add unit test for calculate_pio_clock_divider

The test will fail, because the current implementation doesn't calculate
the fractional part.

1 files changed, 29 insertions, 1 deletions

diff --git a/embassy-rp/src/pio_programs/clock_divider.rs b/embassy-rp/src/pio_programs/clock_divider.rs
index 02e353f53..687fd53ba 100644
--- a/embassy-rp/src/pio_programs/clock_divider.rs
+++ b/embassy-rp/src/pio_programs/clock_divider.rs
@@ -16,10 +16,38 @@ use crate::clocks::clk_sys_freq;
 /// A fixed-point divider value suitable for use in a PIO state machine configuration
 #[inline]
 pub fn calculate_pio_clock_divider(target_hz: u32) -> fixed::FixedU32<U8> {
+    calculate_pio_clock_divider_inner(clk_sys_freq(), target_hz)
+}
+
+#[inline]
+fn calculate_pio_clock_divider_inner(sys_freq: u32, target_hz: u32) -> fixed::FixedU32<U8> {
     // Requires a non-zero frequency
     assert!(target_hz > 0, "PIO clock frequency cannot be zero");
 
     // Calculate the divider
-    let divider = (clk_sys_freq() + target_hz / 2) / target_hz;
+    let divider = (sys_freq + target_hz / 2) / target_hz;
     divider.to_fixed()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn clock_divider_math() {
+        // A simple divider that must have a fractional part.
+        let divider = calculate_pio_clock_divider_inner(125_000_000, 40_000_000);
+        let expected: fixed::FixedU32<U8> = 3.125.to_fixed();
+        assert_eq!(divider, expected);
+
+        // A system clk so high it would overflow a u32 if shifted left.
+        let divider = calculate_pio_clock_divider_inner(2_000_000_000, 40_000);
+        let expected: fixed::FixedU32<U8> = 50000.to_fixed();
+        assert_eq!(divider, expected);
+
+        // A divider that requires all 8 fractional bits.
+        let divider = calculate_pio_clock_divider_inner(134_283_264, 16_777_216);
+        let expected: fixed::FixedU32<U8> = 8.00390625.to_fixed();
+        assert_eq!(divider, expected);
+    }
+}