33812 datasheet(18/25 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33812

FUNCTIONAL DEVICE OPERATION

WATCHDOG TIMER OPERATION

The purpose of the watchdog timer is to provide a RESET

to the MCU whenever the MCU is locked up in a loop or

otherwise hung up, perhaps by executing erroneous code,

such as a HALT instruction. The watchdog timer is initialized

by a power on RESET or a RESET that occurs after a fault

such as an under-voltage event on VCC.

Whenever the watchdog timer is refreshed, it is always

reloaded with the value WDRELOAD which initially has a value

of WDMAX seconds. Whenever a RESET occurs, the

WDRELOAD value is set to WDMAX seconds and the watchdog

timer is re-loaded with this value. When the RESET pulse

returns high, and, if the WDRFSH line is low, the watchdog

timer starts counting. If the watchdog timer reaches the

WDMAX value before the next rising edge on the WDRFSH

line, the watchdog circuit will generate a RESET pulse to the

MCU and reload itself with the maximum time value of

WDRELOAD, which will have been set back to WDMAX

seconds.

In normal operation, the MCU will issue a WDRFSH pulse,

periodically, which re-loads the watchdog timer with the

WDRELOAD value and starts the counting again, thus avoiding

a watchdog timer generated RESET pulse. When the

watchdog timer is refreshed by a WDRFSH pulse, before the

watchdog timer reaches the programmed value, the refresh

will prevent a RESET pulse from being issued to the MCU.

Loading the Watchdog Timer and WDRELOAD

Aside from the RESET case, which always loads the

WDRELOAD value and the watchdog timer with the maximum

time value, WDMAX, there is an additional way that the

watchdog timer and the value WDRELOAD can be re-loaded.

During initialization, if the WDRFSH pulse width is greater

than WDLOAD, both the watchdog timer and the value

WDRELOAD will be loaded with a timer count value,

corresponding to the width of the pulse present on the

WDRFSH input. Once this value is set, no further setting of

the WDLOAD value is possible until a RESET is performed.

Once the WDRFSH input goes low, the watchdog timer will

begin incrementing again, counting up to the new value that

has been loaded into the reload register. The watchdog must

be refreshed by another pulse on the WDRFSH line, before

the watchdog timer counts up to the reload value, or else a

RESET pulse will be generated and sent to the MCU.

If the WDRFSH line is ever kept high for longer than

WDRELOAD seconds, the watchdog will issue an immediate

RESET to the MCU. Upon receiving a RESET input from the

33812, the MCU should always be programmed to bring the

WDRFSH line low to avoid being locked in a “deadly

embrace” condition where the MCU and 33812 alternate

back and forth between the RESET and Normal states.

Disabling the Watchdog Timer

If the WD_INH line is pulled high through a pull-up resistor

of 10 K or less, (i.e. not tied to ground), the watchdog timer

will be inhibited from issuing a RESET to the MCU, while the

line is held in this state. This “watchdog Inhibited” state

should only be used during software testing and development

to avoid being concerned about an inadvertent watchdog

RESET.

Watchdog Timing Diagrams

Watchdog Loaded with WDMAX

Time

WDRFSH

Voltage

Time

Voltage

Time

RESET

Watchdog timer

WDRFSH

PWA<WDLOAD

RESET loads the watchdog timer and WDRELOAD with WDMAX

Refresh pulses, PWA, on WDRFSH load the Watchdog timer with the

WDRELOAD

Voltage

Time

WDRFSH

PWB >WDLOAD

During initialization, for the first WDRFSH pulse only, PWB, that is greater

than WDLOAD but less than WDMAX, the Watchdog timer and WDRELOAD

value will be loaded with a time value corresponding to the width of that

pulse, PWB. All pulses on the WDRFSH line width less than WDRELOAD, will

result in the Watchdog timer being reloaded with the time value correspond-

ing to PWB. This programmability is only allowed once per RESET.

Holding WDRFSH high will trigger RESET every WDMAX time

& WDRELOAD= WDMAX

33812 Datasheet(PDF) 18 Page - Freescale Semiconductor, Inc

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