Registers

It's time to explore what the Gpio API does under the hood.

In a nutshell, it just writes to some special memory regions. Go into the 05-digging-deeper directory and let's run the starter code.

#![no_std]
#![no_main]

extern crate pg;
extern crate f3;

use f3::peripheral;

#[inline(never)]
#[no_mangle]
pub fn main() -> ! {
    let gpioa = unsafe { peripheral::gpioa_mut() };
    let rcc = unsafe { peripheral::rcc_mut() };

    rcc.ahbenr.modify(|_, w| w.iopaen(true));
    gpioa.moder.write(|w| w.moder5(0b01));

    loop {
        f3::delay::ms(1_000);
        gpioa.bsrr.write(|w| w.bs5(true));
        f3::delay::ms(1_000);
        gpioa.bsrr.write(|w| w.br5(true));
    }
}

Wow I just a lower level abstraction, but this is essentially what the Gpio module does. To dig down deeper into the direct register accesses we need to ge a little farther into unsafe land.

#![no_std]
#![no_main]

extern crate pg;

#[inline(never)]
#[no_mangle]
pub fn main() -> ! {
    unsafe {
        // A magic address!
        const GPIOE_BSRR: u32 = 0x4800_1018;

        // Turn on the North LED (red)
        *(GPIOE_BSRR as *mut u32) = 1 << 9;

        // Turn on the East LED (green)
        *(GPIOE_BSRR as *mut u32) = 1 << 11;

        // Turn off the North LED
        *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);

        // Turn on the East LED
        *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
    }

    loop {}
}

What's this magic?

The address 0x4800_1018 points to a register. A register is special region of memory that controls a peripheral. A peripheral is a piece of electronics that sits right next to the processor within the microcontroller package and provides the processor extra functionality. After all, the processor, on its own, can only do math and logic.

This particular register controls General Purpose Input/Output (GPIO) pins (GPIO is a peripheral) and can be used to drive each of those pins low or high.

An aside: LEDs, digital outputs and voltage levels

Drive? Pin? Low? High?

A pin is a electrical contact. Our microcontroller has several of them and some of them are connected to LEDs. An LED, a Light Emitting Diode, will only emit light when voltage is applied to it with a certain polarity.

Luckily for us, the microcontroller's pins are connected to the LEDs with the right polarity. All that we have to do is output some non-zero voltage through the pin to turn the LED on. The pins attached to the LEDs are configured as digital outputs and can only output two different voltage levels: "low", 0 Volts, or "high", 3 Volts. A "high" (voltage) level will turn the LED on whereas a "low" (voltage) level will turn it off.

These "low" and "high" states map directly to the concept of digital logic. "low" is 0 or false and "high" is 1 or true. This is why this pin configuration is known as digital output.

OK. But how can one find out what this register does? Time to Read the Reference Manual!