timers

src/timers.rs

@@ -1,22 +1,98 @@
+//! Module to interface with the GBA's four timer units.
+//!
+//! Similar to the background layers and DMA units, there are four timer units
+//! and they're numbered 0 through 3.
+//!
+//! There's two hardware addresses that control each timer.
+//! * The timer's high address is the [`TimerControl`] bits.
+//! * The timer's low address is a `u16` which *reads* the timer's "count"
+//!   value, but *writes* the timer's "reload" value. In this crate we actually
+//!   represent that as two separate MMIO controls for improved code clarity.
+//!   Just be aware that in mGBA's debugger and in other documentation you'll
+//!   see it as a single address.
+//!
+//! When a timer is disabled, it will continue to read the count value that it
+//! stopped at.
+//!
+//! ## Reloading
+//!
+//! When the timer goes from disabled to enabled, or when the timer overflows,
+//! the last set reload value is copied to the counter value.
+//!
+//! ## Ticking
+//!
+//! When a timer is enabled, the timer will tick every so often. Each tick
+//! increases the counter value by 1. The rate at which the timer ticks depends
+//! on the timer's configuration:
+//!
+//! * If the `cascade` bit is set the timer will tick once per overflow of the
+//!   next lower timer. For example, if timer 3 is set to cascade, it will tick
+//!   once per overflow of timer 2. Note that timer 0 ignores the cascade bit,
+//!   since it doesn't have a "next lower" timer.
+//! * Otherwise, the timer ticks every one or more CPU cycles, according to the
+//!   [`TimerScale`] set in the `scale` field.
+//!
+//! ## Overflows
+//!
+//! When a timer would tick *above* `u16::MAX` then an overflow occurs. This can
+//! trigger an interrupt, and will also cause the timer to copy its reload value
+//! into its counter.
+//!
+//! If you want a timer to overflow every `x` ticks (where `x` is non-zero),
+//! then use the [`wrapping_neg`](u16::wrapping_neg) method to easily get the
+//! right reload value to set:
+//!
+//! ```
+//! # use gba::prelude::*;
+//! let x = 7_u16;
+//! TIMER0_RELOAD.write(x.wrapping_neg());
+//! ```
+//!
+//! ## Timer Tips
+//!
+//! When a timer goes from disabled to enabled it will reset the counter value
+//! to the reload value. If you want to temporarily pause a timer *without*
+//! having the counter value get reset when you resume the timer you can instead
+//! set the `cascade` bit of the timer while the next lower timer is disabled.
+//! This keeps the timer active but prevents it from ticking. Note that this
+//! doesn't work for timer 0 because that timer ignores the cascade bit.
+
 use crate::macros::{pub_const_fn_new_zeroed, u16_bool_field, u16_enum_field};
 
+/// A number of CPU cycles per timer tick.
+///
+/// * The GBA's CPU runs at 16,777,216 cycles per second (16.78 Mhz).
+/// * The GBA's PPU outputs one pixel per 4 CPU cycles.
+/// * It takes 280,896 cycles for one full frame (when you add up all the draw
+///   and blank periods).
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[repr(u16)]
-pub enum TimerPrescale {
+pub enum TimerScale {
+  /// Approximately 59.6 nanoseconds
   #[default]
   _1 = 0,
+  /// Approximately 3.815 microseconds
   _64 = 1,
+  /// Approximately 15.26 microseconds
   _256 = 2,
+  /// Approximately 61.04 microseconds
   _1024 = 3,
 }
 
+/// Timer configuration bits.
+///
+/// * `prescale` is how many CPU cycles per tick
+/// * `cascade` will override the prescale value and instead tick the timer once
+///   per overflow of the next lower timer. Timer 0 ignores the cascade bit.
+/// * `overflow_irq` will cause an IRQ to be sent each overflow.
+/// * `enabled` makes the timer tick.
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[repr(transparent)]
 pub struct TimerControl(u16);
 impl TimerControl {
   pub_const_fn_new_zeroed!();
-  u16_enum_field!(0 - 1: TimerPrescale, prescale, with_prescale);
-  u16_bool_field!(2, chained, with_chained);
+  u16_enum_field!(0 - 1: TimerScale, scale, with_scale);
+  u16_bool_field!(2, cascade, with_cascade);
   u16_bool_field!(6, overflow_irq, with_overflow_irq);
   u16_bool_field!(7, enabled, with_enabled);
 }