Datasheet SHT21
3 Interface Specifications
Pin Name
Comment
2
1
6
5
VSS Ground
4
5
6
3
2
1
SDA Serial Data, bidirectional
SCL Serial Clock, bidirectional
VDD Supply Voltage
3,4 NC Not Connected, floating
Figure 8 Top view of example of mounted SHT2x with slits
milled into PCB to minimize heat transfer.
Table 2 SHT2x pin assignment, NC remain floating
2.5 Light
3.1 Power Pins (VDD, VSS)
The SHT2x is not light sensitive. Prolonged direct
exposure to sunshine or strong UV radiation may age the
sensor.
The supply voltage of SHT2x must be in the range of 2.1 –
3.6V, recommended supply voltage is 3.0V. Power supply
pins Supply Voltage (VDD) and Ground (VSS) must be
decoupled with a 100nF capacitor, that shall be placed as
close to the sensor as possible – see Figure 9.
2.6 Materials Used for Sealing / Mounting
Many materials absorb humidity and will act as a buffer
increasing response times and hysteresis. Materials in the
vicinity of the sensor must therefore be carefully chosen.
Recommended materials are: Any metals, LCP, POM
(Delrin), PTFE (Teflon), PEEK, PP, PB, PPS, PSU, PVDF,
PVF.
3.2 Serial clock (SCL)
SCL is used to synchronize the communication between
microcontroller (MCU) and the sensor. Since the interface
consists of fully static logic there is no minimum SCL
frequency.
For sealing and gluing (use sparingly): Use high filled
epoxy for electronic packaging (e.g. glob top, underfill),
and Silicone. Out-gassing of these materials may also
contaminate the sensor (see Section 1.3). Therefore try to
add the sensor as a last manufacturing step to the
assembly, store the assembly well ventilated after
manufacturing or bake at >50°C for 24h to outgas
contaminants before packing.
3.3 Serial SDA (SDA)
The SDA pin is used to transfer data in and out of the
sensor. For sending a command to the sensor, SDA is
valid on the rising edge of SCL and must remain stable
while SCL is high. After the falling edge of SCL the SDA
value may be changed. For safe communication SDA shall
be valid tSU and tHD before the rising and after the falling
edge of SCL, respectively – see Figure 10. For reading
data from the sensor, SDA is valid tVD after SCL has gone
low and remains valid until the next falling edge of SCL.
2.7 Wiring Considerations and Signal Integrity
Carrying the SCL and SDA signal parallel and in close
proximity (e.g. in wires) for more than 10cm may result in
cross talk and loss of communication. This may be
resolved by routing VDD and/or VSS between the two
SDA signals and/or using shielded cables. Furthermore,
slowing down SCL frequency will possibly improve signal
integrity. Power supply pins (VDD, VSS) must be
decoupled with a 100nF capacitor – see next Section.
VDD
MCU (master)
RP RP
SCL IN
SCL
SCL OUT
SHT2x
(slave)
SDA IN
SDA
GND
SDA OUT
Figure 9 Typical application circuit, including pull-up resistors
RP and decoupling of VDD and VSS by a capacitor.
To avoid signal contention the micro-controller unit (MCU)
must only drive SDA and SCL low. External pull-up
resistors (e.g. 10kΩ), are required to pull the signal high.
For the choice of resistor size please take bus capacity
requirements into account (compare Table 5). It should be
noted that pull-up resistors may be included in I/O circuits
www.sensirion.com
Version 1.0 – January 2010
5/12
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