1-100071-04 View Datasheet(PDF) - Unspecified
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1-100071-04 Datasheet PDF : 11 Pages
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Datasheet SHT7x
Command
Reserved
Measure Temperature
Measure Relative Humidity
Read Status Register
Write Status Register
Reserved
Soft reset, resets the interface, clears the
status register to default values. Wait minimum
11 ms before next command
Code
0000x
00011
00101
00111
00110
0101x-1110x
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 5th 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.
90%
SCK 1 2 3 4 - 8 9
2
10%
Transmission Start
90%
DATA
10%
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-8 Checksum
Calculation” 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 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. A more detailed description is
available in the Application Note “Status Register”.
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 Figures 9 and 10 – the
assignation of the bits is displayed in Table 5.
00000110
Status Register
Figure 9: Status Register Write
00000111
Status Register
Checksum
Figure 10: Status Register Read
Examples of full communication cycle are displayed in
Figures 11 and 12.
000
Command
Wait for
DATA ready
0
0
MSB
LSB
Checksum
Figure 11: Overview of Measurement Sequence. TS = Trans-
mission Start, MSB = Most Significant Byte, LSB = Last
Significant Byte, LSb = Last Significant Bit.
www.sensirion.com
Version 4.1 – July 2008
6/11
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