Alternative to NSS?

steven23 · ‎2015-06-22

Posted on June 23, 2015 at 00:34

Hi. I've never designed with STM before and I have to admit I've not fully studied the datasheets. We're in prelim design phase right now. I tentatively have the pins for an STM32F205 fully specified, and I realize now we have what appears to be a problem. We require both DACs, one of which is on PA4. This pin is also used for SPI1_NSS and SPI3_NSS. SPI1_NSS and SPI3_NSS can also be on PA15. That's fine if we need only SPI1

or

SPI3... But we need all 3 SPI and both DAC.

Is there a way to use a different GPIO or the NSS function? Or I'm stuck either using an external DAC or finding a different chip? I don't have any spare gpio, but we can move up to the 100pin version of the chip if necessary... but that still has the same ''problem'' wrt DAC/NSS.

Thank you.

bjerman · ‎2015-06-22

Posted on June 23, 2015 at 00:52

Yes. You can use a GPIO pin instead of a hardware NSS pin. You just need to manually set it from idle to active (depending on the SPI mode from 1->0 or 0->1) at the start of the transaction and then reset it at the end. Not only can this be done, it is often preferable to the hardware version since it gives you more control over the transaction.

steven23 · ‎2015-06-25

Posted on June 25, 2015 at 19:09

Thank you for your speedy response! I just came back here wondering why my question was ignored, but now I see either this forum doesn't alert on reply, or else I have some setting wrong.

Edit: Figured it out :)