Re: Using 20x4 LCD display

On 22/10/2021 09:52, Olivier wrote:
> Hello,
> 
> I would like to use a 20x4 LCD display attached to a Raspberry Pi
> running FreeBSd 12.2.
> 
> I can talk to the display through the I2C interface, when I send
> something the back light will turn off and on, so the link is
> established.
> 
> What I cannot find is the sequence of codes to send in order to write a
> character on te screen, to move the cursor around, etc.
> 
> All documentation I could find so far relate to Raspbian or Arduino and
> are always using high level libraries (I could not find such on FreeBSD,
> equivalent to WiringPi).
> 
> Whatever random characters I send to the screen, it persistently shows
> alternating empty lines and lines or black boxes.
> 
> Any help pointing me in the right direction will be greatly appreciated.

I have a small utility for talking to a character LCD display via gpio.
I am not sure about your exact hardware, but typically the I2C interface is just 
a GPIO-over-I2C (aka an I2C I/O expander).

The utility is here: https://github.com/avg-I/gpiolcd
It's based on ppilcd utility written for driving such LCDs connected to a 
parallel port.  That utility can be found at share/examples/ppi/ppilcd.c.  It 
uses parallel port ioctl-s for the job.

To use my utility you need the LCD to be presented as a gpio device.
For that you need a driver for a specific I2C to I/O port expander that you 
have.  In my case it's PCF8574.

You can search the internet for information about how HD44780 (and similar) works.

https://www.exploreembedded.com/wiki/LCD_16_x_2_Basics
http://web.alfredstate.edu/faculty/weimandn/lcd/lcd_addressing/lcd_addressing_index.html
https://www.ti.com/product/PCF8574

P.S.
In case you also have PCF8574 or compatible, I have a driver for it but I 
haven't committed it yet.  You can find it here:
https://github.com/avg-I/freebsd/commit/fb31fe9a933d62c7af96b0ab830b98cbfeed5309

-- 
Andriy Gapon