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External Bus Interface
13-40
MPC801 USER’S MANUAL
MOTOROLA
13
13.4.10 Exception Control Cycles
The MPC801 bus architecture requires the TA signal to be asserted from an external device
to indicate bus cycle completion. TA is not asserted when one of the following conditions
occur:
The external device does not respond
Other application-dependent errors occur
The external circuitry can provide TEA when no device responds by asserting TA within an
appropriate period of time after the MPC801 initiates the bus cycle. This allows the cycle to
terminate and the processor to enter exception processing for the error condition, whereas
each of the internal masters causes an internal interrupt. To properly control the termination
of a bus cycle for a bus error, the TEA signal must be asserted simultaneously or before TA
is asserted. TEA should be negated before the second rising edge after it was
sample-asserted to avoid an error for the next initiated bus cycle. TEA is an open-drain pin
that allows the “wire-OR” of any different error generation sources.
13.4.10.1 RETRY
When an external device asserts the RETRY signal during a bus cycle, the MPC801 enters
a sequence in which it terminates the current transaction, relinquishes the ownership of the
bus, and retries the cycle using the same address, address attributes, and data (for a write
cycle). Figure 13-29 illustrates the behavior of the MPC801 when the RETRY signal is found
as a termination of a transfer. In the figure, it is illustrated that when the internal arbiter is
enabled, MPC801 negates the BB signal and asserts the BG signal in the clock cycle
following the retry detection. This allows any external master to gain bus ownership. In the
next clock cycle, a normal arbitration procedure occurs again. The figure also illustrates that
the external master did not use the bus, so the MPC801 initiates a new transfer with the
same address and attributes as before. In Figure 13-30 the same situation is illustrated to
show that the MPC801 is working with an external arbiter. If the clock cycle after the RETRY
signal is asserted, the BR signal is negated together with the BB signal. One clock cycle
later, the normal arbitration procedure occurs again.