Can a STM32L496RG read a 20-bit SSI signal with a SPI interface?

[SoCalJim]

I have to read a 20-bit SSI signal. I connected it to a SPI interface as a “Receive Only Slave”.

It appears that the SPI interface is limited to 16-bits.

We could live with the first 16-bits of a 20-bit word. Can the SPI receiver do that?

[TDK]

> We could live with the first 16-bits of a 20-bit word. Can the SPI receiver do that?

Yes, it could.

A better solution would be to use a 10-bit word size and read two of them. That gets you all 20 bits.

[tjaekel]

Yes, the SPI config should be able to set 4bit... 32bit word size: for instance: set 4bit and wait for 4 words (or as TDK has mentioned (2x10bit, I am not sure if 10bits is possible or just 4,8,12,16,...?).

You might be able to configure also 16bit as word size and receive one "complete" word. The problem is just: you will get 4 additional bits afterwards (nSS is still active, the SPI Slave still receiving). Even you could check that one word (16bits) was received - the next cycle is not really completed. And no idea what what happens if the SPI Receiver remains in an incomplete state: it will potentially not start again to receive a new word (unless you reset it and start it again.

Best is really to check if you can figure as "N times Xbits_as_word" (to make sure the entire transaction would be complete as 2x10 or 5x4, .....).

[SoCalJim]

I am trying to capture this waveform. Note that it only has CLK and DATA.

[SoCalJim]

It looks like the STM32L496’s SPI RX FIFO is either 8-bit or 16-bit.

Would you expect this to capture the above waveform: 

hspi3.Init.DataSize = SPI_DATASIZE_10BIT;

...

uint16_t buf[2];

HAL_SPI_Receive(&hspi3, (uint8_t*)buf, 2, 1000);

[TDK]

> Would you expect this to capture the above waveform: 

Yes. Note that the call needs to happen before data is sent. And it will time out after 1 second.

You will have to reconstruct the 20-bit word from two 10-bit words each stored as a uint16_t. Lower 10 bits of each uint16_t will have half of the data.