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2002 Microchip Technology Inc.

Preliminary

DS30453D-page 23

PIC16C5X

5.2

Device Reset Timer (DRT)

The Device Reset Timer (DRT) provides an 18 ms

nominal time-out on RESET regardless of Oscillator

mode used. The DRT operates on an internal RC oscil-

lator. The processor is kept in RESET as long as the

DRT is active. The DRT delay allows VDD to rise above

VDD min., and for the oscillator to stabilize.

Oscillator circuits based on crystals or ceramic resona-

tors require a certain time after power-up to establish a

stable oscillation. The on-chip DRT keeps the device in

a RESET condition for approximately 18 ms after the

voltage on the MCLR/VPP pin has reached a logic high

(VIH) level. Thus, external RC networks connected to

the MCLR input are not required in most cases, allow-

ing for savings in cost-sensitive and/or space restricted

applications.

The Device Reset time delay will vary from chip to chip

due to VDD, temperature, and process variation. See

AC parameters for details.

The DRT will also be triggered upon a Watchdog Timer

time-out. This is particularly important for applications

using the WDT to wake the PIC16C5X from SLEEP

mode automatically.

5.3

Reset on Brown-Out

A brown-out is a condition where device power (VDD)

dips below its minimum value, but not to zero, and then

recovers. The device should be RESET in the event of

a brown-out.

To RESET PIC16C5X devices when a brown-out

occurs, external brown-out protection circuits may be

built, as shown in Figure 5-6, Figure 5-7 and Figure 5-

FIGURE 5-6:

EXTERNAL BROWN-OUT

PROTECTION CIRCUIT 1

FIGURE 5-7:

EXTERNAL BROWN-OUT

PROTECTION CIRCUIT 2

FIGURE 5-8:

EXTERNAL BROWN-OUT

PROTECTION CIRCUIT 3

This circuit will activate RESET when VDD goes below Vz

+ 0.7V (where Vz = Zener voltage).

33K

10K

40K

VDD

MCLR

PIC16C5X

VDD

This brown-out circuit is less expensive, although

less accurate. Transistor Q1 turns off when VDD

is below a certain level such that:

VDD

R1 + R2

= 0.7V

40K

VDD

MCLR

PIC16C5X

VDD

This brown-out protection circuit employs Micro-

chip

Technology’s

MCP809

microcontroller

supervisor. The MCP8XX and MCP1XX families

of supervisors provide push-pull and open collec-

tor outputs with both "active high and active low"

RESET pins. There are 7 different trip point selec-

tions to accommodate 5V and 3V systems.

MCLR

PIC16C5X

VDD

Vss

RST

MCP809

VDD

bypass

capacitor

VDD

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