ESP32 Forum

Hi,

I am able to drive a TFT display using SPI with 4 wires which are CS, CLK, MOSI, D/C. But recently, I am working on driving another TFT display using SPI with only CS, CLK, MOSI (without D/C pin). The trick is that I need to add once more bit in front of the 8 bits data/command I want to send. And that bit indicates whether the following 8 bits is command or not. Is it possible for esp32 to send 9 bit signal a time. What is the best way to implement it? Is there any limitation to that (i.e. cannot use dma)?

Please comment, thank you.

Thanks,
Jones

It's no different from other ways to control a SPI device, but you have to keep in mind that you need to pack the 9 bits of the data into the 8 bits you feed into the SPI peripheral. So for instance, a 3x9-bit transaction would have this data: dc 7 6 5 4 3 2 1 | 0 dc 7 6 5 4 3 2 | 1 0 dc 7 6 5 4 3 | 2 1 x x x x (Note that I'm not sure about the word or bit ordering, but you get the idea I hope.)

Hi ngjones,

have you found a good way to perform this succesfully?
I am running into the same issue while trying to control an OLED display with 3-wire SPI.

Thanks a lot in advance,

- MZ

Just in case someone else comes here:
I tried to follow @ESP_Sprite's suggestion and it worked great. I was afraid I had to toggle CS line after every 9 bits, but it appears it's not required as long as I keep CS low all the time.
Thank you @ESP_Sprite!

The way I've done it is by creating a buffer to write data into SPI. Using uint16_t is enough.
I append 9 bits on every write attempt and when my buffer length is over 8 bits, then I push those 8-bits into SPI.
I also had to add some dummy write of 0x00 at the end in case when I have less than 8 bits of data when everything is completed.