www.sensirion.com Version 1.9 – July 2015 5/10
3.4 Data transfer format
Data is transferred in byte packets in the I
2
C protocol,
which means in 8-bit frames. Each byte is followed by an
acknowledge bit. Data is transferred with the most
significant bit (MSB) first.
A data transfer sequence is initiated by the master
generating the Start condition (S) and sending a header
byte. The I
2
C header consists of the 7-bit I
2
C device
address and the data direction bit (R/_W).
The value of the R/_W bit in the header determines the
data direction for the rest of the data transfer sequence. If
R/_W = 0 (WRITE) the direction remains master-to-slave,
while if R/_W = 1 (READ) the direction changes to slave-
to-master after the header byte.
4. Command Set and Data Transfer
Sequences
A command is represented by an 8-bit command code.
The data direction may not change after the command
byte, since the R/_W bit of the preceding I
2
C header has
already determined the direction to be master-to-slave. In
order to execute commands in Read mode using I
2
C, the
following principle is used. On successful (acknowledged)
receipt of a command byte, the sensor stores the
command nibble internally. The Read mode of this
command is then invoked by initiating an I
2
C data transfer
sequence with R/_W = 1.
If a correctly addressed sensor recognizes a valid
command and access to this command is granted, it
responds by pulling down the SDA line during the
subsequent SCL pulse for the acknowledge signal (ACK).
Otherwise it leaves the SDA line unasserted (NACK).
The two most important commands are described in this
data sheet, and the data transfer sequences are specified.
Contact Sensirion for advanced sensor options.
4.1 Measurement triggering
Each individual measurement is triggered by a separate
read operation.
Note that two transfer sequences are needed to perform a
measurement. First write command byte hF1 (trigger
measurement) to the sensor, and then execute a read
operation to trigger the measurement and retrieve the flow
or differential pressure information.
On receipt of a header with R/_W=1, the sensor generates
the Hold Master condition on the bus until the first
measurement is completed. After the Hold Master
condition is released, the master can read the result as
two consecutive bytes. A CRC byte follows if the master
continues clocking the SCL line after the second result
byte. The sensor checks whether the master sends an
acknowledge after each byte and aborts the transmission
if it does not.
8-bit command code: hF1
Command: Trigger differential pressure measurement
1312 18 19 2221 27
MSByte MeasData LSByte MeasData
11 14 15 16 17 20 23 24 25 26
ACK
ACK
28 3130 36
Check Byte
29 32 33 34 35
1 2 3 4 5 6 7 8
ACK
9 18
S
I2CAdr
ACK
S
ACK
W
1 2 3 4 5 6 7 8 9
10
Hold Master
P
ACK
10 11 12 13 14 15 16 17
1 1 1 1 0 0 0 1
Command
1 0 0 0 0 0 0
R
1 0 0 0 0 0 0
I2CAdr
0 1
Hatched areas indicate that the sensor controls the SDA line.
Note that the first measurement result after reset is not
valid.
4.2 Soft reset
This command forces a sensor reset without switching the
power off and on again. On receipt of this command, the
sensor reinitializes the control/status register contents
from the EEPROM and starts operating according to these
settings.
8-bit command code: hFE
Command: Soft reset
1 2 3 4 5 6 7 8
S
9
ACK
ACK
18
system reboot
10 11 12 13 14 15 16 17
I2CAdr
W
1 0 0 0 0 0 0 1 1 1 1
Command
1 1 1 00
4.3 CRC-8 Redundant Data Transmission
Cyclic redundancy checking (CRC) is a popular technique
used for error detection in data transmission. The
transmitter appends an n-bit checksum to the actual data
sequence. The checksum holds redundant information
about the data sequence and allows the receiver to detect
transmission errors. The computed checksum can be
regarded as the remainder of a polynomial division, where
the dividend is the binary polynomial defined by the data
sequence and the divisor is a “generator polynomial”.
The sensor implements the CRC-8 standard based on the
generator polynomial
x
8
+ x
5
+ x
4
+1.
Note that CRC protection is only used for date transmitted
from the slave to the master.
For details regarding cyclic redundancy checking, please
refer to the relevant literature.
