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Add comment to calc_delta function as it's non-obvious.

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Jonathan Pallant (42 Technology) 2021-10-11 16:22:11 +01:00
parent f56369e1c7
commit 64207a62e5
1 changed files with 11 additions and 1 deletions
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12
rp2040-hal/examples/rom_funcs.rs
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@ -36,7 +36,8 @@ pub static BOOT2: [u8; 256] = rp2040_boot2::BOOT_LOADER;
/// if your board has a different frequency /// if your board has a different frequency
const XTAL_FREQ_HZ: u32 = 12_000_000u32; const XTAL_FREQ_HZ: u32 = 12_000_000u32;
/// Our Cortex-M systick goes from this value down to zero. /// Our Cortex-M systick goes from this value down to zero. For our timer maths
/// to work, this value must be of the form `2**N - 1`.
const SYSTICK_RELOAD: u32 = 0x00FF_FFFF; const SYSTICK_RELOAD: u32 = 0x00FF_FFFF;
/// Entry point to our bare-metal application. /// Entry point to our bare-metal application.
@ -168,6 +169,15 @@ fn main() -> ! {
} }
} }
/// Calculate the number of systicks elapsed between two counter readings.
///
/// Note: SYSTICK starts at `SYSTICK_RELOAD` and counts down towards zero, so
/// these comparisons might appear to be backwards.
///
/// ```
/// assert_eq!(1, calc_delta(SYSTICK_RELOAD, SYSTICK_RELOAD - 1));
/// assert_eq!(2, calc_delta(0, SYSTICK_RELOAD - 1));
//// ```
fn calc_delta(start: u32, end: u32) -> u32 { fn calc_delta(start: u32, end: u32) -> u32 {
if start < end { if start < end {
(start.wrapping_sub(end)) & SYSTICK_RELOAD (start.wrapping_sub(end)) & SYSTICK_RELOAD