From e3e34aaadbe64df54d5ef8ff77b335e0e7d18412 Mon Sep 17 00:00:00 2001
From: osmarks <osmarks@protonmail.com>
Date: Wed, 9 Oct 2024 20:46:32 +0100
Subject: [PATCH] SCD4x metrics script

---
 scd4x_prometheus.py | 85 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)
 create mode 100644 scd4x_prometheus.py

diff --git a/scd4x_prometheus.py b/scd4x_prometheus.py
new file mode 100644
index 0000000..4bf2b71
--- /dev/null
+++ b/scd4x_prometheus.py
@@ -0,0 +1,85 @@
+# refer to https://sensirion.com/media/documents/48C4B7FB/64C134E7/Sensirion_SCD4x_Datasheet.pdf
+
+import smbus3
+import struct
+import time
+from prometheus_client import start_http_server, Gauge
+import sys
+
+location = sys.argv[1]
+
+def sensiron_bad_crc8(data):
+    crc = 0xFF
+    POLY = 0x31
+    for byte in data:
+        crc ^= byte
+        for _ in range(8):
+            if crc & 0x80:
+                crc = (crc << 1) ^ POLY
+            else:
+                crc = (crc << 1)
+        crc &= 0xFF
+    return crc
+
+bus = smbus3.SMBus(1)
+ADDR = 0x62
+
+def encode_word_crc(hi, lo):
+    return [hi, lo, sensiron_bad_crc8([hi, lo])]
+
+def setup():
+    stop_measurement = smbus3.i2c_msg.write(ADDR, [0x3f, 0x86])
+    bus.i2c_rdwr(stop_measurement)
+    time.sleep(0.5)
+
+    serial_cmd = smbus3.i2c_msg.write(ADDR, [0x36, 0x82]) # serial number
+    serial_read = smbus3.i2c_msg.read(ADDR, 9)
+    bus.i2c_rdwr(serial_cmd, serial_read)
+    serial = bytes(serial_read)
+    print((serial[0:2] + serial[3:5] + serial[6:8]).hex())
+    time.sleep(0.1)
+    # automatic self-calibration breaks badly if CO2 never below 400ppm, which is not guaranteed
+    disable_asc = smbus3.i2c_msg.write(ADDR, [0x24, 0x16] + encode_word_crc(0x00, 0x00))
+    bus.i2c_rdwr(disable_asc)
+    time.sleep(0.1)
+
+    # default offset of 4 degrees C is "probably right" but I don't have the equipment to test this
+    #real_offset = 4.0
+    #offset_bytes = struct.pack(">H", int(real_offset * 65535.0 / 175.0))
+    #set_temp_offset = smbus3.i2c_msg.write(ADDR, [0x24, 0x1d] + encode_word_crc(*offset_bytes))
+    #bus.i2c_rdwr(set_temp_offset)
+    #time.sleep(0.1)
+
+    start_measurement = smbus3.i2c_msg.write(ADDR, [0x21, 0xb1])
+    bus.i2c_rdwr(start_measurement)
+
+setup()
+
+temperature = Gauge("scd4x_temperature", "Temperature in Celsius", ("location",))
+co2_ppm = Gauge("scd4x_co2_ppm", "CO2 parts per million", ("location",))
+rh = Gauge("scd4x_relative_humidity_percent", "Relative humidity in percent", ("location",))
+
+def read_temperature_pressure():
+    measurement_read_write = smbus3.i2c_msg.write(ADDR, [0xec, 0x05])
+    measurement_read_read = smbus3.i2c_msg.read(ADDR, 9)
+    bus.i2c_rdwr(measurement_read_write, measurement_read_read)
+    measurement_read_read = bytes(measurement_read_read)
+
+    print(measurement_read_read)
+
+    co2 = struct.unpack(">H", measurement_read_read[0:2])[0]
+    raw_temperature = struct.unpack(">H", measurement_read_read[3:5])[0]
+    raw_humidity = struct.unpack(">H", measurement_read_read[6:8])[0]
+
+    temperature_celsius = -45 + 175 * (raw_temperature / 65535.0)
+    relative_humidity = raw_humidity / 65535.0 * 100
+
+    temperature.labels(location=location).set(temperature_celsius)
+    rh.labels(location=location).set(relative_humidity)
+    co2_ppm.labels(location=location).set(co2)
+
+start_http_server(9091)
+
+while True:
+    time.sleep(5)
+    read_temperature_pressure()
\ No newline at end of file