TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-17
V2.0, 2007-07
EBU, V2.0
13.3.4
Arbitration Input Signal Sampling
The sampling of the arbitration inputs can be programmed for two modes:
•
Synchronous Arbitration
•
Asynchronous Arbitration
When synchronous arbitration signal sampling is selected (ARBSYNC = 0), the
arbitration input signals are sampled and evaluated in the same clock cycle. This mode
provides the least overhead during arbitration (i.e. when changing bus ownership). The
disadvantage is that the input signals must adhere to setup and hold times with respect
to LMBCLK to prevent the propagation of meta-stable signals in the EBU.
When asynchronous arbitration signal sampling is selected (ARBSYNC = 0), the
arbitration signals are sampled and then fed to an additional latch to be evaluated in the
cycle following that in which they were sampled. This provides the EBU with good
immunity to signals changing state at or around the time at which they are sampled. The
disadvantage is the introduction of additional latency during arbitration (i.e. when
changing bus ownership).
13.3.5
Locking the External Bus
The external bus can be locked to allow the EBU to perform uninterrupted sequences of
external bus accesses. The EBU allows two methods of locking the external bus:
•
Locked PLMB accesses
•
Lock bit EXTLOCK
When the EBU has ownership of the external bus and is performing external bus
accesses in response to a locked PLMB access sequence, the ownership of the external
bus will not be relinquished until the locked PLMB access sequence has been
completed.
When lock bit EXTLOCK = 1, the EBU will hold the ownership of the external bus until
EXTLOCK is subsequently cleared. If EXTLOCK is written to 1 while the EBU is the
owner of the external bus, the EBU is immediately prevented from losing the ownership
of the external bus until EXTLOCK is cleared. If EXTLOCK is written to 1 while the EBU
is not the owner of the external bus, no immediate effect occurs. When the EBU gains
the ownership of the external bus the next time, the external master is prevented from
regaining ownership of the external bus until EXTLOCK is again cleared.
Note: There is no time-out mechanism available for the EXTLOCK bit. When the EBU is
owner of the external bus with EXTLOCK bit set, the external master will remain
locked off the bus until the EXTLOCK bit is cleared by software.