3-wire spi for esp8266

[windowsair]

A few months ago, I already found out that there is a so-called sio field in the registers of the esp8266, which allows us to work in 3 wire spi mode so that we don't need to additionally connect the MISO and MOSI pins together. (3 lines represent CLK, MOSI(MISO), CS, here we don't need CS line.)

So I tried to do that. The results were rather strange：

1. everything was fine for the first transmission, we had a half-duplex transmission on one line at the same time.
2. the second transfer, we do not get any data during the read phase, when the bus is pulled high.

Well I don't know what it is, but the esp8266 technical reference doesn't document this odd use, and there is little mention of it online, perhaps because everyone uses esp32 now...

[windowsair]

Ohhhhhh, I did it.😄

This is some of my experience in using 3-wire SPI, I hope it will be helpful to you:

1. If the read phase is required in a transfer transaction (via MOSI), you need to set sio in the spi register to 1.
2. If you only need to perform write operations which include one or more of the command, address and write phase, you need to clear sio to 0

If you don't do this, there may be timing issues...

The above description is done at the register level. For esp-idf, you need to set or clear the SPI_DEVICE_3WIRE flag at the appropriate time.

Well, I haven't found other esp-idf examples yet, but it's really easy to misuse for a beginner like me.