Datasheet

Datasheet SHT7x
www.sensirion.com Version 4.3 – May 2010 5/11
microcontrollers. See Table 2 for detailed I/O characteristic
of the sensor.
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.
Absolute maximum ratings for VDD versus GND are +7V
and -0.3V. Exposure to absolute maximum rating
conditions for extended periods may affect the sensor
reliability (e.g. hot carrier degradation, oxide breakdown).
Parameter Conditions min typ max Units
Power supply DC
10
2.4 3.3 5.5 V
measuring 0.55 1 mA
average
11
2 28 µA
Supply current
sleep 0.3 1.5 µA
Low level output
voltage
I
OL
< 4 mA 0 250 mV
High level output
voltage
R
P
< 25 kΩ 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 µA
on 4 mA
Output current
Tri-stated (off) 10 20 µA
Table 2: SHT7x DC characteristics. R
P
stands for pull up
resistor, while I
OL
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. Note that DATA valid
read time is triggered by falling edge of anterior toggle.
10
Recommended voltage supply for highest accuracy is 3.3V, due to sensor
calibration.
11
Minimum value with one measurement of 8 bit resolution without OTP reload
per second, typical value with one measurement of 12bit resolution per
second.
Parameter Conditions min typ max Units
VDD > 4.5V 0 0.1 5 MHz
F
SCK
SCK Frequency
VDD < 4.5V 0 0.1 1 MHz
T
SCKx
SCK hi/low time 100 ns
T
R
/T
F
SCK rise/fall time 1 200 * ns
OL = 5pF 3.5 10 20 ns
T
FO
DATA fall time
OL = 100pF 30 40 200 ns
T
RO
DATA rise time ** ** ** ns
T
V
DATA valid time 200 250 *** ns
T
SU
DATA setup time 100 150 *** ns
T
HO
DATA hold time 10 15 **** ns
* T
R_max
+ T
F_max
= (F
SCK
)
-1
– T
SCKH
– T
SCKL
** T
R0
is determined by the R
P
*C
bus
time-constant at DATA line
*** T
V_max
and T
SU_max
depend on external pull-up resistor (R
P
) and total bus
line capacitance (Cbus) at DATA line
**** T
H0_max
< T
V
– max (T
R0
, T
F0
)
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
8
0%
2
0%
8
0%
2
0%
DATA valid
read
DATA valid write
DATA
SCK
8
0%
2
0%
8
0%
2
0%
T
V
T
SCKL
T
SU
T
HO
T
SCK
T
SCKH
T
R
T
F
T
RO
T
FO