PCF8820U Philips Semiconductors, PCF8820U Datasheet - Page 26

• Manufacturer Part Number: PCF8820U
• Description: 67 x 101 Grey-scale/ECB colour dot matrix LCD driver
• Manufacturer: Philips Semiconductors

Philips Semiconductors
It is not possible to measure temperature in Power-down
mode.
During a temperature measurement, the status register
value remains zero until the measurement has completed
and then the register is updated with the current
temperature value (non-zero value). Because the I
interface is asynchronous to the temperature
measurement, the value read from the status register
should be validated by reading the status register a few
times.
During a temperature measurement, the temperature
coefficient (TC) has to be selected.
The ideal temperature read-out can be calculated by the
equation:
where T is the on-chip temperature in C and ‘a’ is the
conversion constant (see Chapter 11).
To improve the accuracy of the temperature
measurement, it is recommended that the temperature
read-out is calibrated during the product’s final assembly.
Calibration of the temperature read-out requires a
measurement to be made at a defined ideal temperature.
The offset between the ideal temperature value and the
measured temperature value is calculated by:
TR
where TR
PCF8820. The offset value must be stored in a non-volatile
register, such as an EEPROM.
A calibrated temperature read-out can be calculated for
each measurement by the equation:
TR
The accuracy after the calibration is 10% 1 bit of the
difference between the measured temperature and the
calibration temperature. For this reason, it is
recommended that a calibration is performed at or near the
most sensitive LCD temperature.
For example: calibration temperature is 25 C and the
measured temperature is 20 C. The relative error
A = 0.10
A = 4.5 1.13
A = 5.63 C.
This calibration accuracy is valid for temperature
measurements made when the supply voltage value is the
same as when it was calibrated.
2000 Dec 07
67 101 Grey-scale/ECB colour dot matrix
LCD driver
offset
cal
= TR
= TR
meas
TR
meas
{25
ideal
ideal
is the actual temperature read-out of the
+ TR
( 20)} 1 bit
=
TR
offset
128
meas
+
T 27 C
–
a
1
-- -
a
2
C-bus
26
8.1.7
The V
bits V
The programmed value for V
reference temperature, called the cut-point temperature
T
V
The values for parameters T
Table 7, and their relationship with the V
register values are shown in Fig.20.
The V
temperature T
and the programmed value for V
temperature T
V
Two overlapping V
(see Table 7 and Fig.20). The maximum voltage that can
be generated depends on the values of V
and the display load current. At a multiplex rate of 1 : 67,
the optimum operating voltage for the LCD can be
calculated by the equation:
where V
material used.
The practical value for V
V
is the typically value when the LCD exhibits approximately
10% contrast.
Table 7 Parameter values for programming V
V
T
a
b
programming
range
cp
LCD
LCD
off(rms)
LCD
cp
SYMBOL
, using the equation:
(at T
(at T
OP6
LCDOUT
LCDOUT
=
with the defined LCD threshold voltage (V
th
------------------------------------------------ -
V
to V
register
cp
oper
is the threshold voltage of the liquid crystal
2
LCD
) = a + b
1
) = V
value can be set by software using the
OP0
generated is dependent on the operating
oper
cp
+
CONTROL REGISTER
1
4.5 to 10.215
and is calculated by the equation:
BIT PRS = 0
–
, the selected temperature coefficient TC
67
of the V
LCD
LCD
------------------- -
67
4.500
0.045
+
23.0
1
(at T
1
ranges are selectable by bit PRS
V
+
OP
LCD
1
LCD
cp
LCD
cp
is determined by equating
VALUE
)
control register.
V
, a and b are given in
th
{1 + TC
10.215 to 15.93 V
has to be calculated for a
LCD
BIT PRS = 1
6.975 V
at the reference
10.215
Product specification
0.045
23.0
LCD
PCF8820
DD2
(T
control
oper
th
and V
LCD
th
), which
V
V
T
control
UNIT
C
DD3
cp
)}
,