Datasheet STS21

www.sensirion.com

Version 2 – December 2011

6/12

Parameter

min

typ

max Units

0

-

0.4

MHz

0.6

-

-

µs

1.3

-

-

µs

100

-

-

ns

0

-

900

ns

0

-

400

ns

0

-

100

ns

0

-

300

ns

0

-

400

pF

fast mode. Entities are displayed in Figure 9. VDD = 2.1V to

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

information

regarding

timing,

please

refer

to

http://www.standardics.nxp.com/support/i2c/.

5 Communication with Sensor

please refer to the following website:

http://www.standardics.nxp.com/support/i2c/.

Furthermore, please note, that Sensirion provides an

exemplary sample code for SHT21 sensors on its home

page – compare www.sensirion.com/SHT21. This sample

code can be used for STS21 sensors with minor

5.1

Start Up Sensor

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

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

power-up, the sensor needs at most 15ms, 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 350µA maximum. Whenever the sensor

is powered up, but not performing a measurement or

communicating, it is automatically in sleep mode (idle

state).

5.2

Start / Stop Sequence

Each transmission sequence begins with Start condition

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

10 and Figure 11.

info@sensirion.com.

Figure 10 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 11 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

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

sensor indicates the proper reception of a byte by pulling

SCL clock. After the issue of a measurement command,

the MCU must wait for the measurement to complete. The

basic commands are summarized in Table 6. Hold master

or no hold master modes are explained in the next

Section.

Command

Comment

Code

Trigger T measurement

hold master

1110‟0011

Trigger T measurement

no hold master 1111‟0011

Write user register

1110‟0110

Read user register

1110‟0111

Soft reset

1111‟1110

Table 6 Basic command set, T stands for temperature

5.4

Hold / No Hold Master Mode

There are two different operation modes to communicate

with the sensor: Hold Master mode or No Hold Master

mode. In the first case the SCL line is blocked (controlled

by sensor) during measurement process while in the latter

case the SCL line remains open for other communication

while the sensor is processing the measurement. No hold

communication tasks on a bus while the sensor is

measuring. A communication sequence of the two modes

is displayed in Figure 12 and Figure 13, respectively.

SDA

SCL

70%

30%

70%

30%

SDA

SCL

70%

30%

70%

30%