Datasheet

Datasheet SHT7x
www.sensirion.com Version 4.3 – May 2010 6/11
Command Code
Reserved 0000x
Measure Temperature 00011
Measure Relative Humidity 00101
Read Status Register 00111
Write Status Register 00110
Reserved 0101x-1110x
Soft reset, resets the interface, clears the
status register to default values. Wait minimum
11 ms before next command
11110
Table 4: SHT7x list of commands
3.3 Measurement of RH and T
After issuing a measurement command (‘00000101’ for
relative humidity, ‘00000011’ for temperature) the
controller has to wait for the measurement to complete.
This takes a maximum of 20/80/320 ms for a 8/12/14bit
measurement. The time varies with the speed of the
internal oscillator and can be lower by up to 30%. To
signal the completion of a measurement, the SHT7x pulls
data line low and enters Idle Mode. The controller must
wait for this Data Ready signal before restarting SCK to
readout the data. Measurement data is stored until
readout, therefore the controller can continue with other
tasks and readout at its convenience.
Two bytes of measurement data and one byte of CRC
checksum (optional) will then be transmitted. The micro
controller must acknowledge each byte by pulling the
DATA line low. All values are MSB first, right justified (e.g.
the 5
th
SCK is MSB for a 12bit value, for a 8bit result the
first byte is not used).
Communication terminates after the acknowledge bit of
the CRC data. If CRC-8 checksum is not used the
controller may terminate the communication after the
measurement data LSB by keeping ACK high. The device
automatically returns to Sleep Mode after measurement
and communication are completed.
Important: To keep self heating below 0.1°C, SHT7x
should not be active for more than 10% of the time – e.g.
maximum one measurement per second at 12bit accuracy
shall be made.
3.4 Connection reset sequence
If communication with the device is lost the following signal
sequence will reset the serial interface: While leaving
DATA high, toggle SCK nine or more times – see Figure 8.
This must be followed by a Transmission Start sequence
preceding the next command. This sequence resets the
interface only. The status register preserves its content.
Figure 8: Connection Reset Sequence
3.5 CRC-8 Checksum calculation
The whole digital transmission is secured by an 8bit
checksum. It ensures that any wrong data can be detected
and eliminated. As described above this is an additional
feature of which may be used or abandoned. Please
consult Application Note “CRC Checksum” for information
on how to calculate the CRC.
3.6 Status Register
Some of the advanced functions of the SHT7x such as
selecting measurement resolution, end-of-battery notice,
use of OTP reload or using the heater may be activated by
sending a command to the status register. The following
section gives a brief overview of these features.
After the command Status Register Read or Status
Register Write – see Table 4 – the content of 8 bits of the
status register may be read out or written. For the
communication compare Figure 9 and Figure 10 – the
assignation of the bits is displayed in Table 5.
0 0 0 0 0 1 1 0 Status Register
TS
ACK Bit 7 ACK
Figure 9: Status Register Write
Figure 10: Status Register Read
Examples of full communication cycle are displayed in
Figure 11 and Figure 12.
0 0 0 Command 0 0 MSB
TS
Wait for
DATA ready
ACK
LSB Checksum
LSb
ACK ACK
Figure 11: Overview of Measurement Sequence. TS = Trans-
mission Start, MSB = Most Significant Byte, LSB = Last
Significant Byte, LSb = Last Significant Bit.
0 0 0 0 0 1 1 1 Status Register Checksum
TS
ACK Bit 7 ACK Bit 7 ACK
DATA
SCK
8
0%
2
0%
8
0%
2
0%
1 2
3
2
4 - 8
9
Transmission Start