www.sensirion.com Version 4 – August 2015 7/14
5.3 Starting a Measurement
A measurement communication sequence consists of a
START condition followed by the I
2
C header with the 7-bit
I
2
C device address and a write bit (write W: ‘0’). The sensor
indicates the proper reception of a byte by pulling the SDA
pin low (ACK bit) after the falling edge of the 8th SCL clock.
Then the sensor is ready to receive a 16-bit measurement
command. Again, the SHTC1 acknowledges the proper
reception of each byte with ACK condition. A complete
measurement cycle is presented in Figure 7.
With the acknowledgement of the measurement command,
the SHTC1 starts measuring humidity and temperature.
5.4 Sensor Behavior during Measurement and
Clock Stretching
In general, the sensor does not respond to any I
2
C activity
during measurement, i.e. I
2
C read and write headers are not
acknowledged (NACK). However, when clock stretching
has been enabled by using a corresponding measurement
command, the sensor responds to a read header with an
ACK and subsequently pulls down the SCL line until the
measurement is complete. As soon as the measurement is
complete, the sensor starts sending the measurement
results.
During measurement, the sensor has a current
consumption according to Table 3.
For best possible repeatability of humidity and temperature
measurements, it is recommended to avoid any
communication on the I2C bus while the SHTC1 is
measuring. For more information, see application note
“SHTC1 Optimization of Repeatibility”.
5.5 Readout of Measurement Results
After a measurement command has been issued and the
sensor has completed the measurement, the master can
read the measurement results by sending a START
condition followed by an I
2
C read header. The sensor will
acknowledge the reception of the read header and send two
bytes of data followed by one byte CRC checksum and
another two bytes of data followed by one byte CRC
checksum. Each byte must be acknowledged by the
microcontroller with an ACK condition for the sensor to
continue sending data. If the SHTC1 does not receive an
ACK from the master after any byte of data, it will not
continue sending data.
Whether the sensor sends out humidity or temperature data
first depends on the measurement command that was sent
to the sensor to initiate the measurement (see Table 9).
The I
2
C master can abort the read transfer with a NACK
condition after any data byte if it is not interested in
subsequent data, e.g. the CRC byte or the second
measurement result, in order to save time.
In case the user needs humidity and temperature data but
does not want to process CRC data, it is recommended to
read the first two bytes of data with the CRC byte (without
processing the CRC data) and abort the read transfer after
reading the second two data bytes with a NACK. This
procedure is more time efficient than starting two different
measurements and aborting the read transfer after the first
two data bytes each time.
5.6 Soft Reset
The SHTC1 provides a soft reset mechanism that forces the
system into a well-defined state without removing the power
supply. If the system is in idle state (i.e. if no measurement
is in progress) the soft reset command can be sent to
SHTC1 according to Figure 8. This triggers the sensor to
reset all internal state machines and reload calibration data
from the memory.
Table 10 Soft reset command.
5.7 Read-out of ID Register
The SHTC1 has an ID register which contains an SHTC1-
specific product code. The read-out of the ID register can
be used to verify the presence of the sensor and proper
communication. The command to read the ID register is
shown in Table 11.
Table 11 Read-out command of ID register.
It needs to be sent to the SHTC1 after an I
2
C write header.
After the SHTC1 has acknowledged the proper reception of
the command, the master can send an I
2
C read header and
the SHTC1 will submit the 16-bit ID followed by 8 bits of
CRC. The structure of the ID is described in Table 12.
bits 5 to 0: SHTC1-specific product code
bits 15 to 6: unspecified information
Table 12 Structure of the 16-bit ID. Bits 15:6 of the ID contain
unspecified information (marked as “x”), which may vary from
sensor to sensor, while bits 5:0 contain the SHTC1-specific
product code.
5.8 Checksum Calculation
The 8-bit CRC checksum transmitted after each data word
is generated by a CRC algorithm with the properties
displayed in Table 13. The CRC covers the contents of the
two previously transmitted data bytes.