bdp_guard: new zero at every ARM

With TPU in a nozzle heating to 150C the pressure kept changing for
minutes (+43, then -56 as it oozed), faster than the sensor's own
baseline update, and the guard stopped the printer at the start of
z_check and home_xy with the nozzle in the air. Every ARM (also a phase
switch) now writes reset_probe (register 52) and waits 0.2 s without
reading, so only force during the guarded moves counts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extras/bdp_guard.py

@@ -8,6 +8,10 @@
 # row at or above THRESHOLD shut Klipper down at once (like M112, without
 # waiting for the running macro to finish).
 #
+# Every ARM (also a phase switch) first sets a new zero (register 52), so
+# slow pressure changes before the guarded moves do not count (TPU in a
+# nozzle that heats up keeps changing pressure for minutes).
+#
 # Arm it only around moves where the nozzle must not touch anything: not
 # around contact homing, extrusion, purges or AFC cuts and loads, which
 # press on the nozzle on purpose. ARM and DISARM act immediately, not in
@@ -95,6 +99,7 @@ class BDPGuard:
         self.sensor = self.i2c = None
         self.available = False
         self.timer = None
+        self.zeroing = False
         self.phase_mode = self.mode
         self._reset_phase('')
         self.printer.register_event_handler('klippy:connect',
@@ -162,6 +167,23 @@ class BDPGuard:
         logging.info(msg)
         gcmd.respond_info(msg)
 
+    def _rezero(self):
+        # A fresh zero at every ARM: only force that appears during the
+        # guarded moves counts, not slow changes before them. Seen with TPU
+        # in a nozzle heating to 150C: the pressure kept rising (+43) or
+        # falling as it oozed (-56) for minutes, faster than the sensor's
+        # own baseline update (only when steady, about every 7 s), and
+        # stopped the printer at the start of a phase with the nozzle in
+        # the air. ARM comes after M400, with the nozzle clear of
+        # everything, so this does not hide a real touch.
+        self.zeroing = True
+        try:
+            self.i2c.i2c_write([REG_RESET_PROBE, 1])
+            self.reactor.pause(self.reactor.monotonic() + 0.2)
+        except Exception:
+            logging.exception("bdp_guard: setting a new zero failed")
+        self.zeroing = False
+
     def _stop_timer(self):
         if self.timer is not None:
             self.reactor.unregister_timer(self.timer)
@@ -179,8 +201,9 @@ class BDPGuard:
                 self._stop_timer()
                 self.phase = ''
                 return self.reactor.NEVER
-            # PA mode (SET_BDPRESSURE START): the delta means nothing then
-            if getattr(self.sensor, 'last_state', 0):
+            # PA mode (SET_BDPRESSURE START): the delta means nothing then;
+            # while ARM sets a new zero the readings are in between
+            if self.zeroing or getattr(self.sensor, 'last_state', 0):
                 return eventtime + self.interval
             data = bytearray(self.i2c.i2c_read([REG_DELTA], 2)['response'])
             if len(data) != 2:
@@ -229,6 +252,7 @@ class BDPGuard:
                                   "extension not found, guard not armed")
                 return
             self._ensure_probe_mode(gcmd)
+        self._rezero()
         self.phase_mode = mode
         self._reset_phase(phase)
         self.armed_until = self.reactor.monotonic() + self.timeout