PIC18F4520ESLASHMLQTP datasheet(248/390 Pages) MICROCHIP

248 / 390 page

PIC18F2420/2520/4420/4520

DS39631A-page 246

 2004 Microchip Technology Inc.

FIGURE 22-3:

HIGH-VOLTAGE DETECT OPERATION (VDIRMAG = 1)

22.5

Applications

In many applications, the ability to detect a drop below

or rise above a particular threshold is desirable. For

example, the HLVD module could be periodically

enabled to detect Universal Serial Bus (USB) attach or

detach. This assumes the device is powered by a lower

voltage source than the USB when detached. An attach

would indicate a high-voltage detect from, for example,

3.3V to 5V (the voltage on USB) and vice versa for a

detach. This feature could save a design a few extra

components and an attach signal (input pin).

For general battery applications, Figure 22-4 shows a

possible voltage curve. Over time, the device voltage

decreases. When the device voltage reaches voltage

VA, the HLVD logic generates an interrupt at time TA.

The interrupt could cause the execution of an ISR,

which would allow the application to perform “house-

keeping tasks” and perform a controlled shutdown

before the device voltage exits the valid operating

range at TB. The HLVD, thus, would give the applica-

tion a time window, represented by the difference

between TA and TB, to safely exit.

FIGURE 22-4:

TYPICAL LOW-VOLTAGE

DETECT APPLICATION

VLVD

VDD

HLVDIF

VLVD

VDD

Enable HLVD

TIVRST

HLVDIF may not be set

Enable HLVD

HLVDIF

HLVDIF cleared in software

HLVDIF cleared in software,

CASE 1:

CASE 2:

HLVDIF remains set since HLVD condition still exists

TIVRST

IRVST

Internal Reference is stable

IRVST

Time

VA = HLVD trip point

VB = Minimum valid device

operating voltage

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