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1-100707-01 Datasheet(PDF) 7 Page - Sensitron

Part No. 1-100707-01
Description  Humidity and Temperature Sensor
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Maker  SENSITRON [Sensitron]
Homepage  http://www.sensitron.com
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Datasheet SHT21
www.sensirion.com
Version 1.0 – January 2010
7/12
commands from the master (MCU). Current consumption
during start up is 350µA maximum.
5.2
Start / Stop Sequence
Each transmission sequence begins with Start condition
(S) and ends with Stop condition (P) as displayed in Figure
11 and Figure 12.
Figure 11 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 12 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. 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
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
I2C
communication tasks on a bus while the sensor is
measuring. A communication sequence of the two modes
is displayed in Figure 13 and Figure 14, respectively.
In the hold master mode, the SHT2x pulls down the SCL
line while measuring to force the master into a wait state.
By releasing the SCL line the sensor indicates that internal
processing is terminated and that transmission may be
continued.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
S 1 0 0 0 0 0 0 0
1 1 1 0 0 1 0 1
I2C address + write
Command (see Table 6)
19
20
21
22
23
24
25
26
27
S 1 0 0 0 0 0 0 1
Measurement
I2C address + read
Hold during measurement
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
0 1 1 0 0 0 1 1
0 1 0 1 0 0 1 0
Data (MSB)
Data (LSB) Stat.
46
47
48
49
50
51
52
53
54
0 1 1 0 0 0 1 1
P
Checksum
Figure 13 Hold master communication sequence – grey blocks
are controlled by SHT2x. Bit 45 may be changed to NACK
followed by Stop condition (P) to omit checksum transmission.
In no hold master mode, the MCU has to poll for the
termination of the internal processing of the sensor. This is
done by sending a Start condition followed by the I2C
header (1000’0001) as shown in Figure 14. If the internal
processing is finished, the sensor acknowledges the poll of
the MCU and data can be read by the MCU. If the
measurement processing is not finished the sensor
answers no ACK bit and the Start condition must be
issued once more.
For both modes, since the maximum resolution of a
measurement is 14 bit, the two last LSBs (bits 43 and 44)
are used for transmitting status information. Bit 1 of the
two
LSBs
indicates
the
measurement
type
(‘0’:
temperature, ‘1’ humidity). Bit 0 is currently not assigned.
SDA
SCL
70%
30%
70%
30%
SDA
SCL
70%
30%
70%
30%




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