Datasheet SHT7x
www.sensirion.com
Version 4.1 – July 2008
5/11
2.4
Electrical Characteristics
The electrical characteristics such as power consumption,
low and high level, input and output voltages depend on
the supply voltage. Table 2 gives electrical characteristics
of SHT7x with the assumption of 5V supply voltage if not
stated otherwise. For proper communication with the
sensor it is essential to make sure that signal design is
strictly within the limits given in Table 3 and Figure 6.
Parameter
Conditions
min
typ
max Units
Power supply DC10
2.4
3.3
5.5
V
measuring
0.55
1
mA
average11
2
28
A
Supply current
sleep
0.3
1.5
A
Low level output
voltage
IOL < 4 mA
0
250
mV
High level output
voltage
RP < 25 k
90%
100% VDD
Low level input
voltage
Negative going
0%
20% VDD
High level input
voltage
Positive going
80%
100% VDD
Input current on
pads
1
A
on
4
mA
Output current
Tri-stated (off)
10
20
A
Table 2: SHT7x DC characteristics. RP stands for pull up
resistor, while IOL is low level output current.
Figure 6: Timing Diagram, abbreviations are explained in
Table 3. Bold DATA line is controlled by the sensor, plain DATA
line is controlled by the micro-controller. Both valid times refer to
the left SCK toggle.
10 Recommended voltage supply for highest accuracy is 3.3V, due to sensor
calibration.
11 Minimum value with one measurement of 8 bit accuracy without OTP reload
per second, typical value with one measurement of 12bit accuracy per
second.
Parameter
Conditions
min
typ
max Units
VDD > 4.5V
0
0.1
5
MHz
FSCK SCK Frequency
VDD < 4.5V
0
0.1
1
MHz
TSCKx SCK hi/low time
100
ns
TR/TF SCK rise/fall time
1
200
*
ns
OL = 5pF
3.5
10
20
ns
TFO
DATA fall time
OL = 100pF
30
40
200
ns
TRO
DATA rise time
**
**
**
ns
TV
DATA valid time
200 250
***
ns
TSU
DATA setup time
100 150
***
ns
THO
DATA hold time
10
15
****
ns
*
TR_max + TF_max = (FSCK)-1 – TSCKH – TSCKL
**
TR0 is determined by the RP*Cbus time-constant at DATA line
***
TV_max and TSU_max depend on external pull-up resistor (RP) and total bus
line capacitance (Cbus) at DATA line
****
TH0_max < TV – max (TR0, TF0)
Table 3: SHT7x I/O signal characteristics, OL stands for Output
Load, entities are displayed in Figure 6.
3 Communication with Sensor
3.1
Start up Sensor
As a first step the sensor is powered up to chosen supply
voltage VDD. The slew rate during power up shall not fall
below 1V/ms. After power-up the sensor needs 11ms to
get to Sleep State. No commands must be sent before
that time.
3.2
Sending a Command
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 – see Figure 7.
Figure 7: "Transmission Start" sequence
The subsequent command consists of three address bits
(only ‘000’ is supported) and five command bits. The
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.
DATA
SCK
90%
10%
90%
10%
DATA valid read
DATA valid write
DATA
SCK
90%
10%
90%
10%
TV
TSCKL
TSU
THO
TSCK
TSCKH
TR
TF
TRO
TFO
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