Low-Power (Standby) Modes - Motorola MC68302 User Manual

clock cycles has elapsed. The hardware watchdog logic has a 10-bit down-
counter and a 4-bit prescaler. When enabled, the watchdog timer commences
counting clock cycles as AS is asserted (for internal or external bus masters).
The count is terminated normally by the negation of AS; however, if the
count reaches zero before AS is negated, SERR will be asserted.
The hardware watchdog logic uses four bits in the SCR.
HWDEN - Hardware Watchdog Enable
0 =The hardware watchdog is disabled.
1 =The hardware watchdog is enabled.
After system reset, this bit defaults to one to enable the hardware watch-
dog.
HWDCN2-HWDCNO - Hardware Watchdog Count 2-0
000 =SERR is asserted after 128 clock cycles (8 µs, 16-MHz clock)
001 =SERR is asserted after 256 clock cycles (16 µs, 16-MHz clock)
010=SERR is asserted after 512 clock cycles (32 µs, 16-MHz clock)
011 =SERR is asserted after 1 K clock cycles (64 µs, 16-MHz clock)
100=SERR is asserted after 2K clock cycles (128 µs, 16-MHz clock)
101 = BERR is asserted after 4K clock cycles (256 µs, 16-MHz clock)
110= BERR is asserted after 8K clock cycles (512 µs, 16-MHz clock)
111
=
BERR is asserted after 16K clock cycles ( 1 ms, 16-MHz clock)
After system reset, these bits default to all ones; thus, BERR will be asserted
after 1 ms for a 16-MHz system clock.
NOTE
Successive timeouts of the hardware watchdoq may vary slightly in
length. The counter resolution is 16 clock cycles.
3.8. 7 Low-Power (Standby) Modes
The low-power modes are used when no processing is required from the
M68000 core processor and when it is desirable to reduce system power
consumption to its minimum value. Each low-power mode is entered by
issuing the M68000 STOP instruction with low power enable (LPEN) set. This
mode stops processor activity while the system control block reduces the
processor clock frequency to achieve minimal power consumption. The user
should disable all unrequired on-chip peripherals to obtain the minimal power
consumption for a given clock frequency (IMP peripherals consume almost
no powerwhen disabled).
MOTOROLA
MC68302 USER'S MANUAL
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