SPI Slave (SPIS) returns 0xD7 instead of default or overrun character before first transaction

[rico]

I am using nrf connect zephyr spis driver initialized via devicetree. My default character is 0xEB which is returned if nothing is in the spi slaves buffers.

But before the first transaction is initialized by the software the spis returns 0xD7 instead. I have checked with ozone that the registers for the spis module are initialized, this is the case. So I assume this actually a hardware bug?

In this state the slave return 0xD7

After doing one transaction the slave return 0xEB correctly:

[rico]

I guess the error is not hardware but zephyr drivers not initializing CPOL and CPHA, which leads to wrong readout. If that is correct that should probably be done in the zephyr driver.

[rico]

I just did a bit more digging, spis is initialized, but not the mode, it is default on mode 0.