>_ GPIO as a state machine: bare-metal ESP32 in C vs Rust

Flashing an LED is the “hello world” of hardware. Everyone does it with the Arduino framework’s digitalWrite and moves on. Boring. So this week I did it from the datasheet on an ESP32 — first in C, then in Rust, both without the Arduino runtime.

Reading the register map

Somewhere in the ESP32 TRM there’s a table that says roughly: to drive GPIO20 as output, you touch two registers. The flow is:

// 1. route the pin (peripheral == simple GPIO)
gpio_pad_select_gpio(GPIO_NUM_20);

// 2. set direction: bits [9:0] of GPIO_ENABLE_REG
REG_WRITE(GPIO_ENABLE_REG, (1u << 20)); // actually: enable GPIO20 as output

// 3. blink by writing the output set/clear registers
while (1) {
    REG_WRITE(GPIO_OUT_W1TS_REG, (1u << 20));
    delay(500);
    REG_WRITE(GPIO_OUT_W1TC_REG, (1u << 20));
    delay(500);
}

The satisfaction is real: you’re driving the output bit yourself, pin by pin, no framework in between.

The same thing in Rust

On the no_std side you hit the classic trio: esp32-hal brings RegisterBlock styled CRATE, borrow, and Peripherals::take(). The equivalent:

#![no_std]
#![no_main]

use esp32_hal::{clock, gpio};
use esp_backtrace as _;

#[entry]
fn main() -> ! {
    let peripherals = esp32_hal::Peripherals::take().unwrap();
    let io = peripherals.GPIO.split();
    let mut led = io.pins.gpio20.into_output();
    let mut delay = ...;
    loop {
        led.toggle().unwrap();
        delay.delay_ms(500u32);
    }
}

What Rust actually costs

• Compile times you can watch. The chip target drags in a lot.
• no_std culture shock: no Vec by default, no panic-to-stdout.
• But: type-safe pin ownership and zero-cost toggle once it compiles.

What it buys back

The borrow checker is a surprisingly good hardware interlock: it refuses to hand the same pin to two timers the way C happily lets you. That’s one less class of “why is my drone spinning?” bug.

Takeaway: the Arduino/C dialect will ship your prototype fastest. The Rust dialect will refuse a thousand little footguns. Knowing both is the actual superpower for robotics work.

# probably the most useful terminal command this week
$ esptool.py -p /dev/ttyUSB0 erase_flash