Datasheet

Datasheet STS21
www.sensirion.com Version 2 – December 2011 6/12
Parameter
min
typ
max
Units
SCL frequency, f
SCL
0
-
0.4
MHz
SCL High Time, t
SCLH
0.6
-
-
µs
SCL Low Time, t
SCLL
1.3
-
-
µs
SDA Set-Up Time, t
SU
100
-
-
ns
SDA Hold Time, t
HD
0
-
900
ns
SDA Valid Time, t
VD
0
-
400
ns
SCL/SDA Fall Time, t
F
0
-
100
ns
SCL/SDA Rise Time, t
R
0
-
300
ns
Capacitive Load on Bus Line, C
B
0
-
400
pF
Table 5 Timing specifications of digital input/output pads for I
2
C
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
STS21 communicates with I
2
C protocol. For information on
I
2
C beyond the information in the following Sections
please refer to the following website:
http://www.standardics.nxp.com/support/i2c/.
Please note that all sensors are set to the same I
2
C
address, as defined in Section 5.3.
8
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
adjustments (I
2
C address, no RH measurement).
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.
8
For sensors with alternative I
2
C address please contact Sensirion via
[e-mail].
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
After sending the Start condition, the subsequent I
2
C
header consists of the 7-bit I
2
C device address „1001‟010‟
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 8
th
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
master mode allows for processing other I
2
C
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%