Optimize delay implementation (#42)

* Optimize delay implementation
* Configure delay with hclk & only for 1MHz+
* Clean up delay implementation
* Add more reasoning and fix documentation
* Add changelog entry for delay optimization

Author: david-sawatzke

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Support for STM32F091 - @jessebraham
 - Support for HSE as a system clocksource (#25 - breaking change) - @zklapow
 
+### Changed
+
+- Optimize delay implemenation (#42) - @david-sawatzke
+
 ### Fixed
 
 - Fixed panic in delay overflow handling for debug builds - @david-sawatzke

src/delay.rs

@@ -1,7 +1,7 @@
 //! API for delays with the systick timer
 //!
-//! Please be aware of potential overflows.
-//! For example, the maximum delay with 48MHz is around 89 seconds
+//! Please be aware of potential overflows when using `delay_us`.
+//! E.g. at 48MHz the maximum delay is 89 seconds.
 //!
 //! Consider using the timers api as a more flexible interface
 //!
@@ -35,27 +35,31 @@ use embedded_hal::blocking::delay::{DelayMs, DelayUs};
 /// System timer (SysTick) as a delay provider
 #[derive(Clone)]
 pub struct Delay {
-    clocks: Clocks,
+    scale: u32,
 }
 
-const MAX_SYSTICK: u32 = 0x00FF_FFFF;
+const SYSTICK_RANGE: u32 = 0x0100_0000;
 
 impl Delay {
     /// Configures the system timer (SysTick) as a delay provider
-    /// As access to the count register is possible without a reference, we can
-    /// just drop it
     pub fn new(mut syst: SYST, clocks: Clocks) -> Delay {
         syst.set_clock_source(SystClkSource::Core);
 
-        syst.set_reload(MAX_SYSTICK);
+        syst.set_reload(SYSTICK_RANGE - 1);
         syst.clear_current();
         syst.enable_counter();
-        Delay { clocks }
+        assert!(clocks.hclk().0 >= 1_000_000);
+        let scale = clocks.hclk().0 / 1_000_000;
+
+        Delay { scale }
+        // As access to the count register is possible without a reference to the systick, we can
+        // just drop it
     }
 }
 
 impl DelayMs<u32> for Delay {
-    // At 48 MHz, calling delay_us with ms * 1_000 directly overflows at 0x15D868 (just over the max u16 value)
+    // At 48 MHz (the maximum frequency), calling delay_us with ms * 1_000 directly overflows at 0x15D86 (just over the max u16 value)
+    // So we implement a separate, higher level, delay loop
     fn delay_ms(&mut self, mut ms: u32) {
         const MAX_MS: u32 = 0x0000_FFFF;
         while ms != 0 {
@@ -68,7 +72,9 @@ impl DelayMs<u32> for Delay {
 
 impl DelayMs<u16> for Delay {
     fn delay_ms(&mut self, ms: u16) {
-        self.delay_us(u32::from(ms) * 1_000);
+        // Call delay_us directly, so we don't have to use the additional
+        // delay loop the u32 variant uses
+        self.delay_us(u32(ms) * 1_000);
     }
 }
 
@@ -78,28 +84,28 @@ impl DelayMs<u8> for Delay {
     }
 }
 
+// At 48MHz (the maximum frequency), this overflows at approx. 2^32 / 48 = 89 seconds
 impl DelayUs<u32> for Delay {
     fn delay_us(&mut self, us: u32) {
         // The SysTick Reload Value register supports values between 1 and 0x00FFFFFF.
         // Here less than maximum is used so we have some play if there's a long running interrupt.
-        const MAX_RVR: u32 = 0x007F_FFFF;
+        const MAX_TICKS: u32 = 0x007F_FFFF;
 
-        let mut total_rvr = if self.clocks.sysclk().0 < 1_000_000 {
-            us / (1_000_000 / self.clocks.sysclk().0)
-        } else {
-            us * (self.clocks.sysclk().0 / 1_000_000)
-        };
+        let mut total_ticks = us * self.scale;
 
-        while total_rvr != 0 {
-            let current_rvr = if total_rvr <= MAX_RVR {
-                total_rvr
+        while total_ticks != 0 {
+            let current_ticks = if total_ticks <= MAX_TICKS {
+                total_ticks
             } else {
-                MAX_RVR
+                MAX_TICKS
             };
 
             let start_count = SYST::get_current();
-            total_rvr -= current_rvr;
-            while (start_count.wrapping_sub(SYST::get_current()) % MAX_SYSTICK) < current_rvr {}
+            total_ticks -= current_ticks;
+
+            // Use the wrapping substraction and the modulo to deal with the systick wrapping around
+            // from 0 to 0xFFFF
+            while (start_count.wrapping_sub(SYST::get_current()) % SYSTICK_RANGE) < current_ticks {}
         }
     }
 }