Keep CO2 in check with CDM7160 and ATmega328P

NDIR CO2 Click is based on the CDM7160, a pre-calibrated single light source, dual wavelength, CO2 sensing system from Figaro Engineering. Its light source emits the light, which two IR sensors detect. One light sensor is placed behind the filter, which allows only a part of the IR spectrum affected by the CO2 gas to pass through, while the second sensor is placed behind the filter, which passes the IR spectrum of the light unaffected by the CO2 gas. This forms a differential input for the sensor - an integrated MCU will process the received data by differentiating these readings. This allows the absolute value of the CO2 gas concentration to be obtained and removes any influences of particles and other disturbances, as they affect both sensors equally. This allows consistent readings over various temperatures in various environments, including areas rich with corrosive gases (SO2, H2S, and more), and over a longer period (aging). The CDM7160 sensor can output data in two ways: depending on the status of the MSEL pin, it can use either a UART or I2C communication interface. If this pin is pulled to a LOW logic level, the I2C interface will be selected after the CDM7160 reset cycle. Otherwise, the UART interface will be selected. Since the communication pins are shared between the interfaces (SCL/RX and SDA/TX), they need to be switched to the corresponding pins of the mikroBUS™ whenever a different type of communication is used. Therefore, the Click board™ has a section with three small SMD slide switches labeled COM SEL. Positioning all three switches to the LEFT position will select the I2C interface, while the RIGHT position will select the UART interface. When the I2C interface is

selected, an additional pin is available to set up the I2C address of the device. This pin determines the LSB of the I2C slave address, and when it is pulled to a LOW logic level, this bit becomes 0. This allows up to 2 different devices to be connected to the same I2C bus. This pin is routed to another SMD slide switch labeled the ADD SEL. It is possible to perform two types of calibration for this sensor: zero calibration and background calibration. The zero calibration is performed in the atmosphere with a CO2 concentration of 0 ppm, while the background calibration is performed in the atmosphere with a nominal CO2 value (400 ppm). Since the sensor is influenced by the sea level and the atmospheric pressure, these calibrations should be performed whenever these conditions are changed. This will allow an increased accuracy of the CO2 concentration readings. The CDM7160 sensor offers a pin labeled as CAL for easy calibration: if the CAL pin is pulled to a LOW logic level for about 2 to 11 seconds, a background calibration will be performed. The zero calibration will be performed if pulled to a LOW logic level for more than 12 seconds. This pin should remain HIGH during normal operation. An internal pull-up resistor ensures the pin is always HIGH if it remains floating. This pin is routed to the mikroBUS™ CS pin, labeled as CAL. The ALERT pin of the CDM7160 sensor triggers an interrupt on the host MCU. It will default trigger an interrupt if the CO2 concentration exceeds 1000ppm. The interrupt will be cleared if the concentration drops below 900ppm. These settings can be changed by writing values to the corresponding ALHI and ALLO registers (upper and lower threshold registers). The ALERT pin of the CDM7160 sensor is

routed to the mikroBUS™ INT pin. The BUSY pin of the sensor provides a means to save the sensor from polling sensor registers to verify if the device is ready for communication. By setting an interrupt for the BUSY pin, the MCU can be automatically triggered only when the sensor is ready to accept a new command. A logic LOW level signals the MCU that the sensor cannot accept a new command. The sensor might be unavailable for about 0.3 seconds while internally processing the data. This pin is routed to the mikroBUS™ RST pin, labeled as BSY on the Click board™. Besides UART and I2C communication, the sensor offers a 1KHz PWM signal with the duty cycle, which depends on the CO2 concentration (0 to 5000 ppm of CO2). The Click board™ is equipped with an operational amplifier, which averages the PWM signal, offering analog DC voltage (0 to 5V) on its output, directly proportional to the pulse width of the PWM signal. By switching the SMD jumper labeled as AN ENABLE to EN position, the voltage at the output of this operational amplifier becomes available at the AN pin of the mikroBUS™. By default, the jumper is soldered to the DIS position. The full-scale voltage on the operational amplifier output is 5V (5000 ppm of CO2 equals 5V). To allow communication with 3.3V MCUs, two additional ICs are used: one is the PCA9306, which translates voltage levels of the I2C signals, while the second IC is the TXB0106, used to translate voltage levels of the remaining IC pins, including the UART. These ICs are used on many other designs and have proven reliable solutions.

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