fix: detect stopped fan when tach count is at max value

EMC2305 returns near-max tach count (~1022) when no tach signal is detected.
Added TACH_COUNT_STOPPED_THRESHOLD constant to detect this condition and
return 0 RPM for stopped fans.

Author: moffa90

emc2305/driver/constants.py

@@ -477,6 +477,10 @@
 TACH_COUNT_MAX = 0x1FFF
 """Maximum tachometer count value (13-bit)"""
 
+TACH_COUNT_STOPPED_THRESHOLD = 1000
+"""Tach count threshold for detecting stopped fan (after 3-bit shift).
+EMC2305 returns near-max values (e.g., 1022) when no tach signal is detected."""
+
 # Minimum valid TACH count - fan is considered stalled below this
 DEFAULT_VALID_TACH_COUNT = 0x0FFF
 """Default minimum valid tachometer count for stall detection"""

emc2305/driver/emc2305.py

@@ -1245,6 +1245,11 @@ def get_current_rpm(self, channel: int) -> int:
                 raw_tach = (tach_high << const.TACH_COUNT_HIGH_SHIFT) | tach_low
                 tach_count = raw_tach >> const.TACH_COUNT_LOW_SHIFT
 
+                # Check for max TACH count - indicates no tach signal (fan stopped)
+                # EMC2305 returns 0x1FFF (or near max) when no pulses are detected
+                if tach_count >= const.TACH_COUNT_STOPPED_THRESHOLD:
+                    return 0
+
                 # Convert to RPM
                 config = self._fan_configs.get(channel, FanConfig())
                 rpm = self._tach_count_to_rpm(tach_count, config.edges)