Philips Semiconductors
Product specification
Universal LCD driver for low multiplex
rates
6.1
Power-on reset
6.3
LCD voltage selector
PCF8566
At power-on the PCF8566 resets to a defined starting
condition as follows:
1. All backplane outputs are set to V
DD
2. All segment outputs are set to V
DD
3. The drive mode ‘1 : 4 multiplex with
1
⁄
3
bias’ is selected
4. Blinking is switched off
5. Input and output bank selectors are reset (as defined
in Table 5)
6. The I
2
C-bus interface is initialized
7. The data pointer and the subaddress counter are
cleared.
should be avoided for 1 ms
Data transfers on the
following power-on to allow completion of the reset action.
6.2
LCD bias generator
I
2
C-bus
The LCD voltage selector coordinates the multiplexing of
the LCD according to the selected LCD drive
configuration. The operation of the voltage selector is
controlled by MODE SET commands from the command
decoder. The biasing configurations that apply to the
preferred modes of operation, together with the biasing
characteristics as functions of V
op
= V
DD
−
V
LCD
and the
resulting discrimination ratios (D), are given in Table 2.
A practical value of V
op
is determined by equating V
off(rms)
with a defined LCD threshold voltage (V
th
), typically when
the LCD exhibits approximately 10% contrast. In the static
drive mode a suitable choice is V
op
≥
3 V
th
. Multiplex drive
ratios of 1 : 3 and 1 : 4 with
1
⁄
2
bias are possible but the
discrimination and hence the contrast ratios are smaller
( 3
=
1.732 for 1 : 3 multiplex or 21
⁄
3
=
1.528 for
1 : 4 multiplex). The advantage of these modes is a
reduction of the LCD full scale voltage V
op
as follows:
1 : 3 multiplex (
1
⁄
2
bias):
V
op
=
6V
op(mrs)
=
2.449V
off
(
rms
)
The full-scale LCD voltage (V
op
) is obtained from
V
DD
−
V
LCD
. The LCD voltage may be temperature
compensated externally through the V
LCD
supply to pin 12.
Fractional LCD biasing voltages are obtained from an
internal voltage divider of three series resistors connected
between V
DD
and V
LCD
. The centre resistor can be
switched out of circuit to provide a
1
⁄
2
bias voltage level for
the 1 : 2 multiplex configuration.
1 : 4 multiplex (
1
⁄
2
bias):
V
op
=
4
3
⁄
3 V
off
(
rms
)
=
2.309V
off
(
rms
)
These compare with V
op
= 3 V
off(rms)
when
1
⁄
3
bias is used.
Table 2
Preferred LCD drive modes: summary of characteristics
LCD BIAS
CONFIGURATION
static (2 levels)
1
⁄
1
⁄
1
⁄
1
⁄
2
3
3
3
LCD DRIVE MODE
Static (1 BP)
1 : 2 MUX (2 BP)
1 : 2 MUX (2 BP)
1 : 3 MUX (3 BP)
1 : 4 MUX (4 BP)
V
off
(
rms
)
----------------------
-
V
op
0
2
⁄
4
=
0.354
1
⁄
1
⁄
1
⁄
3
3
3
V
on
(
rms
)
----------------------
-
V
op
1
10
⁄
4
=
0.791
5
⁄
3
=
0.745
33
⁄
9
=
0.638
3
⁄
3
=
0.577
V
on
(
rms
)
D
=
----------------------
-
V
off
(
rms
)
∞
5
=
2.236
5
=
2.236
33
⁄
3
=
1.915
3
=
1.732
(3 levels)
(4 levels)
(4 levels)
(4 levels)
= 0.333
= 0.333
= 0.333
1998 May 04
7
PHILIPS [ NXP SEMICONDUCTORS ]
