Spi lcd 1.3 protocol

The addresses on the SPI bus are 7 bits wide. The lower bit specifies if the transaction is to be a read or a write. Write transactions have the lower bit cleared (0), read transactions have the lower bit set (1).

Each transaction on the SPI bus starts with the address of the board. The spi_lcd board will ignore any transactions on the SPI bus that do not start with its own address.

After the address a single byte indicates the "port" on the board that the data is written to. The software can thus define 256 ports on each board.

write ports

Some ports just set a single value. So writing more than one byte to such a port is redundant. Other ports are logically a stream of bytes. So writing more than one byte is encouraged.

The spi_lcd board defines several ports.

port	function
0x00	display data.
0x01	write data as command to LCD.
0x10	any data clears the screen.
0x11	move the cursor to x, y. y is the top 3 bits, x is the bottom 5 bits of the data.
0x12	set contrast.
0x13	set backlight.
0xf0	change address.

read ports

The spi_lcd board supports two read ports:

port	function
0x01	identification string.
0x02	read eeprom (serial number).