SENSIRION SCD4 Series, Manual, Page: 7 / 24

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3 Digital Interface Description
All SCD4x commands and data are mapped to a 16-bit address space.
SCD4x Hex. Code
I
2
C address 0x62
Table 8 I
2
C device address.
3.1 Power-Up and Communication Start
The sensor starts powering-up after reaching the power-up threshold voltage V
DD,Min
= 2.25 V. After reaching this threshold
voltage, the sensor needs 1000 ms to enter the idle state. Once the idle state is entered it is ready to receive commands from
the master.
Each transmission sequence begins with a START condition (S) and ends with a STOP condition (P) as described in the I
2
C-
bus specification.
3.2 Data type & length
Data sent to and received from the sensor consists of a sequence of 16-bit commands and/or 16-bit words (each to be interpreted
as unsigned integer, most significant byte transmitted first). Each data word is immediately succeeded by an 8-bit CRC. In write
direction it is mandatory to transmit the checksum. In read direction it is up to the master to decide if it wants to process the
checksum (see chapter 3.11).
3.3 Command Sequence Types
The SCD4x features four different I
2
C command sequence types: “ read I
2
C sequences ” , “ write I
2
C sequences ” ,“ send I
2
C
command ” and “ send command and fetch result ” sequences. Figure 2 illustrates how the I
2
C communication for the different
sequence types is built-up.
Figure 2: Command Sequence types: “ write ” sequence, “ send command ” sequence, “ read ”
sequence, and “ send command and fetch result ” sequence.
For “ read ” ” or “send command and fetch results” sequences, after writing the address and/or data to the sensor and sending the
ACK bit, the sensor needs the execution time (see Table 9 ) to respond to the I
2
C read header with an ACK bit. Hence, it is
required to wait the command execution time before issuing the read header. Commands must not be sent while a previous
command is being processed.