Lcd protocol 1.6
This document describes the protocol specific to the LCD modules I2C_LCD, and SPI_LCD. Each can be configured with a 20x4 or a 16x2 display.
Addresses on the SPI and 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 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, similar to the i2c bus version.
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 and i2c versus spi
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. |
| 0x08 | write up to 20 bytes for startupmessage line 1. |
| 0x09 | write up to 20 bytes for startupmessage line 2. |
| 0x0a | write up to 20 bytes for startupmessage line 3. |
| 0x0b | write up to 20 bytes for startupmessage line 4. |
| 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. |
| 0x14 | reinit LCD. |
| 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. |
define custom character
A usage of the 0x01 port is to define custom characters. Here in a less verbose format:
82 01 40 # set CGRAM char 0 line 0 82 00 01 02 04 08 10 10 10 # define character 0 (7 bytes) 82 11 00 # move to home position 82 00 41 42 0 # print characters A B and our newly defined character.
Use 0x48 instead of 0x40 to define character number "1".
the character data "01 02 04 08 10 10 10" is just an example. 11 11 11 1f 11 11 11 is the uppercase "H" that I have on my display right now.
the last two lines are just a example of how to get back to "display" mode. It works for me I don't have the inclination to find other ways.
scrolling the display
Send
82 01 18
to scroll the display one place left. Use 0x1c instead of 0x18 to scroll right. Replace 0x82 by the address of your display (e.g. 0x94 if you have an rpi_ui at the default address).