Explain change in MG

esp-hal/MIGRATING-0.21.md

@@ -256,4 +256,14 @@ the GPIO drivers (`Input`, `Output`, `OutputOpenDrain` and `Flex`) instead.
 +)
 +.with_sda(sda)
 +.with_scl(scl);
-```
+```
+
+### The calculation of I2C timeout has changed
+
+Previously, I2C timeouts were counted in increments of I2C peripheral clock cycles. This meant that
+the configure value meant different lengths of time depending on the device. With this update, the
+I2C configuration now expects the timeout value in number of bus clock cycles, which is consistent
+between devices.
+
+ESP32 and ESP32-S2 use an exact number of clock cycles for its timeout. Other MCUs, however, use
+the `2^timeout` value internally, and the HAL rounds up the timeout to the next appropriate value.