PIC18F2220/2320/4220/4320
22.0
LOW-VOLTAGE DETECT
In many applications, the ability to determine if the
device voltage (V
DD
) is below a specified voltage level
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 (LVD) 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 then 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.
(typically for batteries). Over time, the device voltage
decreases. When the device voltage equals voltage V
A
,
the LVD logic generates an interrupt. This occurs at
time T
A
. The application software then has the time,
until the device voltage is no longer in valid operating
range, to shut down the system. Voltage point V
B
is the
minimum valid operating voltage specification. This
occurs at time T
B
. The difference, T
B
– T
A
, is the total
time for shutdown.
The block diagram for the LVD module is shown in
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 1.2V
internal reference voltage generated by the voltage ref-
erence module. The comparator then generates an
interrupt signal setting the LVDIF bit. This voltage is
software programmable to any one of 16 values (see
the LVDL3:LVDL0 bits (LVDCON<3:0>).
FIGURE 22-1:
TYPICAL LOW-VOLTAGE DETECT APPLICATION
Voltage
V
A
V
B
Legend:
V
A
= LVD trip point
V
B
= Minimum valid device
operating voltage
T
B
Time
T
A
2003 Microchip Technology Inc.
DS39599C-page 231
MICROCHIP [ MICROCHIP TECHNOLOGY ]
