I've been hacking and figuring out how to support SPI Slave in all four modes -- it can be done! (sample code soon to be posted but it isn't ready yet).
Also, hacking to support a simple 8-bit signal. Basically, I want the ESP8266 to emulate an 8-bit shift register chip.
Anyhow, in poking around and understanding all registers .. this one is critical BUT I find no documentation for it. Any ideas?
SPI1P = (1 << 19);
In other headers, this will look like SPI_PIN_REG(HSPI) << BIT(19)
The ESP32 documentation is mostly good for ESP8266 BUT, this particular bit is un-documented/reserved.
When I don't set it, the clock pin actually drives a distorted sine wave around 1.4 volts. The data in pin also has some sort of floating bias.
When I set this pin, as it is set in any SPI Slave sample code Iv'e seen, it all behave sell. Clock pin is an input so is MOSI pin. Both with high impedance.