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PIC16C717/770/771

DS41120B-page 104

2002 Microchip Technology Inc.

10.3

Low Voltage Detect (LVD)

This module is used to generate an interrupt when the

supply voltage falls below a specified “trip” voltage.

This module operates completely under software

control. This allows a user to power the module on

and off to periodically monitor the supply voltage, and

thus minimize total current consumption.

The LVD module is enabled by setting the LVDEN bit in

the LVDCON register. The “trip point” voltage is the

minimum supply voltage level at which the device can

operate before the LVD module asserts an interrupt.

When the supply voltage is equal to or less than the trip

point, the module will generate an interrupt signal set-

ting interrupt flag bit LVDIF. If interrupt enable bit LVDIE

was set, then an interrupt is generated. The LVD inter-

rupt can wake the device from SLEEP. The "trip point"

voltage is software programmable to any one of 16 val-

ues, five of which are reserved (See Figure 10-1). The

trip point is selected by programming the LV<3:0> bits

(LVDCON<3:0>).

Once the LV bits have been programmed for the spec-

ified trip voltage, the low-voltage detect circuitry is then

enabled by setting the LVDEN (LVDCON<4>) bit.

If the bandgap reference voltage is previously unused

by either the brown-out circuitry or the voltage refer-

ence circuitry, then the bandgap circuit requires a time

to start-up and become stable before a low voltage con-

dition can be reliably detected. The low-voltage inter-

rupt flag is prevented from being set until the bandgap

has reached a stable reference voltage.

When the bandgap is stable the BGST (LVDCON<5>)

bit is set indicating that the low-voltage interrupt flag bit

is released to be set if VDD is equal to or less than the

LVD trip point.

10.3.1

EXTERNAL ANALOG VOLTAGE INPUT

The LVD module has an additional feature that allows

the user to supply the trip voltage to the module from

an external source. This mode is enabled when

LV<3:0> = 1111. When these bits are set the compar-

ator input is multiplexed from an external input pin

(RA1/AN1/LVDIN).

Note:

The LVDIF bit can not be cleared until the

supply voltage rises above the LVD trip

point. If interrupts are enabled, clear the

LVDIE bit once the first LVD interrupt

occurs to prevent reentering the interrupt

service routine immediately after exiting

the ISR.

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