MOTOROLA
Chapter 14. MPC860 External Bus Interface
14-37
Part IV. Hardware Interface
Figure 14-28. Reservation on Multilevel Bus Hierarchy
In this case, the bussesÕ interface block implements a reservation ßag for the local bus
master. The reservation ßag is set by the bussesÕ interface when a load with the local bus
master issues a reservation whose address is on the remote bus. The ßag is reset when an
alternative master on the remote bus accesses the same location in a write cycle. If the
MPC860 begins a memory cycle to the previously reserved address (located in the remote
bus) as a result of a stwcx., the following two cases can occur:
¥
If the reservation ßag is set, the local bus interface acknowledges the cycle in a
normal way.
¥
If the reservation ßag is reset, the local bus interface should assert
KR
. However, the
local bus interface should either not perform the remote bus write access or abort it
if the remote bus supports aborted cycles. The failure of stwcx. is reported to the
core.
14.4.10 Bus Exception Control Cycles
The MPC860 bus architecture requires assertion of the TA from an external device to signal
that the bus cycle is complete. TA is not asserted in the following cases:
¥
The external device does not respond
¥
Various other application-dependent errors occur
External circuitry or the internal MPC860 bus monitor can provide TEA when no device
responds by asserting TA within an appropriate period of time after the MPC860 initiates
the bus cycle. This allows the cycle to terminate and the processor to enter exception
processing for the error condition (each one of the internal masters causes an internal
interrupt under this situation). To properly control termination of a bus cycle for a bus error,
TEA must be asserted at the same time or before TA is asserted. TEA should be negated
MPC860
External Bus (Local Bus)
External Bus
Interface
Master in the Remote
Bus Write to the
AT[0Ð3], RSV,
A[0Ð31]
KR
S
R
Q
Reserved Location
BussesÕ
Interface
R/W, TS
Remote Bus
Summary of Contents for MPC860 PowerQUICC
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