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).
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 11.
DATA valid write
DATA valid read
Figure 11: 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.
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
TR/TF SCK rise/fall time
1 200 * ns
TFO DATA fall time
OL = 5pF 3.5 10 20 ns
OL = 100pF 30 40 200 ns
TRO DATA rise time
TV DATA valid time
TSU DATA setup time
THO DATA hold time
** ** ** ns
200 250 *** ns
100 150 *** ns
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: SHT1x I/O signal characteristics, OL stands for Output
Load, entities are displayed in Figure 11.
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
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 12.
Figure 12: "Transmission Start" sequence
The subsequent command consists of three address bits
(only ‘000’ is supported) and five command bits. The
SHT1x 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.
Measure Relative Humidity
Read Status Register
Write Status Register
Soft reset, resets the interface, clears the
status register to default values. Wait minimum
11 ms before next command
Table 4: SHT1x list of commands
3.3 Measurement of RH and T
After issuing a measurement command (‘00000101’ for
relative humidity, ‘00000011’ for temperature) the
controller has to wait for the measurement to complete.
This takes a maximum of 20/80/320 ms for a 8/12/14bit
measurement. The time varies with the speed of the
internal oscillator and can be lower by up to 30%. To
signal the completion of a measurement, the SHT1x pulls
data line low and enters Idle Mode. The controller must
wait for this Data Ready signal before restarting SCK to
readout the data. Measurement data is stored until
readout, therefore the controller can continue with other
tasks and readout at its convenience.
Two bytes of measurement data and one byte of CRC
checksum (optional) will then be transmitted. The micro
controller must acknowledge each byte by pulling the
DATA line low. All values are MSB first, right justified (e.g.
the 5th SCK is MSB for a 12bit value, for a 8bit result the
first byte is not used).
Version 5 – December 2011