Datasheet SHT1x

www.sensirion.com

Version 5 – December 2011

7/12

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, SHT1x

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

13. 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 13: Connection Reset Sequence

3.5

CRC 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 SHT1x 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 14 and Figure 15 – the

assignation of the bits is displayed in Table 5.

0 0 0 0 0 1 1 0

Status Register

Figure 14: Status Register Write

Figure 15: Status Register Read

Examples of full communication cycle are displayed in

Figure 16 and Figure 17.

Wait for

DATA ready

0 0 0 Command

0 0

MSB

LSB

Checksum

Figure 16: Overview of Measurement Sequence. TS = Trans-

mission Start, MSB = Most Significant Byte, LSB = Last

Significant Byte, LSb = Last Significant Bit.

Figure 17: Example RH measurement sequence for value “0000’0100“0011’0001” = 1073 = 35.50%RH (without temperature

compensation). DATA valid times are given and referenced in boxes on DATA line. Bold DATA lines are controlled by sensor while plain

lines are controlled by the micro-controller.

0 0 0 0 0 1 1 1

Status Register

Checksum

DATA

SCK

80%

20%

80%

20%

1

2

3

2

4 - 8

9

Transmission Start

DATA

DATA

SCK

SCK

SCK

DATA

A2

A1

A0

C4

C3

C2

C1

C0

ACK

15

14

13

12

11

10

9

8

ACK

7

6

5

4

3

2

1

0

ACK

7

6

5

4

3

2

1

0

ACK

Transmission Start

Address = ‘000’

Command = ‘00101’

12bit Humidity Data

CRC-8 Checksum

Transmission Start

Measurement

(80ms for 12bit)

Sleep (wait for next

measurement)

Idle Bits

Skip ACK to end transmission

(if no CRC is used)

Sensor pulls DATA line low after

completion of measurement

MSb

LSb

MSb

LSb

15

14

13

12

11

10

9

8

ACK

7

6

5

4

3

2

1

0

A2

A1

A0

C4

C3

C2

C1

C0

ACK

7

6

5

4

3

2

1

0

ACK

ACK