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pcf8574-linux

communicate to a pcf8574 based LCD through i2c on linux machines without any libraries(bit banging). NOTE: this will only work for MH based i2c backpacks

Wiring

LCD-VGA

VGA Pin Description
5 Ground
9 +5V DC
12 SDA
15 SCL

Execution

Ensure you have the i2c-dev module,i2c-toolsand python3 is installed. Load the i2c-dev module:

sudo modprobe i2c-dev

then, list all the i2c adapters by this command:

$ sudo i2cdetect -l
i2c-0	smbus     	SMBus I801 adapter at 6040      	SMBus adapter
i2c-1	i2c       	i915 gmbus ssc                  	I2C adapter
i2c-2	i2c       	i915 gmbus vga                  	I2C adapter
i2c-3	i2c       	i915 gmbus panel                	I2C adapter
i2c-4	i2c       	i915 gmbus dpc                  	I2C adapter
i2c-5	i2c       	i915 gmbus dpb                  	I2C adapter
i2c-6	i2c       	i915 gmbus dpd                  	I2C adapter
i2c-7	i2c       	AUX A/DDI A/PHY A               	I2C adapter
i2c-8	i2c       	AUX C/DDI C/PHY C               	I2C adapter
i2c-9	i2c       	AMDGPU i2c bit bus 0x96         	I2C adapter

What you're after is the DDI adapter which in this case could either be i2c-7 or i2c-8. A simple way to check is to first run a scan before plugging the display in. Scanning i2c-8:

$ sudo i2cdetect -y 8
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --

After plugging the display in:

$ sudo i2cdetect -y 8
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 3e 3f 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --

Notice how two addresses popped up though there can only be one address to interface with, this is usually because the i2c adapter is scanning for 8-bit addresses while the display utilizes a 7-bit address, choose the value which is the greatest. In this case 3f is the chosen value. NOTE: Please ensure you have correctly identified both the adapter and the address otherwise writing to the wrong i2c adapter could brick your computer.

Now run the python script by the following command:

sudo python3 ./pcf8574.py [address] [interface]

sudo python3 ./pcf8574.py 3f 8

Resources

A more thorough explanation by mitxela: https://mitxela.com/projects/ddc-oled