Datasheet SHT25

www.sensirion.com

Version 0.91 – October 2010

7/12

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

0

-

0.4

MHz

0.6

-

-

s

1.3

-

-

s

100

-

-

ns

0

-

900

ns

0

-

400

ns

0

-

100

ns

0

-

300

ns

0

-

400

pF

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

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

5 Communication with Sensor

Sections please refer to the following website:

Furthermore, please note, that Sensirion provides an

exemplary sample code on its home page – compare

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 350 A maximum.

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

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

(‘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. Hold master or no hold master modes are explained in

next Section.

Command

Comment

Code

Trigger T measurement

hold master

1110’0011

Trigger RH measurement

hold master

1110’0101

Trigger T measurement

no hold master 1111’0011

Trigger RH measurement

no hold master 1111’0101

Write user register

1110’0110

Read user register

1110’0111

Soft reset

1111’1110

Table 6 Basic command set, RH stands for relative humidity,

and T stands for temperature

SCL

70%

30%

SDA

70%

30%

SDA valid read

DATA IN

SDA

70%

30%

DATA OUT

SDA valid write

SDA

SCL

70%

30%

70%

30%

SDA

SCL

70%

30%

70%

30%