PIC18F2420/2520/4420/4520
FIGURE 22-3:
HIGH-VOLTAGE DETECT OPERATION (VDIRMAG = 1)
CASE 1:
HLVDIF may not be set
VLVD
VDD
HLVDIF
Enable HLVD
IRVST
TIRVST
HLVDIF Cleared in Software
Internal Reference is Stable
CASE 2:
VLVD
VDD
HLVDIF
Enable HLVD
TIRVST
IRVST
Internal Reference is Stable
HLVDIF Cleared in Software
HLVDIF cleared in software,
HLVDIF remains set since HLVD condition still exists
FIGURE 22-4:
TYPICAL LOW-VOLTAGE
DETECT APPLICATION
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).
VA
VB
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.
TB
VA = HLVD trip point
VB = Minimum valid device
operating voltage
TA
Time
Legend:
DS39631E-page 246
Advance Information
© 2008 Microchip Technology Inc.
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