PIC18F4539-ESLASHMLSQTP datasheet(194/322 Pages) MICROCHIP

194 / 322 page

PIC18FXX39

DS30485A-page 192

Preliminary

 2002 Microchip Technology Inc.

19.2

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 con-

stantly operating. To decrease the current require-

ments, 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:

Write the value to the LVDL3:LVDL0 bits

(LVDCON register), which selects the desired

LVD Trip Point.

Ensure that LVD interrupts are disabled (the

LVDIE bit is cleared or the GIE bit is cleared).

Enable the LVD module (set the LVDEN bit in

the LVDCON register).

Wait for the LVD module to stabilize (the IRVST

bit to become set).

Clear the LVD interrupt flag, which may have

falsely become set until the LVD module has

stabilized (clear the LVDIF bit).

Enable the LVD interrupt (set the LVDIE and the

GIE bits).

Figure 19-4 shows typical waveforms that the LVD

module may be used to detect.

FIGURE 19-4:

LOW VOLTAGE DETECT WAVEFORMS

VLVD

VDD

LVDIF

VLVD

VDD

Enable LVD

Internally Generated

TIVRST

LVDIF may not be set

Enable LVD

LVDIF

LVDIF cleared in software

LVDIF cleared in software,

CASE 1:

CASE 2:

LVDIF remains set since LVD condition still exists

Reference Stable

Internally Generated

Reference Stable

TIVRST

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