PIC16F7X7
time TA. 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 VB is the
minimum valid operating voltage specification. This
occurs at time TB. The difference, TB – TA, is the total
time for shutdown.
15.8 Low-Voltage Detect
In many applications, the ability to determine if the
device voltage (VDD) is below a specified voltage level
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.
The block diagram for the LVD module is shown in
Figure 15-4. 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.
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
interrupt vector address and the software can then
respond to that interrupt source.
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
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 15-4). The trip point is selected by programming
the LVDL3:LVDL0 bits (LVDCON<3:0>).
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 15-3 shows a possible application voltage curve
(typically for batteries). Over time, the device voltage
decreases. When the device voltage equals voltage VA,
the LVD logic generates an interrupt. This occurs at
FIGURE 15-3:
TYPICAL LOW-VOLTAGE DETECT APPLICATION
VA
VB
Legend:
VA = LVD trip point
VB = Minimum valid device
operating voltage
TB
TA
Time
DS30498C-page 174
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
