Typewriter User Manual
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
0002
9
/EXTAL Input Frequency 15.6 ms 30 µs
0012
11
/EXTAL Input Frequency 62.5 ms 122 µs
0102
13
/EXTAL Input Frequency 250 ms 488 µs
0112
15
/EXTAL Input Frequency 1 s 1.95 ms
1002
18
/EXTAL Input Frequency 8 s 15.6 ms
1012
20
/EXTAL Input Frequency 32 s 62.5 ms
1102
22
/EXTAL Input Frequency 128 s 250 ms
1112
24
/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.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
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