TC32MCDBTR データシートの表示（PDF） - Microchip Technology

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TC32MCDBTR

ECONOMONITOR™ – 3-Pin System Supervisor with Power Supply Monitor and Watchdog

Microchip Technology 

3.0 DETAILED DESCRIPTION

The TC32M provides three important functions to

safeguard stable processor operation: precision pro-

cessor monitor, watchdog sanity timer and external

override reset control.

3.1 Processor Monitor

The RS pin is immediately driven low any time VDD is

below the nominal threshold voltage. As a result, this

pin is LOW when power is initially applied, holding the

processor in its reset state. RS remains low for a

minimum of 500msec after VDD is within tolerance to

allow the power supply and processor to stabilize.

3.2 Watchdog Timer

The processor drives the RS pin with an input/output

(I/O) line in series with an resistor voltage divider to

VDD. Pulling the bottom resistor of this divider low

results in an internal voltage change (strobe) sufficient

to reset the watchdog timer, but above the VIL input

threshold of the processor RESET pin. The processor

must continuously apply strobes in this manner within

a set period to verify proper software execution. A

momentary reset (500msec minimum) is generated by

the TC32M if a hardware or software failure keeps RS

from being strobed within the watchdog timeout period.

This action typically initiates the processor's power-up

routine. If the interruption persists, new reset pulses

are generated each timeout period until RS is strobed.

This timeout period is typically 700msec.

TC32M

FIGURE 3-1:

RS

VSTH (MIN)

VSTL (MAX)

WATCHDOG STROBE

tST

VSTL (MIN)

tTD

Note: tTD is the maximum elapsed time between strobes which

will keep the watchdog timer from forcing RS LOW.

(A STROBE is defined as a high-to-low transition from VSTH to VSTL).

The software routine that drives the RS strobe must be

in a section of the program that executes frequently

enough so the time between toggles is less than one

watchdog timeout period. The strobe signal can be

derived from microprocessor address, data and/or

control signals. Typical circuit examples are shown in

Figure 3-4.

3.3 Resistor Value Selection

The values of R1 and R2 must be chosen to ensure a

valid low strobe level (VSTL) on RS when the processor

I/O line is low. The use of 10kΩ, ±5% tolerance

resistors are recommended. These values result in a

nominal strobe level of 2.5 on RS (min/max of 2.13V/

3.08V, assuming VDD = 5.0V ±10%). Other resistor

values can be used, so long as the additive tolerances

of the power supply and resistor values result in a

strobe that falls within VSTH and VSTL under all additive

tolerance conditions.

© 2002 Microchip Technology Inc.

DS21402B-page 5

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