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ST9 USER GUIDE

Figure 33

. Output Pin Block Diagram

4.3.2 Timer Application for Periodic Interrupts

In this application, the clock is internal and the input and output pins are unused. The counter

is set to Continuous mode, and the value of the reload registers chosen so that the overflow

occurs exactly every 128 microseconds.

4.3.2.1 Initialisation of the WDT for a Periodic Interrupt

Very few registers are involved when you use the WDT for this purpose, since there is no input

apart from the internal clock and no output. However, the WDT interrupt handling is a little dif-

ferent from most other peripherals in that it borrows the interrupt circuit named INTA0 that is

normally assigned to external interrupt pin INT0. So you configure the WDT in two steps: ini-

tializing the WDT itself, and also the external interrupt INTA0.

In this example application, the internal frequency is 24 MHz and the interrupt rate must be

8192 Hz. The timer starts with the preset count on the low-to-high transition of the ST_SP bit

of the WDT Control Register.

The initialisation routine for the WDT is then:

WDTCR (R251 page 0)

Timer/Watchdog Control Register

Logical level user definable

while counter is running

WDTOUT

EOC

ST_SP S_C INMD1 INMD2

OUTMD WROUT

INEN

OUTEN

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