ST90135 datasheet(97/190 Pages) STMICROELECTRONICS

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ST90158 - TIMER/WATCHDOG (WDT)

TIMER/WATCHDOG (Cont’d)

9.1.2 Functional Description

9.1.2.1 External Signals

The HW0SW1 pin can be used to permanently en-

able Watchdog mode. Refer to Section 0.1.3.1.

The WDIN Input pin can be used in one of four

modes:

– Event Counter Mode

– Gated External Input Mode

– Triggerable Input Mode

– Retriggerable Input Mode

The WDOUT output pin can be used to generate a

square wave or a Pulse Width Modulated signal.

An interrupt, generated when the WDT is running

as the 16-bit Timer/Counter, can be used as a Top

Level Interrupt or as an interrupt source connected

to channel A0 of the external interrupt structure

(replacing the INT0 interrupt input).

The counter can be driven either by an external

clock, or internally by INTCLK divided by 4.

9.1.2.2 Initialisation

The prescaler (WDTPR) and counter (WDTRL,

WDTRH) registers must be loaded with initial val-

ues before starting the Timer/Counter. If this is not

done, counting will start with reset values.

9.1.2.3 Start/Stop

The ST_SP bit enables downcounting. When this

bit is set, the Timer will start at the beginning of the

following instruction. Resetting this bit stops the

counter.

If the counter is stopped and restarted, counting

will resume from the last value unless a new con-

stant has been entered in the Timer registers

(WDTRL, WDTRH).

A new constant can be written in the WDTRH,

WDTRL, WDTPR registers while the counter is

running. The new value of the WDTRH, WDTRL

registers will be loaded at the next End of Count

(EOC) condition while the new value of the

WDTPR register will be effective immediately.

End of Count is when the counter is 0.

When Watchdog mode is enabled the state of the

ST_SP bit is irrelevant.

9.1.2.4 Single/Continuous Mode

The S_C bit allows selection of single or continu-

ous mode.This Mode bit can be written with the

Timer stopped or running. It is possible to toggle

the S_C bit and start the counter with the same in-

struction.

Single Mode

On reaching the End Of Count condition, the Timer

stops, reloads the constant, and resets the Start/

Stop bit. Software can check the current status by

reading this bit. To restart the Timer, set the Start/

Stop bit.

Note: If the Timer constant has been modified dur-

ing the stop period, it is reloaded at start time.

Continuous Mode

On reaching the End Of Count condition, the coun-

ter automatically reloads the constant and restarts.

It is stopped only if the Start/Stop bit is reset.

9.1.2.5 Input Section

If the Timer/Counter input is enabled (INEN bit) it

can count pulses input on the WDIN pin. Other-

wise it counts the internal clock/4.

For instance, when INTCLK = 20MHz, the End Of

Count rate is:

3.35 seconds for Maximum Count

(Timer Const. = FFFFh, Prescaler Const. = FFh)

200 ns for Minimum Count

(Timer Const. = 0000h, Prescaler Const. = 00h)

The Input pin can be used in one of four modes:

– Event Counter Mode

– Gated External Input Mode

– Triggerable Input Mode

– Retriggerable Input Mode

The mode is configurable in the WDTCR.

9.1.2.6 Event Counter Mode

In this mode the Timer is driven by the external

clock applied to the input pin, thus operating as an

event counter. The event is defined as a high to

low transition of the input signal. Spacing between

trailing edges should be at least 8 INTCLK periods

(or 400ns with INTCLK = 20MHz).

Counting starts at the next input event after the

ST_SP bit is set and stops when the ST_SP bit is

reset.

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