PIC18F24 MICROCHIP [Microchip Technology], PIC18F24 Datasheet - Page 261

• Manufacturer Part Number: PIC18F24
• Description: 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module
• Manufacturer: MICROCHIP [Microchip Technology]

23.0
In many applications, the ability to determine if the
device voltage (V
is a desirable feature. A window of operation for the
application can be created, where the application
software can do “housekeeping tasks” before the
device voltage exits the valid operating range. This can
be done using the Low-Voltage Detect module.
This module is a software programmable circuitry,
where a device voltage trip point can be specified.
When the voltage of the device becomes lower than the
specified point, an interrupt flag is set. If the interrupt is
enabled, the program execution will branch to the
interrupt vector address and the software can then
respond to that interrupt source.
The Low-Voltage Detect circuitry is completely under
software control. This allows the circuitry to be “turned
off” by the software which minimizes the current
consumption for the device.
FIGURE 23-1:
 2004 Microchip Technology Inc.
LOW-VOLTAGE DETECT
DD
V
V
) is below a specified voltage level
A
B
TYPICAL LOW-VOLTAGE DETECT APPLICATION
Time
T
A
T
B
Figure 23-1 shows a possible application voltage curve
(typically for batteries). Over time, the device voltage
decreases. When the device voltage equals voltage V
the LVD logic generates an interrupt. This occurs at
time T
until the device voltage is no longer in valid operating
range, to shutdown the system. Voltage point V
minimum valid operating voltage specification. This
occurs at time T
time for shutdown.
The block diagram for the LVD module is shown in
Figure 23-2. A comparator uses an internally gener-
ated reference voltage as the set point. When the
selected tap output of the device voltage crosses the
set point (is lower than), the LVDIF bit is set.
Each node in the resistor divider represents a “trip point”
voltage. The “trip point” voltage is the minimum supply
voltage level at which the device can operate before the
LVD module asserts an interrupt. When the supply
voltage is equal to the trip point, the voltage tapped off of
the resistor array is equal to the internal reference
voltage generated by the voltage reference module. The
comparator then generates an interrupt signal, setting
the LVDIF bit. This voltage is software programmable to
any one of 16 values (see Figure 23-2). The trip point is
selected by programming the LVDL3:LVDL0 bits
(LVDCON<3:0>).
A
. The application software then has the time,
Legend:
V
V
A
B
= LVD trip point
= Minimum valid device
operating voltage
B
. The difference T
PIC18FXX8
DS41159D-page 259
B
– T
A
is the total
B
is the
A
,