ST90135 datasheet(99/190 Pages) STMICROELECTRONICS

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ST90158 - TIMER/WATCHDOG (WDT)

TIMER/WATCHDOG (Cont’d)

9.1.3.3 Preventing Watchdog System Reset

In order to prevent a system reset, the sequence

AAh, 55h must be written to WDTLR (Watchdog

Timer Low Register). Once 55h has been written,

the Timer reloads the constant and counting re-

starts from the preset value.

To reload the counter, the two writing operations

must be performed sequentially without inserting

other instructions that modify the value of the

WDTLR register between the writing operations.

The maximum allowed time between two reloads

of the counter depends on the Watchdog timeout

period.

9.1.3.4 Non-Stop Operation

In Watchdog Mode, a Halt instruction is regarded

as illegal. Execution of the Halt instruction stops

further execution by the CPU and interrupt ac-

knowledgment, but does not stop INTCLK, CPU-

CLK or the Watchdog Timer, which will cause a

System Reset when the End of Count condition is

reached. Furthermore, ST_SP, S_C and the Input

Mode selection bits are ignored. Hence, regard-

less of their status, the counter always runs in

Continuous Mode, driven by the internal clock.

The Output mode should not be enabled, since in

this context it is meaningless.

Figure 56. Watchdog Timer Mode

TIMER START COUNTING

WRITEWDTRH,WDTRL

WD EN=0

WRITE AAh,55h

INTOWDTRL

RESET

SOFTWAREFAIL

(E.G. INFINITELOOP)

ORPERIPHERALFAIL

VA00220

PRODUCE

COUNT RELOAD

VALUE

COUNT

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