Write humidity and temperature to sensor

The written values are dummies, but I'd like to either have an
additional sensor, or read the values from a config-file on the SD card

src/co2.rs

@@ -9,6 +9,7 @@ const CCS811_I2C_ADDRESS: u16 = 0x5A;
 const CCS811_REGISTER_STATUS: u8 = 0x00;
 const CCS811_REGISTER_MEAS_MODE: u8 = 0x01;
 const CCS811_REGISTER_ALG_RESULT_DATA: u8 = 0x02;
+const CCS811_REGISTER_ENV_DATA: u8 = 0x05;
 
 /// Bootloader I2C Register addresses of the co2 sensor
 const CCS811_REGISTER_BOOTLOADER_APP_START: u8 = 0xF4;
@@ -69,3 +70,45 @@ pub async fn set_measurement_mode(
 ) -> Result<(), embassy_rp::i2c::Error> {
     write_register(i2c, CCS811_REGISTER_MEAS_MODE, [mode]).await
 }
+
+/// Set the ambient temperature and humidity of the measurement environment to
+/// calibrate the sensor output
+///
+/// The function panics for humidity values > 100% or  temperatures < -25
+/// degrees Celsius.
+///
+/// * `temperature` - Current temperature in milidegrees celsius
+/// * `humidity` - Current humidity in per mille
+pub async fn set_environment_values(
+    i2c: &mut I2c<'_, I2C1, Async>,
+    temperature: i32,
+    humidity: u32,
+) -> Result<(), embassy_rp::i2c::Error> {
+    assert!(temperature >= -25000, "`temperature` was lower than -25000");
+    assert!(humidity <= 1000, "`humidity` was higher than 100%");
+
+    // A register value of 0 maps to -25 degrees
+    let temperature: i32 = temperature + 25000;
+    // A single increment in the register maps to 1/512 degrees.
+    // Order of operations is relevant due to integer division
+    let temperature: i32 = (temperature * 512) / 1000;
+
+    assert!(
+        temperature <= (u16::MAX as i32),
+        "temperature register value was higher than u16::MAX"
+    );
+    let temperature: u16 = temperature
+        .try_into()
+        .expect("We checked the value in an assert already");
+
+    // A single increment in the register maps to 1/512%RH
+    let humidity: u16 = ((humidity * 512) / 1000)
+        .try_into()
+        .expect("We checked the value in an assert already");
+
+    let bytes = humidity
+        .to_be_bytes()
+        .into_iter()
+        .chain(temperature.to_ne_bytes().into_iter());
+    write_register(i2c, CCS811_REGISTER_STATUS, bytes).await
+}

src/main.rs

@@ -92,6 +92,9 @@ async fn main(spawner: Spawner) {
 
     // 16 is measurement mode 1 => 1 measurement per second
     co2::set_measurement_mode(&mut i2c, 16).await.unwrap();
+    co2::set_environment_values(&mut i2c, 25000, 660)
+        .await
+        .unwrap();
 
     loop {
         let status = co2::get_status(&mut i2c).await.unwrap();