Datasheet

PIC18F2455/2550/4455/4550
DS39632E-page 288 © 2009 Microchip Technology Inc.
FIGURE 24-3: HIGH-VOLTAGE DETECT OPERATION (VDIRMAG = 1)
24.5 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 24-4 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 applica-
tion a time window, represented by the difference
between T
A and TB, to safely exit.
FIGURE 24-4: TYPICAL
HIGH/LOW-VOLTAGE
DETECT APPLICATION
VHLVD
VDD
HLVDIF
VHLVD
VDD
Enable HLVD
TIRVST
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
IRVST
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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