Specifications

82
Figure 39 SHT21 Normal operating range (Reprinted with permission from
Sensirion)
The SHT21 has a default resolution of 12 bits for relative humidity and 14 bits for
temperature. At this resolution, with a V
DD
of 3.0V and at 25˚C, the sensor has
accuracy for relative humidity and temperature of +-2.0%RH and +-0.3˚C
respectively.
The chip has six pins, that include SDA (Bidirectional Serial Data), VSS
(Ground), VDD (Supply Voltage), SCL (Bidirectional Serial Clock), and two
floating pins that do not connect to anything. The pin layout can be seen below
in Figure 40.
Pin
Name
Comment
1
SDA
Serial Data,
bidirectional
2
VSS
Ground
5
V
DD
Supply Voltage
6
SCL
Serial Clock,
bidirectional
3, 4
NC
Not Connected
Figure 40 SHT21 pin layout (Reprinted with permission from Sensirion)
The supply voltage, V
DD
can range from 2.1V – 3.6V with a typical value of 3V.
With this supply voltage the chip will typically draw 0.15 µA when in sleep mode
(not measuring temperature/humidity) and 300 µA while measuring.
3.3.2 Sensirion SHT21 and the I
2
C Protocol
The SHT21 utilizes the I
2
C, or Inter-Integrated Circuit protocol. The I
2
C format
uses two bidirectional lines. They are the Serial Data Line (SDA) and the Serial
Clock (SCL). I
2
C is appropriate for our design because we are sending a small
amount of data between the sensor and the microcontroller which the I
2
C
interface is able to handle while being extremely cost efficient. Only having two
connections between the sensor and the microcontroller is also a plus since
space on printed circuit boards is usually scarce.