Microchip PIC16F946 Handleiding

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© 2004 Microchip Technology Inc.DS91084A-page 1

TB084

INTRODUCTION

The PIC16F91X is a low-cost flash-based PICmicro®

microcontroller that has the ability to drive LCD glass.

This device also has a wide operational voltage range

and low power, making it an ideal candidate for many

battery powered applications. To get the most use from

the battery, the device must operate over a wide

voltage range. The wide voltage range can cause

contrast problems with the LCD glass, so a solution is

needed that will maintain constant LCD voltage and be

very cost effective. In this technical brief we will discuss

three different contrast control systems; external

voltage regulator, comparator buck regulator and

comparator buck/boost regulator.

SYSTEM REQUIREMENTS

LCD glass requires a specific RMS voltage applied

between a segment and a common to create the

required polarization shift. Different pieces of glass are

designed for different voltages with the most common

type of glass being 5.0V glass. Lower voltage glass is

available, though as the voltage gets below 3V, it is

more difficult to create large panels with acceptable

contrast.

The PIC16F91X has a series of analog switches to

connect the segment and common pins to 4 voltages.

Three of the voltages are applied to the VLCD<3:0>

pins. The fourth voltage is internally connected to VSS.

To operate the LCD module, correct voltages must be

applied to these pins. The voltages applied depend

upon the type of LCD drive and the type of glass.

FIGURE 1:VOLTAGE APPLICATION

Author:Joseph Julicher

Microchip Technology Inc.

Static

Bias 1/2 Bias1/3 Bias

VLCD 0VSS SSSSV V

VLCD 1—VDD DDV

VLCD 2—VDD DDV

VLCD 3VDD VDD DDV

VLCD3

VLCD2

VLCD1

VLCD0(1)

To

LCD

Driver

Ω

LCD Bias 3LCD Bias 2LCD Bias 1

V

DD

*

V

DD

*

V

DD

*

Connections for External R-ladder

Status Bias

1/2 Bias

1/3 Bias

10 k*

Ω

10 k*

Ω

10 k*

Ω

10 k*

Ω

10 k*

V

SS

V

SS

*These values are provided for design guidance only and should be optimized for the application

by the designer.

Note 1:Internal connection.

Contrast Control Circuits for the PIC16F91X

Product specificaties