PIC18F2525_08 Datasheet, PDF (248/412 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F2525/2620/4525/4620
FIGURE 22-3:
HIGH-VOLTAGE DETECT OPERATION (VDIRMAG = 1)
CASE 1:
HLVDIF may not be set
VDD
VLVD
HLVDIF
Enable HLVD
IRVST
CASE 2:
VDD
TIRVST
Internal Reference is stable
HLVDIF cleared in software
VLVD
HLVDIF
Enable HLVD
IRVST
TIRVST
Internal Reference is stable
HLVDIF cleared in software
HLVDIF cleared in software,
HLVDIF remains set since HLVD condition still exists
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 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 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
VA
VB
Time ^{TA TB}
Legend: VA = HLVD trip point
VB = Minimum valid device
operating voltage
DS39626E-page 246
© 2008 Microchip Technology Inc.