Registers

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

In a nutshell, it just writes to some special memory regions. Go into the 07-registers directory and let's run the starter code statement by statement.

#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    aux7::init();

    unsafe {
        // A magic address!
        const GPIOE_BSRR: u32 = 0x48001018;

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

    loop {}
}

What's this magic?

The address 0x48001018 points to a register. A register is a 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 with 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 RTRM (Read the Reference Manual)!