PIC18F2X1X Datasheet, PDF (236/380 Pages) Microchip Technology – 28/40/44-PIN FLASH MICROCONTROLLERS WITH 10-BIT A/D AND NANO WATT TECHNOLOGY

PIC18F2X1X/4X1X
FIGURE 21-3:
CASE 1:
HIGH-VOLTAGE DETECT OPERATION (VDIRMAG = 1)
HLVDIF may not be set
VDD
VLVD
HLVDIF
Enable HLVD
IRVST
CASE 2:
VDD
TIVRST
Internal Reference is stable
HLVDIF cleared in software
VLVD
HLVDIF
Enable HLVD
IRVST
TIVRST
Internal Reference is stable
HLVDIF cleared in software
HLVDIF cleared in software,
HLVDIF remains set since HLVD condition still exists
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
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 application
a time window, represented by the difference between
TA and TB, to safely exit.
FIGURE 21-4:
TYPICAL
HIGH/LOW-VOLTAGE
DETECT APPLICATION
VA
VB
Time ^{TA TB}
Legend: VA = HLVD trip point
VB = Minimum valid device
operating voltage
DS39636A-page 234
 2004 Microchip Technology Inc.