libboard_zynq: find prescaler for GlobalTimer, rename new() to start()

experiments/src/main.rs

@@ -72,7 +72,7 @@ pub fn main_core0() {
     }
     let mut flash = flash.stop();
 
-    let timer = libboard_zynq::timer::GlobalTimer::new();
+    let timer = libboard_zynq::timer::GlobalTimer::start();
 
     let mut ddr = zynq::ddr::DdrRam::new();
     #[cfg(not(feature = "target_zc706"))]

libboard_zynq/src/mpcore.rs

@@ -92,7 +92,7 @@ register!(value_register, ValueRegister, RW, u32);
 register_bits!(value_register, value, u32, 0, 31);
 
 register!(global_timer_control, GlobalTimerControl, RW, u32);
-register_bits!(global_timer_control, prescaler, u16, 8, 15);
+register_bits!(global_timer_control, prescaler, u8, 8, 15);
 register_bit!(global_timer_control, auto_increment_mode, 3);
 register_bit!(global_timer_control, irq_enable, 2);
 register_bit!(global_timer_control, comp_enablea, 1);

libboard_zynq/src/timer/global.rs

@@ -13,7 +13,14 @@ pub struct GlobalTimer {
 }
 
 impl GlobalTimer {
-    pub fn new() -> GlobalTimer {
+    /// Get the potentially uninitialized timer
+    pub unsafe fn get() -> GlobalTimer {
+        let mut regs = mpcore::RegisterBlock::new();
+        GlobalTimer { regs }
+    }
+
+    /// Get the timer with a reset
+    pub fn start() -> GlobalTimer {
         let mut regs = mpcore::RegisterBlock::new();
         Self::reset(&mut regs);
         GlobalTimer { regs }
@@ -33,12 +40,17 @@ impl GlobalTimer {
             mpcore::ValueRegister::zeroed()
         );
 
+        // find a prescaler value that matches CPU speed / 2 to us
+        let clocks = Clocks::get();
+        let mut prescaler = clocks.cpu_3x2x() / 1_000_000;
+        while prescaler > 256 {
+            prescaler /= 2;
+        }
+
         // Start
         regs.global_timer_control.write(
             mpcore::GlobalTimerControl::zeroed()
-                // maximum prescaler is still enough for millisecond
-                // precision while overflowing after centuries.
-                .prescaler(255)
+                .prescaler((prescaler - 1) as u8)
                 .auto_increment_mode(true)
                 .timer_enable(true)
         );