General • Is SPI1 TX equals MOSI for RP2350 as SPI Slave, or does it mean "transmission" irrespective of configuration?

Hello. I have a problem when documentation seems to conflict reality.
I have a setup with RP2350 being a SPI Slave while the other microcontroller being a SPI Master.

Qeustion: Is SPI1 TX equals MOSI for RP2350 as SPI Slave, or does it mean "transmission" irrespective of current slave configuration? I have ordered a PCB off jlcpcb and it seems the pins are swapped, explanation below.

For reasons, I have the following pin assignment

#define SPI_PIN_SCK 42
#define SPI_PIN_MOSI 43
#define SPI_PIN_MISO 44
#define SPI_PIN_CS 45

As RP2350 has the following GPIO (page 19 of RP2350 datasheet):
Multiple sources confirm that TX is from the "master" perspective therefor it 43 is MOSI while 44 is MISO.

I see this traffic, to confirm the SPI is functioning okay.
By double checking 10 times I confirm that SCK goes to pin 42, MOSI goes to pin 43, MISO goes to pin 44.
However, while reading this traffic I see only zeros. When I tie pin 43 (MISO, presumably pin 44 or RX) to 3.3v, i suddenly read FF.

When I tied MOSI/MISO together like that:
I also get traffic. So that leads me to believe that TX is always "for transmission" and RX for "receiving", as opposed to MOSI/MISO swapping roles depending on slave configuration.

Could anyone clarify this?

Statistics: Posted by desertkun — Sat May 17, 2025 5:30 pm — Replies 3 — Views 194

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