PIC18F2331 Datasheet, PDF (266/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
21.2 Operation
Depending on the power source for the device voltage,
the voltage normally decreases relatively slowly. This
means that the LVD module does not need to be
constantly operating. To decrease the current
requirements, the LVD circuitry only needs to be
enabled for short periods, where the voltage is
checked. After doing the check, the LVD module may
be disabled.
Each time that the LVD module is enabled, the circuitry
requires some time to stabilize. After the circuitry has
stabilized, all status flags may be cleared. The module
will then indicate the proper state of the system.
The following steps are needed to set up the LVD
module:
1. Write the value to the LVDL3:LVDL0 bits
(LVDCON register), which selects the desired
LVD Trip Point.
2. Ensure that LVD interrupts are disabled (the
LVDIE bit is cleared or the GIE bit is cleared).
3. Enable the LVD module (set the LVDEN bit in
the LVDCON register).
4. Wait for the LVD module to stabilize (the IRVST
bit to become set).
5. Clear the LVD interrupt flag, which may have
falsely become set until the LVD module has
stabilized (clear the LVDIF bit).
6. Enable the LVD interrupt (set the LVDIE and the
GIE bits).
Figure 21-4 shows typical waveforms that the LVD
module may be used to detect.
FIGURE 21-4:
CASE 1:
VDD
LOW-VOLTAGE DETECT WAVEFORMS
LVDIF may not be set
LVDIF
VLVD
Enable LVD
Internally Generated
Reference Stable
TIVRST
LVDIF cleared in software
CASE 2:
VDD
LVDIF
Enable LVD
Internally Generated
Reference Stable
VLVD
TIVRST
LVDIF cleared in software
LVDIF cleared in software,
LVDIF remains set since LVD condition still exists
DS39616B-page 264
Preliminary
 2003 Microchip Technology Inc.