SHT71 SENSIRION, SHT71 Datasheet - Page 6

• Manufacturer Part Number: SHT71
• Description: Humidity Sensor
• Manufacturer: SENSIRION

Specifications of SHT71

• Sensing Accuracy: 3%
• Sensor Output: Digital
• Sensor Terminals: Through Hole
• Peak Reflow Compatible (260 C): Yes
• Terminal Type: PCB Surface Mount
• Humidity Range: 0 To 100% RH
• Response Time: 8s
• Humidity Accuracy ±: 3%
• Rohs Compliant: Yes
• Leaded Process Compatible: Yes
• Package / Case: 4-SIP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
Table 4: SHT7x list of commands
3.3
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
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
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.
www.sensirion.com
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Measurement of RH and T
Connection reset sequence
SCK is MSB for a 12bit value, for a 8bit result the
Code
0000x
00011
00101
00111
00110
0101x-1110x
11110
Version 4.3 – May 2010
Figure 8: Connection Reset Sequence
3.5
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
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.
Figure 9: Status Register Write
Figure 10: Status Register Read
Examples of full communication cycle are displayed in
Figure 11 and Figure 12.
Figure 11: Overview of Measurement Sequence. TS = Trans-
mission Start, MSB = Most Significant Byte, LSB = Last
Significant Byte, LSb = Last Significant Bit.
SCK
DATA
0 0 0 0 0 1 1 0
0 0 0 0 0 1 1 1
0 0 0 Command
CRC-8 Checksum calculation
Status Register
1
LSB
2
DATA ready
3
2
Wait for
Status Register
Status Register
4 - 8
0 0
Checksum
9
MSB
Transmission Start
Checksum
6/11
20%
80%
80%
20%