PIC18F242 datasheet(191/332 Pages) MICROCHIP

191 / 332 page

DS39564C-page 189

PIC18FXX2

18.0

LOW VOLTAGE DETECT

In many applications, the ability to determine if the

device voltage (VDD) is below a specified voltage level

is a desirable feature. A window of operation for the

application can be created, where the application soft-

ware can do “housekeeping tasks” before the device

voltage exits the valid operating range. This can be

done using the Low Voltage Detect module.

This module is a software programmable circuitry,

where a device voltage trip point can be specified.

When the voltage of the device becomes lower then the

specified point, an interrupt flag is set. If the interrupt is

enabled, the program execution will branch to the inter-

rupt vector address and the software can then respond

to that interrupt source.

The Low Voltage Detect circuitry is completely under

software control. This allows the circuitry to be “turned

off” by the software, which minimizes the current

consumption for the device.

Figure 18-1 shows a possible application voltage curve

(typically for batteries). Over time, the device voltage

decreases. When the device voltage equals voltage VA,

the LVD logic generates an interrupt. This occurs at

time TA. The application software then has the time,

until the device voltage is no longer in valid operating

range, to shutdown the system. Voltage point VB is the

minimum valid operating voltage specification. This

occurs at time TB. The difference TB - TA is the total

time for shutdown.

FIGURE 18-1:

TYPICAL LOW VOLTAGE DETECT APPLICATION

The block diagram for the LVD module is shown in

Figure 18-2. A comparator uses an internally gener-

ated reference voltage as the set point. When the

selected tap output of the device voltage crosses the

set point (is lower than), the LVDIF bit is set.

Each node in the resistor divider represents a “trip

point” voltage. 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 the trip point, the voltage

tapped off of the resistor array is equal to the 1.2V

internal reference voltage generated by the voltage

reference module. The comparator then generates an

interrupt signal setting the LVDIF bit. This voltage is

software programmable to any one of 16 values (see

Figure 18-2).

The

trip

point

selected

programming the LVDL3:LVDL0 bits (LVDCON<3:0>).

Time

VA = LVD trip point

VB = Minimum valid device

operating voltage

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