
Bus Operation
MOTOROLA
MC68307 USER’S MANUAL
3-27
3.4 BUS ERROR AND HALT OPERATION
In a bus architecture that requires a handshake from an external device, such as the asyn-
chronous bus used in the M68000 family, the handshake may not always occur. A bus mon-
itor is provided to terminate a bus cycle in error when the expected signal is not asserted.
Different systems and different devices within the same system require different maximum-
response times. This internal circuitry asserts the internal EC000 core bus error signal after
the appropriate delay following the assertion of address strobe.
3.4.1 Bus Error Operation
When the bus error condition is recognized, the current bus cycle is terminated in S7 for a
read cycle, a write cycle, or the read portion of a read-modify-write cycle. For the write por-
tion of a read-modify-write cycle, the current bus cycle is terminated in S19.
After the aborted bus cycle is terminated, the processor enters exception processing for the
bus error exception. During the exception processing sequence, the following information is
placed on the supervisor stack:
1. Status register
2. Program counter (two words, which may be up to five words past the instruction being
executed)
3. Error information
The first two items are identical to the information stacked by any other exception. The
EC000 core stacks bus error information to help determine and to correct the error.
After the processor has placed the required information on the stack, the bus error exception
vector is read from vector table entry 2 (offset $08) and placed in the program counter. The
processor resumes execution at the address in the vector, which is the first instruction in the
bus error handler routine. Refer to
Figure 3-26 for an example bus error timing diagram.