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Bus Operation
7-46 M68060 USER’S MANUAL MOTOROLA
7.9 BUS EXCEPTION CONTROL CYCLES
The MC68060 bus architecture requires assertion of TA from an external device to signal
that a bus cycle is complete. TA is not asserted in the following cases:
The external device does not respond.
No interrupt vector is provided.
Various other application-dependent errors occur.
External circuitry can provide TEA when no device responds by asserting TA within an
appropriate period of time after the processor begins the bus cycle. This allows the cycle to
terminate and the processor to enter exception processing for the error condition. A retry
may be indicated by asserting TEA in combination with TA in the MC68040 acknowledge
termination mode or by asserting TRA if in the native-MC68060 acknowledge termination
mode.
To properly control termination of a bus cycle for a bus error or retry condition, TA and TEA
must be asserted and negated about the same rising edge of BCLK when using the
MC68040 acknowledge termination mode. Table 7-5 lists the control signal combinations
and the resulting bus cycle terminations. Bus error and retry terminations during burst cycles
operate as described in 7.7.2 Line Read Transfer and 7.7.4 Line Write Cycles
7.9.1 Bus Errors
The system hardware can use the TEA signal to abort the current bus cycle when a fault is
detected. A bus error is recognized during a bus cycle when TA is negated and TEA is
asserted (MC68040 acknowledge termination mode) or during a bus cycle when TEA is
asserted (native-MC68060 acknowledge termination mode). Also, for the MC68040
acknowledge termination mode, a retry termination during the 2nd, 3rd, or 4th long word of
a line transfer is interpreted as a bus error termination. This rule applies also for the second,
third, and fourth long-word transfer on a line transfer that was burst inhibited.
Table 7-5. Termination Result Summary
Acknowledge
Termination
Mode
TA TEA TRA Result
MC68040 High Low High Bus Error—Terminate and Take Bus Error Exception,
Possibly Deferred
Native-MC68060 Don’t Care Low Don’t Care
MC68040 1Low Low High
Retry Operation—Terminate and Retry
Native-MC680602Don’t Care High Low
Either Low High High Normal Cycle Terminate and Continue
Either High High High Insert Wait States
MC68040 Don’t Care Don’t Care Low Illegal operation, Not Supported
NOTES:
1. A retry termination in MC68040-mode is valid only for the first long word of a line transfer and is considered a
bus error termination otherwise. Note that for burst-inhibited line transfers, the resulting long-word bus cycles
are considered part of the original line transfer and would therefore cause a bus error termination as well.
2. A retry termination in native-MC68060-mode is valid only for the first long word of a line transfer it is ignored
otherwise. Note that for burst-inhibited line transfers, the resulting long-word bus cycles are considered part of
the original line transfer and would therefore ignore the retry termination as well.
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