PCF8578USLASH2 datasheet(9/44 Pages) PHILIPS

9 / 44 page

1998 Sep 08

Philips Semiconductors

Product speciﬁcation

LCD row/column driver for dot matrix

graphic displays

PCF8578

7.3

Multiplexed LCD bias generation

The bias levels required to produce maximum contrast

depend on the multiplex rate and the LCD threshold

voltage (Vth). Vth is typically defined as the RMS voltage at

which the LCD exhibits 10% contrast. Table 2 shows the

optimum voltage bias levels for the PCF8578 as functions

of Vop (Vop =VDD − VLCD), together with the discrimination

ratios (D) for the different multiplex rates. A practical value

for Vop is obtained by equating Voff(rms) with Vth. Figure 4

shows the first 4 rows of Table 2 as graphs. Table 3 shows

the relative values of the resistors required in the

configuration of Fig.5 to produce the standard multiplex

rates.

Table 2

Optimum LCD voltages

Table 3

Multiplex rates and resistor values for Fig.5

PARAMETER

MULTIPLEX RATE

1:8

1:16

1:24

1:32

0.739

0.800

0.830

0.850

0.522

0.600

0.661

0.700

0.478

0.400

0.339

0.300

0.261

0.200

0.170

0.150

0.297

0.245

0.214

0.193

0.430

0.316

0.263

0.230

1.447

1.291

1.230

1.196

3.370

4.080

4.680

5.190

RESISTORS

MULTIPLEX RATE (n)

n = 8

n = 16, 24, 32

---------

off rms

()

-----------------------

on rms

()

-----------------------

on rms

()

off rms

()

-----------------------

---------

–

() R

–

() R

–

() R

7.4

Power-on reset

At power-on the PCF8578 resets to a defined starting

condition as follows:

1. Display blank

2. 1 : 32 multiplex rate, row mode

3. Start bank, 0 selected

4. Data pointer is set to X, Y address 0, 0

5. Character mode

6. Subaddress counter is set to 0

7. I2C-bus interface is initialized.

Data transfers on the I2C-bus should be avoided for 1 ms

following power-on, to allow completion of the reset action.

Fig.4

Vbias/Vop as a function of the multiplex rate.

1:8

1:16

1:32

1.0

0.8

MSA838

1:24

0.6

0.4

0.2

multiplex rate

V bias

Vop

Vbias =V2, V3, V4, V5. See Table 2.

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