PCF85133USLASH2DASLASH1 datasheet(5/41 Pages) NXP

5 / 41 page

PCF85133_1

Product data sheet

Rev. 1 — 17 February 2009

5 of 41

NXP Semiconductors

PCF85133

Universal LCD driver for low multiplex rates

7.1 Power-on reset

At power-on the PCF85133 resets to the following starting conditions:

• All backplane and segment outputs are set to VLCD

• The selected drive mode is 1:4 multiplex with 1

⁄

3 bias

• Blinking is switched off

• Input and output bank selectors are reset

• The I2C-bus interface is initialized

• The data pointer and the subaddress counter are cleared (set to logic 0)

• The display is disabled

Remark: Do not transfer data on the I2C-bus for at least 1 ms after a power-on to allow the

reset action to complete.

7.2 LCD bias generator

Fractional LCD biasing voltages are obtained from an internal voltage divider of three

series resistors between VLCD and VSS. The center resistor can be bypassed to provide a

⁄

2 bias voltage level for the 1:2 multiplex conﬁguration.

7.3 LCD voltage selector

The LCD voltage selector coordinates the multiplexing of the LCD in accordance with 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

VLCD and the resulting discrimination ratios (D), are given in Table 5.

A practical value for VLCD is determined by equating Voff(RMS) with a deﬁned LCD

threshold voltage (Vth), typically when the LCD exhibits approximately 10 % contrast. In

the static drive mode a suitable choice is VLCD > 3Vth.

Multiplex drive modes of 1:3 and 1:4 with 1

⁄

2 bias are possible but the discrimination and

hence the contrast ratios are smaller.

Bias is calculated by

, where the values for a are

a = 1 for 1

⁄

2 bias

a = 2 for 1

⁄

3 bias

Table 5.

Discrimination ratios

LCD drive mode Number of:

LCD bias

conﬁguration

Backplanes

Levels

static

∞

1:2 multiplex

⁄

0.354

0.791

2.236

1:2 multiplex

⁄

0.333

0.745

2.236

1:3 multiplex

⁄

0.333

0.638

1.915

1:4 multiplex

⁄

0.333

0.577

1.732

off RMS

()

LCD

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

on RMS

()

LCD

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

on RMS

()

off RMS

()

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

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

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