PIC17C752-08SLASHPQ datasheet(28/320 Pages) MICROCHIP

28 / 320 page

PIC17C75X

DS30264A-page 28

Preliminary

5.1.5

BROWN-OUT RESET (BOR)

PIC17C75X devices have an on-chip Brown-out Reset

circuitry. This circuitry places the device into a reset

when the device voltage falls below a trip point (BVDD).

This ensures that the device does not continue pro-

gram execution outside the valid operation range of the

device. Brown-out resets are typically used in AC line

applications or large battery applications where large

loads may be switched in (such as automotive).

A conﬁguration bit, BODEN, can disable (if clear/pro-

grammed) or enable (if set) the Brown-out Reset cir-

cuitry. If VDD falls below BVDD (Typically 4.0V,

parameter D005 in electrical speciﬁcation section), for

greater than parameter D035, the brown-out situation

will reset the chip. A reset is not guaranteed to occur if

VDD falls below BVDD for less than parameter D035.

The chip will remain in Brown-out Reset until VDD rises

above BVDD. The Power-up Timer will now be invoked

and will keep the chip in reset an additional 96 ms. If

VDD drops below BVDD while the Power-up Timer is

running, the chip will go back into a Brown-out Reset

and the Power-up Timer will be initialized. Once VDD

rises above BVDD, the Power-up Timer will execute a

96 ms time delay. Figure 5-10 shows typical Brown-out

situations.

In some applications the Brown-out reset trip point of

the device may not be at the desired level. Figure 5-8

and Figure 5-9 are two examples of external circuitry

that may be implemented. Each needs to be evaluated

to determine if they match the requirements of the

application.

Note: Before using the on-chip brown-out for a

voltage

supervisory

function,

please

review the electrical speciﬁcations to

ensure that they meet your requirements.

FIGURE 5-8: EXTERNAL BROWN-OUT

PROTECTION CIRCUIT 1

FIGURE 5-9: EXTERNAL BROWN-OUT

PROTECTION CIRCUIT 2

VDD

33k

10k

40 k

Ω

VDD

MCLR

PIC17CXXX

This circuit will activate reset when VDD goes below

(Vz + 0.7V) where Vz = Zener voltage.

This brown-out circuit is less expensive, albeit less

accurate. Transistor Q1 turns off when VDD is below a

certain level such that:

VDD •

R1 + R2

= 0.7V

40 k

Ω

VDD

MCLR

PIC17CXXX

VDD

FIGURE 5-10: BROWN-OUT SITUATIONS

96 ms

BVDD Max.

BVDD Min.

VDD

Internal

Reset

BVDD Max.

BVDD Min.

VDD

Internal

Reset

96 ms

< 96 ms

96 ms

BVDD Max.

BVDD Min.

VDD

Internal

Reset

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