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1-100093-04

Humidity and Temperature Sensor IC

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Datasheet SHT1x

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.

SCK

TSU

TSCK

TSCKH

TSCKL TR

TF

DATA valid write

THO

DATA valid read

TV

DATA

80%

20%

TRO

TFO

80%

20%

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.

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

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

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 12.

80%

SCK

20%

80%

DATA

20%

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.

Command

Reserved

Measure Temperature

Measure Relative Humidity

Read Status Register

Write Status Register

Reserved

Soft reset, resets the interface, clears the

status register to default values. Wait minimum

11 ms before next command

Code

0000x

00011

00101

00111

00110

0101x-1110x

11110

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).

www.sensirion.com

Version 5 – December 2011

6/12

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