PCF8576CUSLASH2 datasheet(17/56 Pages) NXP

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PCF8576C_9

Product data sheet

Rev. 09 — 9 July 2009

17 of 56

NXP Semiconductors

PCF8576C

Universal LCD driver for low multiplex rates

7.5 Oscillator

The internal logic and the LCD drive signals of the PCF8576C are timed by the frequency

fclk, which equals either the built-in oscillator frequency fosc or the external clock frequency

fclk(ext).

The clock frequency (fclk) determines the LCD frame frequency (ffr) and the maximum rate

for data reception from the I2C-bus. To allow I2C-bus transmissions at their maximum data

rate of 100 kHz, fclk should be chosen to be above 125 kHz.

7.5.1 Internal clock

The internal oscillator is enabled by connecting pin OSC to pin VSS. In this case, the

output from pin CLK is the clock signal for any cascaded PCF8576s or PCF8566s in the

system.

Remark: The PCF8576C is backwards compatible with the PCF8576 (Voper up to 9 V).

Where resistor Rext (on pin OSC) to VSS is present, the internal oscillator is selected.

7.5.2 External clock

Connecting pin OSC to VDD enables an external clock source. Pin CLK then becomes the

external clock input.

Remark: A clock signal must always be supplied to the device. Removing the clock,

freezes the LCD in a DC state, which is not suitable for the liquid crystal.

7.6 Timing

The timing of the PCF8576C sequences the internal data ﬂow of the device. This includes

the transfer of display data from the display RAM to the display segment outputs. In

cascaded applications, the synchronization signal (SYNC) maintains the correct timing

relationship between the PCF8576Cs in the system. The timing also generates the LCD

frame frequency which is derived as an integer division of the clock frequency (see

Table 6). The frame frequency is set by the mode set commands when an internal clock is

used or by the frequency applied to the pin CLK when an external clock is used.

[1]

The possible values for fclk see Table 20.

[2]

For fclk = 200 kHz.

[3]

For fclk = 31 kHz.

The ratio between the clock frequency and the LCD frame frequency depends on the

mode in which the device is operating. In the power-saving mode the reduction ratio is six

times smaller; this allows the clock frequency to be reduced by a factor of six. The

reduced clock frequency results in a signiﬁcant reduction in power consumption.

Table 6.

LCD frame frequencies [1]

PCF8576C mode

Frame frequency

Nominal frame frequency (Hz)

Normal mode

69 [2]

Power saving mode

65 [3]

clk

2880

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

clk

480

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