PCF8537 datasheet(9/82 Pages) NXP

9 / 82 page

PCF8537

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Product data sheet

Rev. 1 — 31 May 2012

9 of 82

NXP Semiconductors

PCF8537

Industrial LCD driver for multiplex rates up to 1:8

8.1.3.2

Internal oscillator

When the internal oscillator is used, it is possible to make the clock signal available on pin

CLK by using the Oscillator-ctrl command (see Table 7). If this is not intended, the pin

CLK should be left open. At power-on the signal at pin CLK is disabled and pin CLK is in

3-state.

If the internal charge pump is enabled, then the internal oscillator starts and is used to run

the charge pump. An external oscillator can still be applied for driving the display

waveforms.

The duty cycle of the output clock provided on the CLK pin is not always 50 : 50. Table 17

on page 13 shows the expected duty cycle for each of the chosen frame frequencies.

8.1.3.3

External clock

In applications where an external clock must be applied to the PCF8537, bit OSC (see

Table 7) must be set logic 1. In this case, pin CLK becomes an input.

The CLK signal is a signal that is fed into the VDD1 domain. Therefore it must have an

amplitude equal to the VDD1 voltage supplied to the chip and be referenced to VSS.

The clock frequency (fclk) determines the LCD frame frequency.

Remark: If an external clock is used then this clock signal must always be supplied to the

device. Removing the clock can freeze the LCD in a DC state. Removal of the clock is

possible when following the correct procedures (see Figure 11 on page 21 and Figure 12

on page 22).

8.1.4 Command: Charge-pump-ctrl

The Charge-pump-ctrl command enables or disables the internal VLCD generation and

controls the charge pump voltage multiplier setting.

[1]

Default value.

Table 8.

Charge-pump-ctrl - charge pump control command bit description

For further information, see Table 11 on page 11 and Section 8.4.3 on page 26.

Bit

Symbol

Binary value

Description

7 to 2

110000

fixed value

1CPE

charge pump switch

0[1]

charge pump disabled;

no internal VLCD generation;

external supply of VLCD

charge pump enabled

0CPC

charge pump voltage multiplier setting

0[1]

VLCD = 2  VDD2

1VLCD = 3  VDD2

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