impl Rust for Pico - Embedded Programming with Rust

Ferris on LCD Display

In this section, we will draw Ferris on a character LCD. This is my attempt at making it look like a crab. If you come up with a better design, feel free to send a pull request.

Although a single custom character is limited to one 5x8 cell, we are not restricted to just one cell. By combining 4 or even 6 adjacent grids, we can display a larger symbol. How far you take this is entirely up to your creativity.

We will use the custom glyph generator from the previous page to design Ferris. The generator produces the byte array that we can directly use in our code.

Project from template

Generate a new project using the custom Embassy template.

cargo generate --git https://github.com/ImplFerris/rp2040-embassy-template.git --tag v0.1.4

liquid_crystal crate

The hd44780-driver crate that we used earlier does not support defining custom glyphs. To work with custom glyphs stored in CGRAM, we will use the liquid_crystal crate.

This crate supports custom glyphs and also provides an async API, which we will use in this chapter.

Update Cargo.toml

Enable the async feature when adding the dependency:

liquid_crystal = { version = "0.2.0", features = ["async"] }

Additional imports

Add these imports at the top of your main.rs:

#![allow(unused)]
fn main() {
// Interrupt Binding
use embassy_rp::peripherals::I2C0;
use embassy_rp::{bind_interrupts, i2c};

// I2C
use embassy_rp::i2c::{Config as I2cConfig, I2c};

// LCD Driver
use liquid_crystal::I2C;
use liquid_crystal::LiquidCrystal;
use liquid_crystal::prelude::*;

use embassy_time::Delay;
}

Bind I2C Interrupt

Bind the I2C0_IRQ interrupt to the Embassy I2C interrupt handler for I2C0:

#![allow(unused)]
fn main() {
bind_interrupts!(struct Irqs {
    I2C0_IRQ => i2c::InterruptHandler<I2C0>;
});
}

Initialize I2C

First, set up the I2C bus to communicate with the display:

#![allow(unused)]
fn main() {
let sda = p.PIN_16;
let scl = p.PIN_17;

let mut i2c_config = I2cConfig::default();
i2c_config.frequency = 100_000; // 100kHz

let i2c_bus = I2c::new_async(p.I2C0, scl, sda, Irqs, i2c_config);
}

Initialize the LCD interface

Once the I2C interface is set up, we initialize the LCD.

#![allow(unused)]
fn main() {
let mut lcd = LiquidCrystal::new(&mut i2c_interface, Bus4Bits, LCD16X2);
lcd.begin(&mut Delay);
}

Generated byte array for the custom glyph

#![allow(unused)]
fn main() {
const FERRIS: [u8; 8] = [
    0b01010, 0b10001, 0b10001, 0b01110, 0b01110, 0b01110, 0b11111, 0b10001,
];
// Define the character
lcd.custom_char(&mut Delay, &FERRIS, 0);
}

Displaying

Displaying the character is straightforward. You just need to use the CustomChar enum and pass the index of the custom character. We’ve defined only one custom character, which is at position 0.

#![allow(unused)]
fn main() {
lcd.write(&mut Delay, CustomChar(0));
lcd.write(&mut Delay, Text(" implRust!"));
}

Clone the existing project

You can clone (or refer) project I created and navigate to the custom-glyph folder.

git clone https://github.com/ImplFerris/rp2040-projects
cd rp2040-projects/embassy/lcd/custom-glyph/

The Full code

#![no_std]
#![no_main]

use embassy_executor::Spawner;
use embassy_time::{Delay, Timer};

// defmt Logging
use defmt::info;
use defmt_rtt as _;

use panic_probe as _;

// Interrupt Binding
use embassy_rp::peripherals::I2C0;
use embassy_rp::{bind_interrupts, i2c};

// I2C
use embassy_rp::i2c::{Config as I2cConfig, I2c};

// LCD Driver
use liquid_crystal::I2C;
use liquid_crystal::LiquidCrystal;
use liquid_crystal::prelude::*;

bind_interrupts!(struct Irqs {
    I2C0_IRQ => i2c::InterruptHandler<I2C0>;
});

#[embassy_executor::main]
async fn main(_spawner: Spawner) {
    let p = embassy_rp::init(Default::default());

    info!("Initializing the program");

    let sda = p.PIN_16;
    let scl = p.PIN_17;

    let mut i2c_config = I2cConfig::default();
    i2c_config.frequency = 100_000; // 100kHz

    let i2c_bus = I2c::new_async(p.I2C0, scl, sda, Irqs, i2c_config);

    // LCD Init
    let mut i2c_interface = I2C::new(i2c_bus, 0x27);
    let mut lcd = LiquidCrystal::new(&mut i2c_interface, Bus4Bits, LCD16X2);
    lcd.begin(&mut Delay);

    const FERRIS: [u8; 8] = [
        0b01010, 0b10001, 0b10001, 0b01110, 0b01110, 0b01110, 0b11111, 0b10001,
    ];
    // Define the character
    lcd.custom_char(&mut Delay, &FERRIS, 0);

    lcd.write(&mut Delay, CustomChar(0));
    lcd.write(&mut Delay, Text(" implRust!"));

    loop {
        Timer::after_secs(1).await;
    }
}