MC68340 datasheet(104/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

4- 7

interrupt (as programmed by the SWRI bit in the SYPCR). The address of the interrupt

service routine for the software watchdog interrupt is stored in the software interrupt vector

the clock control circuits for the periodic interrupt timer.

The watchdog clock rate is determined by the SWP bit in the periodic interrupt timer

timeout periods.

The software watchdog service sequence consists of the following steps: 1) write $55 to

the software service register (SWSR) and 2) write $AA to the SWSR. Both writes must

occur in the order listed prior to the watchdog timeout, but any number of instructions or

accesses to the SWSR can be executed between the two writes.

CLOCK

DISABLE

PRESCALER (2 )

CLOCK

MUX

EXTAL

15 STAGE DIVIDER CHAIN (2 )

LPSTOP

SWP

PTP

FREEZE

SWCLK

PITCLK

PRECLK

RESET

MODULUS COUNTER

PIT

INTERRUPT

PITR

Figure 4-3. Software Watchdog Block Diagram

4.2.2.6 PERIODIC INTERRUPT TIMER. The periodic interrupt timer consists of an 8-bit

modulus counter that is loaded with the value contained in the PITR (see Figure

4-3). The modulus counter is clocked by a signal derived from the EXTAL input pin unless

an external frequency source is used. When an external frequency source is used

(MODCK low during reset), the default state of the prescaler control bits (SWP and PTP)

in the PITR is changed to enable both prescalers.

Either clock source (EXTAL or EXTAL

÷ 512) is divided by 4 before driving the modulus

counter (PITCLK). When the modulus counter value reaches zero, an interrupt is

generated. The level of the generated interrupt is programmed into the PIRQL bits in the

periodic interrupt control register (PICR). During the IACK cycle, the SIM40 places the

periodic interrupt vector, programmed into the PIV bits in the PICR, onto the internal bus.

The value of bits 7–0 in the PITR is then loaded again into the modulus counter, and the

counting process starts over. If a new value is written to the PITR, this value is loaded into

the modulus counter when the current count is completed.

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