MCP131X datasheet(30/44 Pages) MICROCHIP

30 / 44 page

MCP131X/2X

DS21985B-page 30

4.5

Watchdog Timer

The purpose of the Watchdog Timer (WDT) is to

increase system reliability. The Watchdog Timer fea-

ture can be used to detect when the Host Controller’s

program flow is not as expected. The Watchdog Timer

monitors for activity on the Watchdog Input pin (WDI).

The WDI pin is expected to be strobed within a given

time frame. When this time frame is exceeded, without

an edge transition on the WDI pin, the Reset pin is

driven active to reset the system. This stops the Host

Controller from continuing its erratic behavior (“run-

away” code execution).

The Watchdog Timer is external to the main portion of

the control system and monitors the operation of the

system. This feature is enabled by a falling edge on the

WDI pin (after device POR). Monitoring is then done by

requiring the embedded controller to force an edge

transition (falling or rising) on the WDI pin (“pet the

Watchdog”) within a predetermined time frame (TWD).

If the MCP131X/2X does not detect an edge on the

WDI pin within the expected time frame, the MCP131X/

2X device will force the Reset pin active.

The Watchdog Timer is in the disabled state when:

• The Device Powers up

• A POR event occurred

• A WDT event occurred

• A Manual Reset (MR) event occurred

When the Watchdog Timer is in the disabled state, the

WDI pin has an internal smart pull-up resistor enabled.

This pull-up resistor has a typical value of 52 k

Ω. This

pull-up resistor holds the WDI signal in the high state,

until it is forced to another state.

After the embedded controller has initialized, if the

Watchdog Timer feature is to be used, then the embed-

ded controller can force the WDI pin low (VIL). This also

enables the Watchdog Timer feature and disables the

WDI pull-up resistor.

Disabling the pull-up resistor

reduces the device’s current consumption. The pull-up

resistor will remain disconnected until the device has a

power-on, a Reset event occurs, or after the WDT time

out.

Once the Watchdog Timer has been enabled, the Host

Contoller must force an edge transition (falling or rising)

on the WDI pin before the minimum Watchdog Timer

time out to ensure that the Watchdog Timer does not

force the Reset pins (RST/RST) to the active state.

If an edge transition does not occur before the maxi-

mum time out occurs, then the MCP131X/2X will force

the Reset pins to their active state.

The MCP131X/2X supports four time outs. The stan-

dard offering devices have a typical Watchdog Timer

period (TWDT) of 1.6 s. Table 4-3 shows the available

Watchdog Timer periods. The tWDT time out is a

function of the device voltage and temperature.

Figure 4-19 shows a block diagram for using the

MCP131X/2X with a PIC® microcontroller (MCU) and

the Watchdog input.

TABLE 4-3:

WATCHDOG TIMER

PERIODS (1)

FIGURE 4-19:

Watchdog Timer.

The software routine that strobes WDI is critical. The

code must be in a section of software that is executed

frequently enough so the time between edge transi-

tions is less than the Watchdog time out period. One

common technique controls the Host Controllers I/O

line from two sections of the program. The software

might set the I/O line high while operating in the Fore-

ground mode and set it low while in the Background or

Interrupt modes. If both modes do not execute cor-

rectly, the Watchdog Timer issues reset pulses.

tWDT

Units

Min

Typ

Max

4.3

6.3

9.3

102

153

1.12

1.6

2.4

sec

17.9

25.6

38.4

sec

↑↑

If the time between

WDI edges is less

than this, it

ensures that the

MCP131X/2X

never forces a

reset

If the time

between WDI

edges is greater

than this, it

ensures that the

MCP131X/2X

always forces a

reset

Note 1:

Shaded rows are custom ordered Watch-

dog Timer Periods (tWDT) time outs. For

information on ordering devices with

these tWDT time outs, please contact your

local Microchip sales office. Minimum

purchase volumes are required.

VCC

GND

RST

WDI

MCLR

+5V

MCP13XX

0.1

10 k

I/O

PIC®

3-Terminal

Regulator

+5V

µF

MCU

(example:

MCP1700)

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