Microchip PIC16F946 Manual

Microchip Ikke kategoriseret PIC16F946

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Side 1/8
© 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 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)
To
LCD
Driver
Ω
LCD Bias 3 LCD Bias 2 LCD 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
TB084
DS91084A-page 2 © 2004 Microchip Technology Inc.
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
R1
R2
R3
banksel CMCON0
MOVLW 0x0D
MOVWF CMCON0 ;enter the contrast
;control mode

Produkt Specifikationer

Mærke: Microchip
Kategori: Ikke kategoriseret
Model: PIC16F946

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