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.

Also see the general SPI protocol

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 line l, position p. l is the top 3 bits p 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. (terminated with 0).
0x02	read eeprom (serial number).

examples

read identification

read the identification string of the board. ('spi_lcd 1.3').

data sent	data recieved	explanation
0x83	xx	select destination with address 0x82 for READ.
0x01	xx	identify
xx	0x73	's'
xx	0x70	'p'
xx	0x69	'i'
xx	...	etc.

Send text to display

Display the string "Hello World!" (only the first 5 bytes of the string shown).

data sent	data recieved	explanation
0x82	xx	select destination with address 0x82 for WRITE
0x00	xx	datastream
0x48	xx	'H'
0x65	xx	'e'
0x6c	xx	'l'
0x6c	xx	'l'
0x6f	xx	'o'
xx	...	etc.

set cursor position

move to line 1, character 5:

data sent	data recieved	explanation
0x82	xx	select destination with address 0x82 for WRITE
0x11	xx	port 0x11 = set cursor position.
0x25	xx	0x25 = 001 00101 = line 1 position 5.

Spi lcd 1.3 protocol

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