3-wire SPI interface

[Locky]

I am trying to connect an accelerometer to my board. Unfortunately I only have enough GPIO's available to configure for a 3-wire SPI interface. However I can't find any information on how to configure the nRF51822 for a 3-wire SPI interface, or how it should be wired. Can anyone help?

Thanks guys

[RK]

Rather depends what you mean. You only need 3 GPIOs to implement SPI; MISO, MOSI and SCLK, as the nrf51 doesn't support chip select.

If you only have 2 GPIOs available and only need transmission in one direction, you can just connect clock + whichever of the two data lines you need and leave the other one unconfigured in the register.

If however you are talking about the 1/2 duplex version of SPI which uses one data line (SISO) and one clock line for bi-directional comms, you can't do it with the nrf51822 SPI.

[Locky]

The wording the RM was a little abstract in its description of 3-wire configuration. I understand that the CS must be handled by the application. What I meant was if there was a solution which involved a standard SPI 3-wire interface which combines SDI & SDO.

I am currently using a GPIO to power this external IC. I may hardwire the accelerometer to Vcc and use this GPIO for the CS.

Thanks for the help :)

[RK]

and as I said, no there is no way to combine SDI and SDO. So if you want bi-directional comms, you need at least 3 GPIOs, CLK, SDI and SDO.