Add "good" timing

I don't get why the delay is off by a factor of 32. None of the
prescaling factors are even 32, it goes 8, 64, 256, 1024. Where does 32
come from? Why is it not in Arduino's code? I have a 16 MHz crystal.
What the heck man? Oh well.

src/main.zig

@@ -1,29 +1,34 @@
-const std = @import("std");
 const gpio = @import("gpio.zig");
 const pins = @import("pins.zig");
+const time = @import("time.zig");
 
 pub fn main() void {
-    // This baud should match what you pass in build
+    time.init();
     // Onboard LED
     gpio.pinMode(pins.led_builtin, .out);
-    // Wired LED
-    gpio.pinMode(8, .out);
-    // Button
-    gpio.pinMode(3, .in);
-
     while (true) {
         gpio.toggle(pins.led_builtin);
-        if (gpio.digitalRead(3) == .low) {
-            gpio.digitalWrite(8, .low);
-        } else {
-            gpio.digitalWrite(8, .high);
-        }
+        time.delay(1000);
+    }
+}
 
-        // Hard code a busy wait. Will add delays eventually.
-        var i: u32 = 0;
-        while (i < 100000) : (i += 1) {
-            // For some reason need to nop the busy wait
-            asm volatile ("nop");
+// Interrupt stuff in root for timer
+pub const interrupts = struct {
+    pub fn TIMER0_OVF() void {
+        // Dunno if we need locals like in Arduino standard library to keep in
+        // registers. But I'll do it anyway
+        var m = time.timer0_millis;
+        var f = time.timer0_fract;
+
+        m += time.millis_inc;
+        f += time.fract_inc;
+        if (f >= time.fract_max) {
+            f -= time.fract_max;
+            m += 1;
         }
+
+        time.timer0_fract = f;
+        time.timer0_millis = m;
+        time.timer0_overflow_count += 1;
     }
-}
+};

src/time.zig

@@ -1,36 +1,57 @@
 //! Timing functions such as delay.
 
-const serial = @import("serial.zig");
-const chip = @import("microzig").chip;
+const micro = @import("microzig");
 const regs = @import("microzig").chip.registers;
 const std = @import("std");
-const Allocator = std.mem.Allocator;
 
-const cpu_freq = 16000000;
-const clock_cycles_per_microsecond = cpu_freq / 1000000;
+const clock_cycles_per_microsecond = micro.clock.get().cpu / 1000000;
+const microseconds_per_timer0_overflow = clock_cycles_to_microseconds(64 * 256);
+pub const millis_inc = (microseconds_per_timer0_overflow / 1000);
 
-var timer0_overflow_count = 0;
+pub const fract_inc: u8 = ((microseconds_per_timer0_overflow % 1000) >> 3);
+pub const fract_max: u8 = (1000 >> 3);
 
-fn init() void {
-    regs.TC0.TCCR0B.modify(.{ .CS0 = 0x5 });
+pub var timer0_overflow_count: u64 = 0;
+pub var timer0_millis: u64 = 0;
+pub var timer0_fract: u8 = 0;
+
+fn clock_cycles_to_microseconds(a: u64) u64 {
+    return ((a) / clock_cycles_per_microsecond);
+}
+
+fn microseconds_to_clock_cycles(a: u64) u64 {
+    return ((a) * clock_cycles_per_microsecond);
+}
+
+// This will eventually move from time. Probably. Just most of this stuff
+// is enabling timers and interrupts on them
+pub fn init() void {
+    // Enable interrupts
+    regs.CPU.SREG.modify(.{ .I = 0x1 });
+    regs.TC0.TIMSK0.modify(.{ .TOIE0 = 0x1 });
+    // Set clock mode
+    regs.TC0.TCCR0B.modify(.{ .CS0 = 0x1 });
     regs.TC0.TCCR0A.modify(.{ .WGM0 = 0x1 });
 }
 
-fn micros() u32 {
-    // TODO: Need to care more about flags, interrupts, stuff like that.
-    // TODO: Care about overflow in timer register
-    return regs.TC0.TCNT0.*;
+fn micros() u64 {
+    const oldSREG = regs.CPU.SREG.*;
+    micro.cpu.cli();
+    var m = timer0_overflow_count;
+    const t = regs.TC0.TCNT0.*;
+    if (regs.TC0.TIFR0.read().TOV0 == 1 and t < 255) {
+        m += 1;
+    }
+    regs.CPU.SREG.* = oldSREG;
+    return ((m << 8) + t) * (64 / clock_cycles_per_microsecond);
 }
 
 pub fn delay(ms: u64) void {
-    init();
-    var var_ms = comptime ms;
+    // Please tell me why in the world I need this 32 here
+    var var_ms = comptime ms * 32;
     var start = micros();
     while (var_ms > 0) {
-        //if (micros() == start) {
-        //var_ms -= 1;
-        //}
-        while ((var_ms > 0) and (micros() >= start)) {
+        while ((var_ms > 0) and ((micros() - start) >= 1000)) {
             var_ms -= 1;
             start += 1000;
         }