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MC68349 USER'S MANUAL
MOTOROLA
2.5 EXCEPTION CONTROL SIGNALS
These signals are used by the MC68349 to recover from an exception.
2.5.1 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 SIM49) resets the MC68349 and all
external devices. A reset signal from the CPU32+ (asserted as part of the RESET
instruction) resets external devices; the internal state of the CPU32+ is not affected. The
on-chip modules are reset, except for the SIM49. However, the module configuration
register for each on-chip module is not altered. When asserted by the MC68349, this
signal is guaranteed to be asserted for a minimum of 512 clock cycles. Refer to Section 3
Bus Operation for a description of reset operation and Section 5 CPU030 for information
about the reset exception.
2.5.2 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 Section
3 Bus Operation for a description of the effects of BERR on bus operation.
2.5.3 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 the processor has halted due to a double bus fault. Refer to
Section 3 Bus Operation for a description of the effects of HALT on bus operation.
2.6 CHIP SELECTS (
CS3—CS0)
These pins can be programmed to be chip select output signals, port B parallel I/O and
autovector input, or additional interrupt request lines. Refer to Section 4 System
Integration Module for more information on these signals.
CS3–CS0
The chip select output signals enable peripherals at programmed addresses. These
signals are inactive high (not high impedance) after reset. CS0 is the chip select for a
boot ROM containing the reset vector and initialization program and functions as the
boot chip select immediately after reset.
IRQ4, IRQ2, IRQ1
Interrupt request lines are external interrupt lines to the CPU32+. These additional
interrupt request lines are selected by the FIRQ bit in the module configuration register.
Port B4, B2, B1,
AVEC
This signal group functions as three bits of parallel I/O and the autovector input. AVEC
requests an automatic vector during an interrupt acknowledge cycle.