Specifications

5
AVR064
2529A–AVR–11/02
LCD Display Liquid Crystal Displays (LCDs) are categorized as non-emissive display devices. In that
respect, they do not produce any form of light like a Cathode Ray Tube (CRT). LCDs are
composed of a polarized liquid crystalline material in between two plates of glass. Typi-
cally, one plate is called the common or backplane, and the other is called a segment or
frontplane. In a reflective LCD panel (one that has no back light) a voltage difference
applied across the two electrodes will result in a polarization which will prevent the light
from reflecting back to the observer. This will appear as a dark segment and is, there-
fore, considered ON. A lack of voltage difference will allow the light to reflect back and is
considered OFF.
For more information on the LCD driver, see application note “AVR065: LCD Driver for
the STK502 LCD"
NTC Thermistor Various types of sensors can be used to measure temperature. One of these is the ther-
mistor, or temperature-sensitive resistor. Most thermistors have a negative temperature
coefficient (NTC), meaning the resistance goes up as temperature goes down. Of all
passive temperature measurement sensors, thermistors have the highest sensitivity
(resistance change per degree of temperature change). Thermistors do not have a lin-
ear temperature/resistance curve.
The NTC thermistor used with this application has a resistance of 10 k
Ω at 25°C (T
AMB
),
beta-value of 3450 and a tolerance of ±1%. The voltage over the NTC can be found
using the A/D converter in the ATmega169. See the ATmega169 data sheet for how to
use the ADC. And by the use of the following equation, the temperature can be
calculated.
β = 3450
V
ADC
= Voltage calculated from the A/D conversion
V
REF
= 1.263V
T
ZERO
= 273°K
T
AMB
= 298°K (273° + 25°)
Temperature
β
V
ADC
V
REF
V
ADC
–
---------------------------------
ln
β
T
AMB
------------- -+
-----------------------------------------------------------------
T
ZERO
–=