PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 257

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• 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 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 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 inter-
rupt 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:
2002 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
Preliminary
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
B
operating voltage
. The difference T
PIC18FXX8
DS41159B-page 255
B
– T
A
is the total
B
is the
A
,