Merge pull request #4 from Sensirion/add-frc-example

Add frc example

CHANGELOG.md

@@ -4,6 +4,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased] 
+- Fix FRC delay if a binary gas >= 0x10 is used.
+- Use 110ms measurement delay as default.
+- Add force recalibration (FRC) example
 
 ### Added
 - Serial print of SHT4x temperature and temperature

examples/exampleUsageFrc/.arduino-ci.yml

@@ -0,0 +1,3 @@
+compile:
+  libraries:
+    - "Sensirion I2C SHT4x"

examples/exampleUsageFrc/README.md

@@ -0,0 +1,75 @@
+# Forced Recalibration (FRC) Example
+
+After soldering, during assembly, board-test or at a later stage, a forced recalibration (FRC)
+must be performed to ensure performance within specifications.
+
+The STC31-C sensor does not have any non-volatile memory. Therfore the sensor config (state) 
+needs to be stored by the micro-controller in non-volatile memory. On a soft reset or power
+cylce, those states need to be restored.
+
+The post-soldering FRC procedure is documented in the paragraph 5 of the
+[Desing guide](https://sensirion.com/resource/application_note/design_guide_STC31-C)
+
+All used commands and formulas used in this exampel can be found in the
+[Datasheet](https://sensirion.com/resource/datasheet/stc31-c)
+
+## Post-soldering Forced Recalibration Process
+
+The post soldering FRC process must be performed in a controlled environment with a
+known CO2 concentration in vol%. In this example we use 0.0 vol% which is what we
+measure for the atmospheric CO2 concentration. The atmospheric CO2 concentration is
+0.04 vol% and as such below the STC31-C accuracy.
+
+The FRC process is done in two loops. In the first loop, we apply the FRC value and
+in the second loop we verify that the FRC was successful.
+
+### Measurement Setup
+For this example we use the STC31-C evaluation kit, with a STC31-C and SHT40 sensor
+on a flex PCB.
+
+* Measurement frequency: 1Hz
+* Weak filter enabled (note: the filter configuration is not stored in the state and
+  therefore needs to be set separately after each soft-reset or power-cylce).
+* automatic self-calibration (ASC) enabled.
+* Reference CO2 concentration: 0.0 vol% (atmospheric CO2 concentration)
+* Measurement duration for stabilization phase: 20s
+* Binary gas mode: 0x13 (19). CO2 concentration between 0 to 40% in air.
+
+The configuration can be changed, by changing the various `#define` that can be found
+at the beginning of the [exampleUsageFrc.ino](exampleUsageFrc.ino) file.
+
+
+### 1st Loop: Forced Recalibration
+
+Before the first loop is started, we send a I2C soft reset command to ensure that
+the sensor is in a clean state.
+
+The first loop is implemented in the function `fistFrcLoop()`. It consists of the
+following steps.
+
+1. Enable automatic self-calibration (ASC)
+2. Set binary gas mode to 0x13
+3. Enable weak filter (implemented in the function `configureFilter`)
+4. Measure for 20s at 1Hz to stabilize the readings (section 4.1.1 in the Desing Guide).
+   humidity and temperature compensation from SHT40 sensor are set before each measurement.
+   If you have a pressure sensor on the board, you should read out the absolute pressure value
+   and set it as well (commented out code in the `measureAndPrint` function)
+5. Apply the FRC with a CO2 reference value of 0.0 vol% (raw value: 16384)
+6. Get the sensor state and store it in the memory of the Arduino. This value should be stored
+   on non-volatile memory in your product.
+7. Trigger an I2C soft reset to initiate the 2nd loop (verify)
+
+
+### 2nd Loop: Verify
+
+The second loop is implemented in the function `secondFrcLoop()`.
+
+1. Write and apply the sensor state we previously stored. This will restore all configurations
+   except for the filters.
+2. Enable weak filter (implemented in the function `configureFilter`)
+3. Measure for 20s at 1Hz to stabilize the readings (See 1st loop step 4).
+4. Take one more measurement and verify that the CO2 concentration is within specification (0.0 vol%).
+   In this example we allow a margin of +- 1.0 vol%.
+
+If the CO2 concentration is within specification, the FRC was successful and we can use the saved
+state. If the value is out of specification, the FRC process must be repeated.