Fix 5 bugs in BME280.py verified against Bosch BME280_SensorAPI and Adafruit references

1. get_power_mode() missing parentheses in set_oversamp_pressure and
   set_oversamp_temperature — compared method object instead of calling it,
   so sleep-mode check never worked correctly.

2. set_oversamp_humidity else branch wrote ctrl_hum_reg into CTRL_MEAS
   register instead of ctrl_meas_reg (per datasheet §5.4.3, humidity
   settings require a subsequent write to ctrl_meas with the correct value).

3. dig_H6 stored as unsigned byte — should be int8_t per Bosch
   bme280_defs.h and Adafruit_BME280.h.

4. dig_H4/H5 sign extension wrong — used to_s16() on 12-bit signed values.
   Now sign-extends from MSB byte first (to_s8) then combines with LSB
   nibble, matching Bosch parse_humidity_calib_data() and Adafruit
   readCoefficients().

5. Humidity compensation formula was missing var5 multiplication factor
   vs Bosch compensate_humidity() (BME280_DOUBLE_ENABLE path). Rewritten
   with Bosch variable names for easy cross-referencing.

1 files changed, 40 insertions, 11 deletions

diff --git a/BME280.py b/BME280.py
index af9afac..8628eb2 100644
--- a/BME280.py
+++ b/BME280.py
@@ -2,6 +2,7 @@ import time
 import smbus
 
 to_s16 = lambda x: (x + 2**15) % 2**16 - 2**15
+to_s8 = lambda x: (x + 2**7) % 2**8 - 2**7
 to_u16 = lambda x: x % 2**16
 
 BME280_I2C_ADDR = 0x76
@@ -119,9 +120,19 @@ class BME280:
         params = self.read(cali_regs['BME280_HUMIDITY_CALIB_DIG_H2_LSB_REG'], 7)
         self.param['dig_H2'] = to_s16((params[1] << 8) | params[0])
         self.param['dig_H3'] = params[2]
-        self.param['dig_H4'] = to_s16((params[3] << 4) | (params[4] & 0x0f))
-        self.param['dig_H5'] = to_s16((params[5] << 4) | (params[4] >> 4))
-        self.param['dig_H6'] = params[6]
+        # FIX: dig_H4/H5 are 12-bit signed values. Must sign-extend from the
+        # MSB byte first, then combine with the LSB nibble — matching the Bosch
+        # BME280_SensorAPI parse_humidity_calib_data() which does:
+        #   dig_h4_msb = (int16_t)(int8_t)reg_data[3] * 16;
+        #   dig_h4 = dig_h4_msb | (reg_data[4] & 0x0F);
+        # The old code used to_s16() which only sign-extends at the 16-bit
+        # boundary, giving wrong results when the 12-bit value is negative.
+        self.param['dig_H4'] = (to_s8(params[3]) << 4) | (params[4] & 0x0f)
+        self.param['dig_H5'] = (to_s8(params[5]) << 4) | (params[4] >> 4)
+        # FIX: dig_H6 is int8_t per Bosch API (bme280_defs.h) and Adafruit.
+        # Was stored unsigned, which gives wrong humidity for sensors where
+        # this calibration coefficient is negative.
+        self.param['dig_H6'] = to_s8(params[6])
 
     def get_power_mode(self):
         return self.read(regs['BME280_CTRL_MEAS_REG'], 1)[0] & 0x03
@@ -160,7 +171,12 @@ class BME280:
             self.write(regs['BME280_CTRL_MEAS_REG'], [self.ctrl_meas_reg])
         else:
             self.write(regs['BME280_CTRL_HUMIDITY_REG'], [v_data])
-            self.write(regs['BME280_CTRL_MEAS_REG'], [self.ctrl_hum_reg])
+            # FIX: Was writing ctrl_hum_reg into CTRL_MEAS register (wrong
+            # value in wrong register). Per datasheet section 5.4.3, humidity
+            # settings only take effect after a write to ctrl_meas — but we
+            # must write the actual ctrl_meas value, not the humidity one.
+            # Matches Adafruit setSampling() and Bosch set_osr_humidity_settings().
+            self.write(regs['BME280_CTRL_MEAS_REG'], [self.ctrl_meas_reg])
 
         self.oversamp_humidity = sampling
         self.ctrl_meas_reg = self.read(regs['BME280_CTRL_MEAS_REG'], 1)[0]
@@ -170,7 +186,11 @@ class BME280:
     def set_oversamp_pressure(self, sampling):
         v_data = (self.ctrl_meas_reg & ~0x1c) | ((sampling << 2) & 0x1c)
 
-        if self.get_power_mode != power_mode['BME280_SLEEP_MODE']:
+        # FIX: Was missing () on get_power_mode — compared the method object
+        # (always truthy) to an int, so the sleep-mode check never worked.
+        # Matches set_power_mode() and set_oversamp_humidity() which both
+        # correctly call self.get_power_mode().
+        if self.get_power_mode() != power_mode['BME280_SLEEP_MODE']:
             self.soft_rst()
             time.sleep(0.003)
             self.write(regs['BME280_CONFIG_REG'], [self.config_reg])
@@ -187,7 +207,8 @@ class BME280:
     def set_oversamp_temperature(self, sampling):
         v_data = (self.ctrl_meas_reg & ~0xe0) | ((sampling << 5) & 0xe0)
 
-        if self.get_power_mode != power_mode['BME280_SLEEP_MODE']:
+        # FIX: Same missing () bug as set_oversamp_pressure — see above.
+        if self.get_power_mode() != power_mode['BME280_SLEEP_MODE']:
             self.soft_rst()
             time.sleep(0.003)
             self.write(regs['BME280_CONFIG_REG'], [self.config_reg])
@@ -217,11 +238,19 @@ class BME280:
 
         return (x1 >> 12) / 1024.0
         """
-        h = float(self.t_fine) - 76800.0
-        h = (raw_val - (float(self.param['dig_H4']) * 64.0 + float(self.param['dig_H5']) / 16384.0 * h)) * (
-        float(self.param['dig_H2']) / 65536.0 * (1.0 + float(self.param['dig_H6']) / 67108864.0 * h * (
-        1.0 + float(self.param['dig_H3']) / 67108864.0 * h)))
-        h = h * (1.0 - float(self.param['dig_H1']) * h / 524288.0)
+        # FIX: Humidity compensation rewritten to match Bosch BME280_SensorAPI
+        # compensate_humidity() (BME280_DOUBLE_ENABLE path in bme280.c).
+        # The old code was missing the var5 multiplication factor, producing
+        # incorrect humidity. The formula below uses the same variable names
+        # as the Bosch reference for easy cross-referencing.
+        var1 = float(self.t_fine) - 76800.0
+        var2 = float(self.param['dig_H4']) * 64.0 + (float(self.param['dig_H5']) / 16384.0) * var1
+        var3 = raw_val - var2
+        var4 = float(self.param['dig_H2']) / 65536.0
+        var5 = 1.0 + (float(self.param['dig_H3']) / 67108864.0) * var1
+        var6 = 1.0 + (float(self.param['dig_H6']) / 67108864.0) * var1 * var5
+        var6 = var3 * var4 * (var5 * var6)
+        h = var6 * (1.0 - float(self.param['dig_H1']) * var6 / 524288.0)
         if h > 100:
             h = 100
         elif h < 0: