Spooky action at a distance

BSRR is not the only register that can control the pins of Port E. The ODR register also lets you change the value of the pins. Furthermore, ODR also lets you retrieve the current output status of Port E.

ODR is documented in:

Section 11.4.6 GPIO port output data register - Page 239

Let's look at this program. The key to this program is fn iprint_odr. This function prints the current value in ODR to the ITM console

#![no_main]
#![no_std]

use core::ptr;

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

// Print the current contents of odr
fn iprint_odr(itm: &mut ITM) {
    const GPIOE_ODR: u32 = 0x4800_1014;

    unsafe {
        iprintln!(
            &mut itm.stim[0],
            "ODR = 0x{:04x}",
            ptr::read_volatile(GPIOE_ODR as *const u16)
        );
    }
}

#[entry]
fn main() -> ! {
    let mut itm= aux7::init().0;

    unsafe {
        // A magic addresses!
        const GPIOE_BSRR: u32 = 0x4800_1018;

        // Print the initial contents of ODR
        iprint_odr(&mut itm);

        // Turn on the "North" LED (red)
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 9);
        iprint_odr(&mut itm);

        // Turn on the "East" LED (green)
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 11);
        iprint_odr(&mut itm);

        // Turn off the "North" LED
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (9 + 16));
        iprint_odr(&mut itm);

        // Turn off the "East" LED
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (11 + 16));
        iprint_odr(&mut itm);
    }

    loop {}
}

If you run this program

$ cargo run
(..)
Breakpoint 1, registers::__cortex_m_rt_main_trampoline () at src/07-registers/src/main.rs:22
22      #[entry]

(gdb) continue
Continuing.

You'll see on itmdump's console:

$ # itmdump's console
(..)
ODR = 0x0000
ODR = 0x0200
ODR = 0x0a00
ODR = 0x0800
ODR = 0x0000

Side effects! Although we are reading the same address multiple times without actually modifying it, we still see its value change every time BSRR is written to.

Discovery

Spooky action at a distance