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Sheet目录3886

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PIC18F2450/4450

2006 Microchip Technology Inc.

Advance Information

DS39760A-page 185

17.2

HLVD Setup

The following steps are needed to set up the HLVD

module:

Disable the module by clearing the HLVDEN bit

(HLVDCON<4>).

Write the value to the HLVDL3:HLVDL0 bits that

selects the desired HLVD trip point.

Set the VDIRMAG bit to detect high voltage

(VDIRMAG = 1) or low voltage (VDIRMAG = 0).

Enable the HLVD module by setting the

HLVDEN bit.

Clear

the

HLVD

Interrupt

Flag,

HLVDIF

(PIR2<2>), which may have been set from a

previous interrupt.

Enable the HLVD interrupt, if interrupts are

desired, by setting the HLVDIE and GIE/GIEH

bits (PIE2<2> and INTCON<7>). An interrupt

will not be generated until the IRVST bit is set.

17.3

Current Consumption

When the module is enabled, the HLVD comparator

and voltage divider are enabled and will consume static

current. The total current consumption, when enabled,

is specified in electrical specification parameter D022

(Section 268 “DC Characteristics”).

Depending on the application, the HLVD module does

not need to be operating constantly. To decrease the

current requirements, the HLVD circuitry may only

need to be enabled for short periods where the voltage

is checked. After doing the check, the HLVD module

may be disabled.

17.4

HLVD Start-up Time

The internal reference voltage of the HLVD module,

specified in electrical specification parameter D420 (see

Table 21-4 in Section 21.0 “Electrical Characteris-

tics”), may be used by other internal circuitry, such as

the Programmable Brown-out Reset. If the HLVD or

other circuits using the voltage reference are disabled to

lower the device’s current consumption, the reference

voltage circuit will require time to become stable before

a low or high-voltage condition can be reliably detected.

This start-up time, TIRVST, is an interval that is

independent of device clock speed. It is specified in

electrical specification parameter 36 (Table 21-10).

The HLVD interrupt flag is not enabled until TIRVST has

expired and a stable reference voltage is reached. For

this reason, brief excursions beyond the set point may

not be detected during this interval. Refer to Figure 17-2

or Figure 17-3.

FIGURE 17-2:

LOW-VOLTAGE DETECT OPERATION (VDIRMAG = 0)

VHLVD

VDD

HLVDIF

VHLVD

VDD

Enable HLVD

TIRVST

HLVDIF may not be set

Enable HLVD

HLVDIF

HLVDIF cleared in software

HLVDIF cleared in software,

CASE 1:

CASE 2:

HLVDIF remains set since HLVD condition still exists

TIRVST

Internal Reference is stable

IRVST

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