Datasheet SHT21

www.sensirion.com / D1

Version 5 – October 2022

10/14

5.8

I2C Communication Recommendation

Uncontrolled operation in the No-Hold Master Mode (see

section 5.4) can very rarely result in false offsets of the

temperature or relative humidity measurement values14.

These offsets can occur, if a rising edge on the SCL falls

within 15 µs to 18.5 µs after the sensor has sent an

ACK bit, which confirms the correct reception of a

measurement command. In this time window the sensor’s

calibration data stored in the OTP memory, might be

corrupted during transmission to the working memory.

Once the offsets manifest in the measurement data, it will

remain until the next soft or hard reset. These offsets can

be completely avoided using one of the following four

workarounds:

1.

Recommended: Avoiding of rising edges in clock line

(SCL) during critical time window. To this end the stop

condition from the master should be sent >18.5 µs

after the ACK bit is send from the slave. This prevents

the offset to occur under all circumstances.

Figure 19 Stop condition after critical time window

2.

Sending the stop condition before the critical time

window, i.e. <15 µs and other communications

after 18.5 µs.

Figure 20 Stop condition before critical time window

3.

Operating in the Hold-Master mode only (see section

5.4).

4.

Performing an OTP reload each time a measurement

command is issued. This entails copying the content

of the OTP into the working memory, which results in

an increased measurement time of around 2.5 ms.

This can be achieved by changing the relevant bit in

the user register. As part of the OTP reload, all values

in the user register are reinitialized to their default

values (except the heater bit). Therefore, the bit

(BIT 1) in the user register that controls the OTP must

be changed prior to every measurement (see section

5.6).

14 If the sensor is operated using Sensibus commands the offset will not occur

5.9

Serial Number

SHT21 provides an electronic identification code. For

instructions on how to read the identification code please

refer to the Application Note “Electronic Identification

Code” – to be downloaded from the web page

www.sensirion.com/products/catalog/SHT21/.

6 Conversion of Signal Output

Default resolution is set to 12 bit relative humidity and 14

bit temperature reading. Measured data are transferred in

two byte packages, i.e. in frames of 8 bit length where the

most significant bit (MSB) is transferred first (left aligned).

Each byte is followed by an acknowledge bit. The two

status bits, the last bits of LSB, must be set to ‘0’ before

calculating physical values. In the example of Figure 15

and Figure 16, the transferred 16 bit relative humidity data

is ‘0110’0011’0101’0000’ = 25424.

6.1

Relative Humidity Conversion

With the relative humidity signal output SRH the relative

humidity RH is obtained by the following formula (result in

%RH), no matter which resolution is chosen:

16

RH

2

S

125

6

RH



+

−

=

In the example given in Figure 15 and Figure 16 the

relative humidity results to be 42.5 %RH.

The physical value RH given above corresponds to the

relative humidity above liquid water according to World

Meteorological Organization (WMO). For relative humidity

above ice RHi the values need to be transformed from

relative humidity above water RHw at temperature t. The

equation is given in the following, compare also

Application Note “Introduction to Humidity”:













+















+





=

t

λ

t

β

t

λ

t

β

RH

RH

i

i

w

w

w

i

exp

exp

Units are %RH for relative humidity and °C for

temperature. The corresponding coefficients are defined

as follows: βw = 17.62, λw = 243.12°C, βi = 22.46, λi =

272.62°C.

6.2

Temperature Conversion

The temperature T is calculated by inserting temperature

signal output ST into the following formula (result in °C), no

matter which resolution is chosen:

16

T

2

S

175.72

46.85

T



+

−

=