From reading the documentation spi_transaction_t::length should be the length of bytes that will be transmitted by the SPI driver (and spi_transaction_t::rxlength how many bytes will be received).

In the code which implements SPI reading ( cmt_spi3_read ) the (tx) length is set to 8 but nothing ...

Easiest way might be to work around it: start two transactions, one for TX and one for RX.


Thank you for your hint. I need a bit more help from you to tackle this.

So I have looked into the code and SPI documentation a bit deeper.
Do I understand it correctly that a call to spi_device_polling ...

ESP-IDF v3 is old and deprecated by now; we're at 5.3 or something now. Are you sure that's the esp-idf version you're using?

Ah, sorry, my bad. I meant ESP-IDF v5.1.4 (which is used by Arduino-ESP32 v3).

I am currently trying to port code from ESP-IDF v4 to v5 (from Arduino-ESP32 v2 to v3), and I am stuck with its SPI part which speaks to the CMT2300a using half duplex SPI.

My ESP32-C6 is crashing with the error

E (2072) spi_master: check_trans_valid(793): SPI half duplex mode is not supported ...