28018 Parallax Inc, 28018 Datasheet - Page 3

• Manufacturer Part Number: 28018
• Description: MODULE SENSOR TEMP+HUMIDITY
• Manufacturer: Parallax Inc
• Series: Sensirion SHT11r

Specifications of 28018

• Output Type: Digital - 2-Wire
• Sensor Type: Humidity and Temperature
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

SHT1x / SHT7x Relative Humidity & Temperature Sensor System
2.2.3
To initiate a transmission, a “Transmission Start” sequence
has to be issued. It consists of a lowering of the DATA line
while SCK is high, followed by a low pulse on SCK and
raising DATA again while SCK is still high.
Figure 3
The subsequent command consists of three address bits
(only “000” is currently supported) and five command bits.
The SHT1x/SHT7x indicates the proper reception of a
command by pulling the DATA pin low (ACK bit) after the
falling edge of the 8th SCK clock. The DATA line is released
(and goes high) after the falling edge of the 9th SCK clock.
Table 2
2.2.4
After issuing a measurement command (‘00000101’ for RH,
‘00000011’ for Temperature) the controller has to wait for the
measurement to complete. This takes approximately
11/55/210ms for a 8/12/14bit measurement. The exact time
varies by up to ±15% with the speed of the internal oscillator.
To signal the completion of a measurement, the SHT1x pulls
down the data line. The controller must wait for this “data
ready” signal before starting to toggle SCK again.
DATA
SCK
DATA
SCK
DATA
SCK
Figure 5
www.sensirion.com
DATA
SCK
Command
Reserved
Measure Temperature
Measure Humidity
Read Status Register
Write Status Register
Reserved
Soft reset, resets the interface, clears the
status register to default values
wait minimum 11ms before next command
Measurement is finished when the
SHTxx pulls down the DATA line
Transmission Start
Sending a command
Measurement sequence (RH and T)
"Transmission Start" sequence
SHTxx list of commands
Example RH measurement sequence for value “0000’1001 ’ 0011’0001”= 2353 = 75.79%RH @ 25°C
MSB
7
low
CRC-8 Checksum
6
low
5
low
4
low
a2
Address='000'
3
11
MSB
a1
2
10
a0
Code
0000x
00011
00101
00111
00110
0101x-1110x
11110
1
9
C4
LSB
0
Command='00101'
8
C3
v2.0 March 2003
ack
ack
C2
wait for next measurement
Skip acknowledge to
end transmission
7
C1
SCK
Two bytes of measurement data and one byte of CRC
checksum will then be transmitted. The uC must
acknowledge each byte by pulling the DATA line low. All
values are MSB first, right justified. (e.g. the 5
for a 12bit value, for a 8bit result the first byte is not used).
Communication terminates after the acknowledge bit of the
CRC data. If CRC-8 checksum is not used the controller may
terminate the communication after the measurement data
LSB by keeping ack high.
The device automatically returns to sleep mode after the
measurement and communication have ended.
Warning: To keep self heating below 0.1°C the SHTxx
should not be active for more than 15% of the time (e.g. max.
3 measurements / second for 12bit accuracy).
2.2.5
If communication with the device is lost the following signal
sequence will reset its serial interface:
While leaving DATA high, toggle SCK 9 or more times. This
must be followed by a “Transmission Start” sequence
preceding the next command. This sequence resets the
interface only. The status register preserves its content.
DATA
Figure 4
2.2.6
The whole digital transmission is secured by a 8 bit
checksum. It ensures that any wrong data can be detected
and eliminated.
Please consult application note “CRC-8 Checksum
Calculation” for information on how to calculate the CRC.
12 bit humidity data
C0
6
Connection reset sequence
CRC-8 Checksum calculation
1
5
ack
Connection reset sequence
4
2
3
Bold = SHT1xx controls DATA line
Plain = uC controls DATA line
3
~55ms for 12 bit
~11ms for 8 bit
Transmission Start
Measurement
2
4 -8
1
Skip acknowledge to end transmission
LSB
0
(if no CRC is used)
9
Transmission Start
ack
th
SCK is MSB
3/9