I2c lcd protocol
The addresses on the I2C 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 I2C bus starts with the address of the board. The i2c_lcd board will ignore any transactions on the I2C 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 I2C 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 i2c_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. ('i2c_lcd 1.4').
| data sent | explanation |
|---|---|
| START | create a START condition on the bus. |
| 0x82 | select destination with address 0x82 for WRITE. |
| 0x01 | identify port |
| STOP | create a STOP condition on the bus. |
| START | create a START condition on the bus. |
| 0x82 | select destination with address 0x82 for READ |
| 0x69 | 'i' |
| 0x32 | '2' |
| 0x63 | 'c' |
| ... | etc. |
Send text to display
Display the string "Hello World!" (only the first 5 bytes of the string shown).
| data sent | explanation |
|---|---|
| START | create a START condition on the bus. |
| 0x82 | select destination with address 0x82 for WRITE |
| 0x00 | datastream |
| 0x48 | 'H' |
| 0x65 | 'e' |
| 0x6c | 'l' |
| 0x6c | 'l' |
| 0x6f | 'o' |
| xx | etc. |
set cursor position
move to line 1, character 5:
| data sent | explanation |
|---|---|
| START | create a START condition on the bus. |
| 0x82 | select destination with address 0x82 for WRITE |
| 0x11 | port 0x11 = set cursor position. |
| 0x25 | 0x25 = 001 00101 = line 1 position 5. |
| STOP | create a STOP condition on the bus. |