TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-19
V2.0, 2007-07
EBU, V2.0
As shown in
, this event also triggers the EBU to arbitrate with the external
master in order to attempt to gain ownership of the external bus so that the request can
be serviced when it is re-submitted by the master. This strategy ensures that the PLMB
remains available while the EBU arbitrates for the external bus.
13.3.6.1 Pending Access Time-Out
The strategy of issuing a retry (when the EBU is not the owner of the external bus) as
described in the previous section may result in the occurrence of a time-out condition.
When a PLMB master issues a request for an external bus access that is rejected by the
EBU with a retry (in order to retain PLMB availability) and arbitration for external bus
ownership is started, the external bus ownership is given once to the EBU. Now the EBU
waits until the next external bus access occurs on the PLMB. If the requesting PLMB
master or any other PLMB master subsequently performs no external bus accesses (e.g.
fails to re-submit the original access request), the EBU has ownership of the bus for an
indefinite time and it would become impossible for an external master to access the
external bus.
To avoid such a bus-locking condition, the EBU contains a time-out mechanism. When
the EBU has gained ownership of the external bus, it will retain ownership only until a
PLMB-to-external bus access occurs or a programmable number of LMBCLK clock
cycles has elapsed. If one of these conditions has occurred, the pending access is
cancelled and the EBU will continue to arbitrate the external bus in the normal fashion.
The desired time-out time (number of LMBCLK cycles) is programmed using bit field
TIMEOUTC. The time-out value can be in the range 1
×
8 up to 255
×
8 LMBCLK clock
cycles.