
MOTOROLA
MC68330 USER’S MANUAL
2- 7
2.8.5 Byte Write Enable (
UWE, LWE)
On a write cycle to a 16-bit port, these active-low output signals indicate when the upper
or lower eight bits of the data bus contain valid data. See 3.1.7 Byte Write Enable for a
description of byte write enable operation.
2.9 EXCEPTION CONTROL SIGNALS
These signals are used by the integrated processor unit to recover from an exception.
2.9.1 Reset (
RESET)
This active-low, open-drain, bidirectional signal is used to initiate a system reset. An
external reset signal (as well as a reset from the SIM) resets the MC68330 as well as all
external devices. A reset signal from the CPU32 (asserted as part of the RESET
instruction) resets external devices only — the internal state of the CPU32 is not affected;
other on-chip modules are reset, but the configuration is not altered. When asserted by the
MC68330, this signal is guaranteed to be asserted for a minimum of 512 clock cycles.
Refer to 3.7 Reset Operation for a description of bus reset operation and Section 5
CPU32 for information about the reset exception.
2.9.2 Halt (
HALT)
This active-low, open-drain, bidirectional signal is asserted to suspend external bus
activity, to request a retry when used with
BERR, or to perform a single-step operation. As
an output,
HALT indicates a double bus fault by the CPU32. Refer to 3.5 Bus Exception
Control Cycles for a description of the effects of
HALT on bus operation.
2.9.3 Bus Error (
BERR)
This active-low input signal indicates that an invalid bus operation is being attempted or,
when used with
HALT, that the processor should retry the current cycle. Refer to 3.5 Bus
Exception Control Cycles for a description of the effects of
BERR on bus operation.
2.10 CLOCK SIGNALS
These signals are used by the MC68330 for controlling or generating the system clocks.
Refer to 4.2.3 Clock Synthesizer for more information on the various clock signals.
2.10.1 System Clock (CLKOUT)
This output signal is the system clock and is used as the bus timing reference by external
devices. CLKOUT can be slowed in low-power stop mode. See 4.3.3 Clock Synthesizer
Control Register (SYNCR) for more information.
2.10.2 Crystal Oscillator (EXTAL, XTAL)
These two pins are the connections for an external crystal to the internal oscillator circuit.
If an external oscillator is used, it should be connected to EXTAL, with XTAL left open.
See 4.2.3 Clock Synthesizer for more information.
2.10.3 External Filter Capacitor (XFC)