DM163006 datasheet(203/320 Pages) MICROCHIP

203 / 320 page

 2001 Microchip Technology Inc.

Advance Information

DS39541A-page 203

PIC18C601/801

18.0

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 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 18-1 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 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. TB - TA is the total time for shut-down.

FIGURE 18-1:

TYPICAL LOW VOLTAGE

DETECT APPLICATION

Figure 18-2 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 18-2). The trip point is selected by

programming the LVDL3:LVDL0 bits (LVDCON<3:0>).

FIGURE 18-2:

LOW VOLTAGE DETECT

(LVD) BLOCK DIAGRAM

Time

VA = LVD trip point

VB = Minimum valid device operating range

Legend:

LVDIF

VDD

LVDEN

LVD Control

Internally Generated

Reference Voltage

LVDIN

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