Datasheet

PIC18F2X1X/4X1X
DS39636D-page 236 © 2009 Microchip Technology Inc.
FIGURE 21-3: HIGH-VOLTAGE DETECT OPERATION (VDIRMAG = 1)
21.5 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 a 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 21-4 shows a
possible voltage curve. Over time, the device voltage
decreases. When the device voltage reaches voltage
V
A, 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 T
B. The HLVD thus would give the application
a time window, represented by the difference between
T
A and TB, to safely exit.
FIGURE 21-4: TYPICAL
HIGH/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
HLVDIF cleared in software,
CASE 1:
CASE 2:
HLVDIF remains set since HLVD condition still exists
T
IVRST
IRVST
Internal Reference is stable
Internal Reference is stable
IRVST
Time
Voltage
VA
VB
TA
TB
VA = HLVD trip point
V
B = Minimum valid device
operating voltage
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