Fix issues with watermark sizes

The M_CAN users manual v330 states that Rx FIFO Watermarks, FnWM, can
take values 1-64 inclusive. Any value greater than 64 is interpreted as
"Watermark interrupt disabled", p.31 and p.34.

It also states that the Tx Event FIFO Watermark, EFWM, can take values
1-32 inclusive, Any value greated than 32 is interpreted as "Watermark
interrupt disabled", p.44.

This change makes the implementation consistent with the users manual on
those specifics.

mcan/CHANGELOG.md

@@ -3,6 +3,7 @@
 Tagging in git follows a pattern: `mcan/<version>`.
 
 ## [Unreleased]
+- Fix some issues with watermark sizes for Rx FIFOs and Tx Event FIFO.
 - Add Can::aux::initialization_mode
 
 ## [0.3.0] - 2023-04-24

mcan/src/bus.rs

@@ -339,9 +339,10 @@ impl<'a, Id: mcan_core::CanId, D: mcan_core::Dependencies<Id>, C: Capacities>
         reg.rxf0.c.modify(|_, w| {
             let w = w.fom().bit(config.rx_fifo_0.mode.into());
             let mut watermark = config.rx_fifo_0.watermark;
-            // According to the spec, any value >= 64 is interpreted as watermark disabled
-            if watermark >= 64 {
-                watermark = 64;
+            // According to the spec, any value > 64 is interpreted as watermark interrupt
+            // disabled, as is 0.
+            if watermark > 64 {
+                watermark = 0;
             }
             // Safety: The value is sanitized before the write
             unsafe { w.fwm().bits(watermark) }
@@ -351,9 +352,10 @@ impl<'a, Id: mcan_core::CanId, D: mcan_core::Dependencies<Id>, C: Capacities>
         reg.rxf1.c.modify(|_, w| {
             let w = w.fom().bit(config.rx_fifo_1.mode.into());
             let mut watermark = config.rx_fifo_1.watermark;
-            // According to the spec, any value >= 64 is interpreted as watermark disabled
-            if watermark >= 64 {
-                watermark = 64;
+            // According to the spec, any value > 64 is interpreted as watermark interrupt
+            // disabled, as is 0.
+            if watermark > 64 {
+                watermark = 0;
             }
             // Safety: The value is sanitized before the write
             unsafe { w.fwm().bits(watermark) }
@@ -366,9 +368,10 @@ impl<'a, Id: mcan_core::CanId, D: mcan_core::Dependencies<Id>, C: Capacities>
         // Configure Tx Event Fifo
         reg.txefc.modify(|_, w| {
             let mut watermark = config.tx.tx_event_fifo_watermark;
-            // According to the spec, any value >= 32 is interpreted as watermark disabled
-            if watermark >= 32 {
-                watermark = 32;
+            // According to the spec, any value > 32 is interpreted as watermark interrupt
+            // disabled, as is 0.
+            if watermark > 32 {
+                watermark = 0;
             }
             // Safety: The value is sanitized before the write
             unsafe { w.efwm().bits(watermark) }