MC68340 datasheet(122/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

4- 25

SWT1, SWT0—Software Watchdog Timing

These bits, along with the SWP bit in the PITR, control the divide ratio used to establish

the timeout period for the software watchdog. The software watchdog timeout period is

given by the following formula:

divide count

EXTAL frequency

The software watchdog timeout period, listed in Table 4-7, gives the formula to derive the

software watchdog timeout for any clock frequency. The timeout periods are listed for a

32.768-kHz crystal used with the VCO and for a 16.777-MHz external oscillator.

Table 4-7. Deriving Software Watchdog Timeout

SWP

SWT1

SWT0

Software Timeout Period

32.768-kHz

Crystal Period

16.777-MHz External

Clock Period

29/EXTAL Input Frequency

15.6 ms

µs

211 /EXTAL Input Frequency

62.5 ms

122

µs

213 /EXTAL Input Frequency

250 ms

488

µs

215 /EXTAL Input Frequency

1 s

1.95 ms

218 /EXTAL Input Frequency

8 s

15.6 ms

220 /EXTAL Input Frequency

32 s

62.5 ms

222 /EXTAL Input Frequency

128 s

250 ms

224 /EXTAL Input Frequency

512 s

1 s

NOTE:

When the SWP and SWT bits are modified to select a software timeout other than the default, the

software service sequence ($55 followed by $AA written to the software service register) must be

performed before the new timeout period takes effect. Refer to 4.2.2.5 Software Watchdog for

more information.

DBFE—Double Bus Fault Monitor Enable

1 = Enable double bus fault monitor function.

0 = Disable double bus fault monitor function.

For more information, see 4.2.2.3 Double Bus Fault Monitor and Section 5 CPU32.

BME—Bus Monitor External Enable

1 = Enable bus monitor function for an internal-to-external bus cycle.

0 = Disable bus monitor function for an internal-to-external bus cycle.

For more information see 4.2.2.2 Internal Bus Monitor.

BMT1, BMT0—Bus Monitor Timing

These bits select the timeout period for the bus monitor (see Table 4-8). Upon reset, the

bus monitor is set to 64 system clocks.

Freescale Semiconductor, Inc.

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