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Reset through the nReset Pin

Pulling the nReset pin low generates a reset similar to a hard reset. The nReset pin is internally

connected to VDD through a pull-up resistor and hence active low. The nReset pin has to be pulled

low for a minimum of 1 µs to generate a reset of the sensor.

Hard Reset

A hard reset is achieved by switching the supply voltage to the VDD Pin off and then on again. In

order to prevent powering the sensor over the ESD diodes, the voltage to pins 1 (SDA), 4 (SCL)

and 2 (ADDR) also needs to be removed.

4.10 Heater

The SHT33 is equipped with an internal heater, which is meant for plausibility checking only (see

commands in Table 17). The temperature increase achieved by the heater depends on various

parameters and lies in the range of a few degrees centigrade. It can be switched on and off by

command, see table below. The status is listed in the status register. After a reset the heater is

disabled (default condition).

Command

Hex Code

MSB

LSB

Heater Enable

0x30

6D

Heater Disabled

66

Table 17. Heater command (Clear blocks are controlled by the microcontroller, grey blocks by the sensor).

4.11 Serial Number

The Serial number allows for an unambiguous identification of each individual device, it is stored

in the one-time programmable (OTP) memory of the chip. Table 18 shows the command structure

to read the serial number. After issuing the measurement command and sending the ACK bit the

sensor needs the time tIDLE = 1ms to respond to the I2C read header with an ACK bit. Hence it is

recommended to wait tIDLE = 1ms before issuing the read header. The Get Serial Number command

returns two words, every word is followed by a CRC Checksum. Together the two words (SNB_3

to SNB_0 Table 18, SNB_0 is the LSB, whereas SNB_3 is the MSB) constitute a unique serial

number with a length of 32 bit. This serial number can be used to individually identify each sensor.

Command

Hex Code

Get Serial Number

0x 37 80

0x 36 82

Table 18. Command to read out the serial number (Clear block are controlled by microcontroller, grey blocks

by the sensor)

t

IDLE

9

36

SNB_3

SNB_2

CRC

Serial Number Word 1

Checksum

37

63

SNB_1

SNB_0

CRC

Serial Number Word 2

Checksum

P

SNB_3

I2C Address

Command MSB

Command LSB

16-bit command

16-bit command

I2C write header

I2C read header

I2C Address

S

W

S

R

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23

24 25 26 27

1

2

3

4

5

6

7

8

9