SPI MOSI default high? NRF SDK 12.2.0

[Nick Bentzen]

I am attempting to interface a Winbond flash device with the NRF52. The datasheet does not specify the idle state of MOSI when reading from the device. However, I cannot make the flash chip work with the NRF52. In the attached image, an Arduino successfully sends a command to the devices (0x06H) then reads a register (0x05H) and gets the expected result (0x02H).

As you can see here, a NRF52 sending the same sequence of commands (roughly speaking) does not get a response. I've tried a number of different frequencies and both mode 0 and 3, which are the two modes the flash chip supports.

The one obvious difference between the Arduino's output and the NRF52's output is the state of MOSI in between transfers. Although the flash memory specifies a D/C during idle periods I would like to try setting MOSI high during idle periods.

Is there a way to do this?

[Turbo J]

Try setting the corresponding bit to 1in NRF_GPIO->OUTregister.

[MattyIce]

@Turbo J, where is that change made?

[Jørn]

Unfortunately setting the MOSI pin high will not change the pins polarity between transmissions. The SPI hardware takes control of the pin, and it will keep the pin low between transmissions. After the SPI is uninitialized the pin will maintain its configuration (low) until the application changes this. The only way I can think of to achieve a high MOSI pin between transmissions is to unititialize the SPI between every transmission and "manually" set the pin high.

[Nick Bentzen]

I was not able to solve this problem - I did end up electrically inverting the MOSI signal (and flipped bits in code to compensate) and it didn't solve my problem - turns out it was EMI.

[MattyIce]

It turns out my issue was due to the full-duplex nature of the SPI driver. Namely, for every byte sent on the MOSI line, a byte is received on the MISO line. So if you send 4 bytes and want to read one byte, in response, you need to make the rx buffer 5 bytes long and read the 5th byte as the "response" from the slave.