This repo contains a port of the linux tpm-tis-spi driver. The original
source is in linux/drivers/char/tpm/tpm_tis_spi_main.c. It is intended
to be used in conjunction with https://github.com/tpm2-software/tpm2-tss.
One uC to TPM command consists of at least two independent spi-api calls. The specification defines that the CS may not be released between those two calls. The image below illustrates a read of 4 bytes (vendor-id). The first 4 bytes are sent by the uC on D2 (MOSI). The Response is received on D1 (MISO) – also 4 bytes. D3 Is the chip select and is held down for the entire time (the glitch at the beginning is caused by the uC startup and can be ignored in this demonstration). D0 is the clock signal at 1 MHz.
I was not able to achieve this behavior with a in hardware chip select. The current implementation therefore switches the pin into GPIO alt mode.