SPI three lines mode

RE: SPI three lines mode

Bastien Reynard — Mon, 11 Sep 2017 08:18:59 GMT

Worked for me too, thanks atune.

RE: SPI three lines mode

atune — Mon, 26 Jun 2017 15:15:57 GMT

Sorry for digging up an old question, but I just had to make a sensor with only "3-wire SPI" work with nRF52. Didn't see a similar answer (sorry if I missed it) and this seemed as good as any to blurt out my own solution.

devzone.nordicsemi.com/.../ is a hardware solution that should work as RK mentioned, but I had to preferably solve it in software.

A software solution that worked for me (reading a register) using the vanilla nrf_drv_spi:

(SPI is uninitialised when not in use)
Initialise SPI with MOSI as [SDIO_PIN] and MISO & CS as NRF_DRV_SPI_PIN_NOT_USED
Assert CS
Make a pure tx transfer (instance, tx_buf, tx_len, NULL, 0)
Uninitialise SPI
Initialise SPI with MISO as [SDIO_PIN] and MOSI & CS as NRF_DRV_SPI_PIN_NOT_USED
Make a pure rx transfer (instance, NULL, 0, rx_buf, rx_len)
Deassert CS
(Uninitialise SPI)

I guess some sensors might throw a hissy fit in between the transfers if there's too much delay, but at least I got it working with my own setup.

Anyhoo hope this helps someone in the same situation in the future.

RE: SPI three lines mode

bjorn-spockeli — Tue, 12 Jul 2016 13:26:16 GMT

Could sound like Maxim's 3-Wire/Microwire.

RE: SPI three lines mode

RK — Tue, 12 Jul 2016 12:18:29 GMT

No the nRF52 (and the nRF51) don't support 3-wire SPI mode. Not that many chips do, it's not a very usual protocol. I did see a diagram somewhere (and it was Nordic-related) someone was using two resistors to join the MISO and MOSI of a standard SPI interface and hook it up to a 3-wire SPI slave. I wasn't entirely sure it would work.

RE: SPI three lines mode

Wojtek — Tue, 12 Jul 2016 12:00:48 GMT

you sure this is SPI? what is the name of the module?