Specifications

84
sensor without having to turn off the power. The soft reset function takes less
than 15ms.
The user register is an eight bit register that comes from the factory with default
settings. It initially sets the resolution of relative humidity to 12 bits and
temperature to 14 bits. The default register settings are shown in Figure 43.
Bit
# Bits
Description / Coding
Default
7, 0
2
Measurement
resolution
RH
T
„00‟
12
bit
14
bit
„01‟
8
bit
12
bit
„10‟
10
bit
13
bit
„11‟
11
bit
11
bit
„00‟
6
1
Status: End of
battery
15
„0‟: V
DD
> 2.25 V
„1‟:V
DD
< 2.25 V
„0‟
3, 4, 5
3
Reserved
2
1
Enable on-chip heater
„0‟
1
1
Disable OTP Reload
„1‟
Figure 43 SHT21 Default register settings (Reprinted with permission from
Sensirion)
Once the bits measurement is taken and the bits are transferred, it must be
converted into a number that makes sense to the user. The binary number must
be converted to a decimal number (called S
RH
or S
T
for relative humidity, or
temperature, respectively). Once converted into decimal, the value must be
plugged into a formula to get the final output. The formulas for relative humidity
and temperature conversion are shown below. The units for relative humidity are
%RH and the units for temperature are ˚C.
RH = -6 + 125 * (S
RH
/ 2
16
)
T = -46.85 + 175.72 * (S
T
/ 2
16
)
3.4 Evervision LCD Panel
This LCD touch screen panel from the Reach Technologies Inc website is what
has been chosen to be implemented into our design as the user interface for the
efficient HVAC control and feedback system. Since this is one of the most
expensive components of the design, we presented the sponsors with a list of
similar options and they picked the one they thought was the best compromise.
They want to have a large, touch screen user interface while keeping the cost
down to both stay within our budget and keep the overall cost of the system low.
We have chosen to purchase the development kit which is composed of a 7 inch