SHT1x / SHT7x Relative Humidity & Temperature Sensor System

www.sensirion.com

v2.02 July 2004

3/9

2.2.3 Sending a command

To initiate a transmission, a “Transmission Start” sequence

has to be issued. It consists of a lowering of the DATA line

while SCK is high, followed by a low pulse on SCK and

raising DATA again while SCK is still high.

DATA

SCK

Figure 3 "Transmission Start" sequence

The subsequent command consists of three address bits

(only “000” is currently supported) and five command bits.

The SHTxx indicates the proper reception of a command by

pulling the DATA pin low (ACK bit) after the falling edge of

the 8th SCK clock. The DATA line is released (and goes

high) after the falling edge of the 9th SCK clock.

Table 2

SHTxx list of commands

2.2.4 Measurement sequence (RH and T)

After issuing a measurement command (‘00000101’ for RH,

‘00000011’ for Temperature) the controller has to wait for the

measurement to complete. This takes approximately

11/55/210 ms for a 8/12/14bit measurement. The exact time

varies by up to

±15% with the speed of the internal oscillator.

To signal the completion of a measurement, the SHTxx pulls

down the data line 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 as convenient.

Two bytes of measurement data and one byte of CRC

checksum will then be transmitted. The uC must

acknowledge each byte by pulling the DATA line low. All

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 the

measurement and communication have ended.

Warning: To keep self heating below 0.1 °C the SHTxx

should not be active for more than 10% of the time

(e.g. max. 2 measurements / second for 12bit accuracy).

2.2.5 Connection reset sequence

If communication with the device is lost the following signal

sequence will reset its serial interface:

While leaving DATA high, toggle SCK 9 or more times. 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.

DATA

SCK

Transmission Start

12

3

4 -8

9

Figure 4 Connection reset sequence

2.2.6 CRC-8 Checksum calculation

The whole digital transmission is secured by a 8 bit

checksum. It ensures that any wrong data can be detected

and eliminated.

Please consult application note “CRC-8 Checksum

Calculation” for information on how to calculate the CRC.

DATA

SCK

a2

a1

C0

a0

C4

C3

C2

C1

ack

DATA

SCK

low

low

8

low

low

11

10

9

ack

7

6

0

5

4

3

2

1

ack

MSB

LSB

60

5

4

3

2

1

ack

LSB

DATA

SCK

7

MSB

Transmission Start

Address=’000’

Command=’00101’

Measurement

~55ms for 12 bit

~11ms for 8 bit

Bold = SHT1xx controls DATA line

Plain = uC controls DATA line

Measurement is finished when the

SHTxx pulls down the DATA line

Skip acknowledge to end transmission

(if no CRC is used)

12 bit humidity data

CRC-8 Checksum

Skip acknowledge to

end transmission

wait for next measurement

Transmission Start

Command

Code

Reserved

0000x

Measure Temperature

00011

Measure 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