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PCF8558

Universal LCD driver for small graphic panels

Philips Electronics 

Philips Semiconductors

Universal LCD driver for small graphic

panels

Objective specification

PCF8558

FUNCTIONAL DESCRIPTION

LCD bias voltage generator

The intermediate bias voltages for the LCD display are

generated and buffered on-chip. This removes the need

for an external resistor bias chain and significantly reduces

the system power consumption.

Oscillator

The on-chip oscillator provides the clock signal for the

display system. No external components are required and

the OSC pin must be connected to VDD.

External clock

If an external clock is to be used it is input at the OSC pin.

The resulting display frame frequency is given by

fframe = 3-f--O-0---S7---C-2- .

Only in the power-down state is the clock allowed to be

stopped (OSC connected to VSS), otherwise the LCD will

be frozen in a state where a DC voltage is applied to it.

Power-on reset

The on-chip power-on reset block initializes the chip after

power-on or power failure. This is a synchronous reset and

requires 2 oscillator cycles to execute. These oscillator

cycles must be provided from the external clock source if

the internal oscillator is not used. If this is not done, the

device may not respond to command sequences

transmitted via the I2C-bus interface.

Power-down

The chip can be put into power-down mode where all static

currents are switched off (no internal oscillator, no internal

power-on reset, no bias level generation and all LCD

outputs are internally connected to VDD) when

PD = logic 1.

During power-down the information in the RAMs and the

internal chip states are preserved. Instruction execution

during power-down is possible if an externally clock signal

is applied to pad OSC.

Registers

The PCF8558 has one 8-bit register, time shared as a

Command Register (CR) and a Data Register (DR).

The command register stores the command code such as

display on or display off and address information for the

Display Data RAM (DDRAM). Both registers can be written

to but not read from by the system controller.

Address Counter (AC)

The address counter assigns addresses to the DDRAM for

writing and is set by Y2 to Y0 in the command and

X6 to X0 in the address. After a write operation the

address counter is automatically incremented by 1 in

accordance with the V flag.

Display Data RAM (DDRAM)

The PCF8558 contains a 40 × 101-bit static RAM which

stores the display data. The RAM is divided into 5 banks of

101 bytes (5 × 8 × 101 bits). During RAM access, data is

transferred to the RAM via the I2C-bus. There is a direct

correspondence between the X address and the column

output number.

Timing generator

The timing generator produces the various signals

required to drive the internal circuitry. Internal chip

operation is not disturbed by operations on the data buses.

Display control

The display is generated by continuously shifting rows of

RAM data to the dot matrix LCD via the column outputs.

The display status (all dots on/off and normal/inverse

video) is set by bits E and D in the command word.

LCD row and column drivers

The PCF8558 contains 40 row and 101 column drivers,

which connect the appropriate LCD bias voltages in

sequence to the display in accordance with the data to be

displayed. Figure 3 illustrates typical waveforms. Unused

outputs should be left unconnected.

The bias voltage levels, V2 to V5, are chosen to give

optimum display contrast for a multiplex rate of 1 : 40.

Table 1 Voltage bias levels

LEVEL

V2

V3

V4

V5

VOLTAGE

0.8635 × (VDD − VLCD)

0.7270 × (VDD − VLCD)

0.2730 × (VDD − VLCD)

0.1365 × (VDD − VLCD)

1998 Apr 07

5

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