Datasheet SHT21

www.sensirion.com / D1

Version 5 – October 2022

7/14

Figure 12

Timing Diagram for Digital Input/Output Pads,

abbreviations are explained in Table 5. SDA directions are seen

from the sensor. Bold SDA line is controlled by the sensor, plain

SDA line is controlled by the micro-controller. Note that SDA

valid read time is triggered by falling edge of anterior toggle.

Parameter

min

typ

max Units

SCL frequency, fSCL

0

-

0.4

MHz

SCL High Time, tSCLH

0.6

-

-

µs

SCL Low Time, tSCLL

1.3

-

-

µs

SDA Set-Up Time, tSU

100

-

-

ns

SDA Hold Time, tHD

0

-

900

ns

SDA Valid Time, tVD

0

-

400

ns

SCL/SDA Fall Time, tF

0

-

100

ns

SCL/SDA Rise Time, tR

0

-

300

ns

Capacitive Load on Bus Line, CB

0

-

400

pF

Table 5 Timing specifications of digital input/output pads for I2C

fast mode. Entities are displayed in Figure 12. VDD = 2.1 V to

3.6 V, T = -40°C to 125°C, unless otherwise noted.

5 Communication with Sensor

SHT21 communicates with I2C protocol. For information on

I2C beyond the information in the following Sections

please look for the NXP I2C-bus specifications and user

manual.

Please note that all sensors are set to the same I2C

address, as defined in Section 5.3.

Furthermore, please note, that Sensirion provides an

exemplary sample code on its home page – compare

www.sensirion.com/products/catalog/SHT20/.

Please note that in case VDD is set to 0 V (GND), e.g. in

case of a power off of the SHT2x, the SCL and SDA pads

are also pulled to GND. Consequently, the I2C bus is

blocked while VDD of the SHT2x is set to 0 V.

5.1

Start Up Sensor

As a first step, the sensor is powered up to the chosen

supply voltage VDD (between 2.1 V and 3.6 V). After

power-up, the sensor needs at most 15 ms, while SCL is

high, for reaching idle state, i.e. to be ready accepting

commands from the master (MCU). Current consumption

during start up is 350 µA maximum. Whenever the sensor

is powered up, but not performing a measurement or

communicating, it is automatically in idle state (sleep

mode).

5.2

Start / Stop Sequence

Each transmission sequence begins with Start condition

(S) and ends with Stop condition (P) as displayed in Figure

13 and Figure 14.

Figure 13 Transmission Start condition (S) - a high to low

transition on the SDA line while SCL is high. The Start condition

is a unique state on the bus created by the master, indicating to

the slaves the beginning of a transmission sequence (bus is

considered busy after a Start).

Figure 14 Transmission Stop condition (P) - a low to high

transition on the SDA line while SCL is high. The Stop condition

is a unique state on the bus created by the master, indicating to

the slaves the end of a transmission sequence (bus is

considered free after a Stop).

5.3

Sending a Command

After sending the Start condition, the subsequent I2C

header consists of the 7-bit I2C device address ‘1000’000’

and an SDA direction bit (Read R: ‘1’, Write W: ‘0’). The

sensor indicates the proper reception of a byte by pulling

the SDA pin low (ACK bit) after the falling edge of the 8th

SCL clock. After the issue of a measurement command

(‘1110’0011’ for temperature, ‘1110’0101’ for relative

humidity’), the MCU must wait for the measurement to

complete. The basic commands are summarized in Table

6.

SCL

70%

30%

tSCLL

1/fSCL

tSCLH

tR

tF

SDA

70%

30%

tSU

tHD

SDA valid read

DATA IN

tR

SDA

70%

30%

DATA OUT

tVD

tF

SDA valid write

SDA

SCL

70%

30%

70%

30%

SDA

SCL

70%

30%

70%

30%