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3.5 Basic Commands

This section lists the basic SCD4x commands that are necessary to start a periodic measurement and subsequently read out

the sensor outputs.

The typical communication sequence between the I2C master (e.g., a microcontroller) and the SCD4x sensor is as follows:

The sensor is powered up

The I2C master sends a start_periodic_measurement command. Signal update interval is 5 seconds.

The I2C master periodically reads out data with the read measurement sequence.

To put the sensor back to idle mode, the I2C master sends a stop periodic measurement command.

While a periodic measurement is running, no other commands must be issued with the exception of read_measurement,

get_data_ready_status, stop_periodic_measurement and set_ambient_pressure.

3.5.1

start_periodic_measurement

Description: start periodic measurement, signal update interval is 5 seconds.

Table 10: start_periodic_measurement

I2C sequence description

Write

(hexadecimal)

Input parameter: -

Response parameter: -

Max. command

duration [ms]

length [bytes]

signal conversion

length [bytes]

signal conversion

0x21b1

not applicable

Example: start periodic measurement

Write

0x21b1

(hexadecimal)

Command

3.5.2

read_measurement

Description: read sensor output. The measurement data can only be read out once per signal update interval as the buffer is

emptied upon read-out. If no data is available in the buffer, the sensor returns a NACK. To avoid a NACK response, the

get_data_ready_status can be issued to check data status (see chapter 3.8.2 for further details). The I2C master can abort the

read transfer with a NACK followed by a STOP condition after any data byte if the user is not interested in subsequent data.

Table 11: read_measurment I2C sequence description

Write

(hexadecimal)

Input parameter: -

Response parameter: CO2, Temperature, Relative

Humidity

Max.

command

duration [ms]

length [bytes]

signal conversion

length [bytes]

signal conversion

0xec05

CO2 [ppm] = word[0]

T = - 45 + 175 * word[1] / 216

RH = 100 * word[2] / 216

Example: read sensor output (500 ppm, 25 °C, 37 % RH)

Write

0xec05

(hexadecimal)

Command

Wait

1 ms

command execution time

Response

0x01f4

0x7b

0x6667

0xa2

0x5eb9

0x3c

(hexadecimal)

CO2 = 500 ppm

CRC of 0x01f4

Temp. = 25 °C

CRC of 0x6667

RH = 37 %

CRC of 0x5eb9

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