Fix TWI clock Configuration (#59)

Updated the frequency configuration and related documentation.
Added comment in I2cConfiguration noting that it will not be accurate for higher frequencies.

CHANGELOG.md

@@ -14,6 +14,7 @@
 ### Removed
 
 ### Fixed
+- TWIHS: Fix issue with clock frequency calculation.
 
 ## [v0.4.2] 2022-11-06
 

hal/src/serial/twi.rs

@@ -109,15 +109,15 @@ impl<M: TwiMeta> Twi<M> {
         // Try to find a valid clock configuration. From §43.8.5 we
         // have
         //
-        //    DIV * 2^CKDIV = (f_pid / f_twi) - 3
+        //    DIV * 2^CKDIV = (f_pid / f_twi / 2) - 3
         //
         // where DIV = CHDIV = CLDIV.
         //
         // We thus iterate over possible values of CKDIV and use the
         // first valid permutation of (CKIV, DIV), unless options are
         // exhausted.
         let calc_div = |ckdiv| {
-            (clk.freq() / conf.freq)
+            (clk.freq() / conf.freq / 2)
                 .checked_sub(3)
                 .map(|v| v / 2u32.pow(ckdiv))
         };
@@ -232,6 +232,9 @@ impl<M: TwiMeta> Twi<M> {
 pub struct I2cConfiguration {
     /// The frequency of the I²C communication. The periods of high
     /// and low clock cycles are equal.
+    ///
+    /// NOTE: freq is an estimate where the rise time is assumed to be negligible, which is only
+    /// valid for lower frequencies.
     pub freq: Hertz,
 }