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MC68349 USER'S MANUAL
MOTOROLA
3.5.4 Double Bus Fault
A double bus fault results when a bus error or an address error occurs during the
exception processing sequence for any of the following:
A previous bus error
A previous address error
A reset
For example, the MC68349 attempts to stack several words containing information about
the state of the machine while processing a bus error exception. If a bus error exception
occurs during the stacking operation, the second error is considered a double bus fault.
When a double bus fault occurs, the MC68349 halts and asserts HALT. Only a reset
operation can restart a halted MC68349. However, bus arbitration can still occur (see 3.6
Bus Arbitration). A second bus error or address error that occurs after exception
processing has completed (during the execution of the exception handler routine or later)
does not cause a double bus fault. A bus cycle that is retried does not constitute a bus
error or contribute to a double bus fault. The MC68349 continues to retry the same bus
cycle as long as the external hardware requests it.
Reset can also be generated internally by the halt monitor (see Section 5 CPU030).
3.6 BUS ARBITRATION
The bus design of the MC68349 provides for a single bus master at any one time, either
the MC68349 or an external device. One or more of the external devices on the bus can
have the capability of becoming bus master for the external bus, but not the MC68349
internal bus. Bus arbitration is the protocol by which an external device becomes bus
master; the bus controller in the MC68349 manages the bus arbitration signals so that the
MC68349 has the lowest priority. External devices that need to obtain the bus must assert
the bus arbitration signals in the sequences described in the following paragraphs.
Systems having several devices that can become bus master require external circuitry to
assign priorities to the devices so that, when two or more external devices attempt to
become bus master at the same time, the one having the highest priority becomes bus
master first. The sequence of the protocol is as follows:
1. An external device asserts BR.
2. The MC68349 asserts BG to indicate that the bus is available.
3. The external device asserts BGACK to indicate that it has assumed bus mastership.
NOTE
The MC68349 does not place CS3–CS0 in a high-impedance
state after reset or when the bus is granted to an external
master.