Microchip PIC16F946 Manual

Microchip Ikke kategoriseret PIC16F946

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Side 1/8

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 Bias 1/3 Bias

VLCD 0 VSS SS SSV V

VLCD 1 — VDD DDV

VLCD 2 — VDD DDV

VLCD 3 VDD VDD DDV

VLCD 3

VLCD 2

VLCD 1

VLCD 0(1)

LCD

Driver

LCD Bias 3 LCD Bias 2 LCD Bias 1

Connections for External R-ladder

Status Bias

1/2 Bias

1/3 Bias

10 k *

* 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

TB084

EXTERNAL VOLTAGE REGULATOR

A simple solution for contrast control is to use an

external voltage regulator to generate the required

glass voltage. If low voltage glass is used, the voltage

regulator simply steps the power supply down to the

required glass voltage.

Circuit Description

The basic format for the external regulator voltage

generator is as shown in Figure 2. The regulator must

be chosen to satisfy the requirements of glass voltage

and power usage.

FIGURE 2: EXTERNAL VOLTAGE REGULATOR GENERATOR

COMPARATOR BUCK REGULATOR

The external comparator solution is a simple one, but

not without its issues. The first issue is that most battery

applications are cost sensitive, so the addition of a

regulator IC may not be practical. Another one is the

regulator’s dropout voltage. Linear regulators require a

higher input voltage than their output. This difference

may be very small, but it will limit the useful battery life

of a product. The last issue is power consumption. The

linear regulator will burn a small amount of current to

provide its function. These problems can be solved with

a PFM switched capacitor regulator, if it can use

features already present in the microcontroller.

Fortunately, the comparator in the PIC16F91X devices

is designed to accommodate this function.

When Comparator mode 5 (CMCON<2:0> = 101) is

used, it is possible to use an internal 0.6V band gap as

the voltage reference. With this mode, creating a PFM

buck regulator is simple.

Circuit Description

The circuit in Figure 4 uses the internal comparator to

create a PFM buck power supply. The voltage between

R4 and R5 is compared to the internal 0.6V reference.

The comparator output is filtered by the capacitors and

R6. R1-R3 divide the output voltage to an appropriate

value for the VLCD inputs.

Software Description

No software is required for running this system, but the

comparator must be initialized into the correct mode.

The following code can be used to initialize this circuit.

EXAMPLE 1: INITIALIZATION CODE

VDD

PIC16F91X

V 3LCD

V 2LCD

VLCD1

Regulator

LCD

Glass

banksel CMCON0

MOVLW 0x0D

MOVWF CMCON0 ;enter the contrast

;control mode

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	PIC16F946

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