PIC18C658 MICROCHIP [Microchip Technology], PIC18C658 Datasheet - Page 247

• Manufacturer Part Number: PIC18C658
• Description: High-Performance Microcontrollers with CAN Module
• Manufacturer: MICROCHIP [Microchip Technology]

21.0
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 soft-
ware 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 software programmable circuitry, where
a device voltage trip point can be specified (internal ref-
erence voltage or external voltage input). 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 vec-
tor 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 con-
sumption for the device.
Figure 21-1 shows a possible application voltage curve
(typically for batteries). Over time, the device voltage
decreases. When the device voltage equals voltage
V
time T
until the device voltage is no longer in valid operating
range, to shut down the system. Voltage point V
minimum valid operating voltage specification. This
occurs at time T
FIGURE 21-1: TYPICAL LOW VOLTAGE
A
2000 Microchip Technology Inc.
, the LVD logic generates an interrupt. This occurs at
Legend:
V
V
A
B
A
V
V
. The application software then has the time,
A
B
LOW VOLTAGE DETECT
= LVD trip point
= Minimum valid device operating range
B
. T
DD
DETECT APPLICATION
B
) is below a specified voltage level
- T
A
Time
is the total time for shutdown.
T
A
T
B
Advanced Information
B
is the
Figure 21-2 shows the block diagram for the LVD mod-
ule. A comparator uses an internally generated refer-
ence voltage as the set point. When the selected tap
output of the device voltage crosses the set point (is
lower than), the LVDIF bit (PIR registers) is set.
Each node in the resister 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 (or external LVDIN input
pin) is equal to the voltage generated by the internal
voltage reference module. The comparator then gener-
ates an interrupt signal setting the LVDIF bit. This volt-
age is software programmable to any one of 16 values
(See Figure 21-2). The trip point is selected by pro-
gramming the LVDL3:LVDL0 bits (LVDCON<3:0>).
FIGURE 21-2: LOW VOLTAGE DETECT
LVDEN
V
DD
(LVD) BLOCK DIAGRAM
LVDIN
PIC18CXX8
Internally Generated
Reference Voltage
LVD Control
DS30475A-page 247
Register
LVDIF