and that seems quite tolerable, even for a coin cell.

DrAzzy:
Typically devices will be fine with SS being tied low all the time - but read over the datasheet to make sure it doesn't say anything about having to bring SS high for something.

Thanks, I will check the datasheet.

Beedoo:
Sorry, I'm not quite understanding. Do you mean that it's negligible as long as the slave select is not left high while the slave is switched off?

Yes. As long as the slave's power is connected, it's fine to leave it high.

If the slave is not powered, then it would draw power through the SS pin (through the protection diode, aka "backpowering" ); this is contrary to spec for almost all parts - they're normally spec'ed for all pins being between Vcc+some fraction of a volt and Gnd - some fraction of a volt. If the device isn't powered and Vcc is 0v, your slave select had better be ~0V too. The fact that this would waste power is secondary to the fact that it can damage the part.

Thanks for explaining that.

That's not my experience, so I think it must depend on the SPI slave.

eg the MCP41010 digital potentiometers I was fiddling around with a couple of months ago.

Putting CS low makes them listen for commands. The commands are executed when CS goes back high.

see page 17 of the datasheet