TS68230 datasheet(47/61 Pages) STMICROELECTRONICS

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5.2.4.2. External Clock.

This configuration allows measurement (counting)

of the number of input pulses occurring in an interval

in which the counter is enabled. The TIN input pin

provides the input pulses. Generally the TOUT and

TIACK pins are not used.

This configuration is similar to the elapsed time

measurement/system clock configuration except

that the TIN pin is used to provide the input frequen-

cy. It can be connected to a simple oscillator and the

same methods could be used. Alternately, it could

be gated off and on externally and the number of cy-

cles occurring while in the run state can be counted.

However, minimum pulse width high and low speci-

fications must be met.

5.2.5. DEVICE WATCHDOG.

This configuration provides the watchdog function

needed in many systems. The TIN pin is the timer

input whose period at the high (one) level is to be

checked. Once allowed by the processor, the TIN in-

put pin controls the run/halt mode. The TOUT pin is

connected to external circuitry requiring notification

when the TIN pin has been asserted longer than the

programmed time. The TIACK pin (timer interrupt

acknowledge) is only needed if the TOUT pin is

connected to the interrupt circuitry.

The processor loads the counter preload register

and timer control register, and then enables the ti-

mer. When the TIN input is asserted (one, high) the

timer transfers the contents of the counter preload

TIN input is negated before zero detect is reached,

the TOUT output and the ZDS status bit remain ne-

gated. If zero detect is reached while the TIN input

is still asserted, the ZDS status bit is set and the

TOUT output is asserted. (The counter rolls over

and keeps counting). In either case, when the TIN

input is negated the ZDS status bit is zero, the TOUT

output is negated, the counting stops, and the pres-

caler is forced to all ones (see figure 5.5).

543

Timer

Enable

TOUT/TIACK

Control

Z.D

Control

Clock

Control

X Changed

Timer

Enable

TOUT/TIACK

Control

Z.D

Control

Clock

Control

1 Changed

Figure 5.5 : Device Watchdog Example.

TS68230

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