TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-7
V2.0, 2007-07
EBU, V2.0
13.3
External Bus Arbitration
External bus arbitration is provided to allow the EBU to share its external bus with other
master devices. This capability allows other external master devices to obtain ownership
of the external bus, and to use the bus to access external devices connected to this bus.
The scheme provided by the EBU is compatible with other TriCore and C166 devices
and therefore allows the use of such devices as (external bus) masters together with the
TC1796.
Note: In this section, the term “external master” is used to denote a device which is
located on the
external
bus and is capable of generating accesses across the
external bus (i.e. is capable of driving the external bus). An external master is not
able to access units that are located inside the TC1796.
13.3.1
External Bus Modes
The EBU can operate in two bus modes on the external bus:
•
Owner Mode
•
Hold Mode
When in Owner Mode, the EBU operates as the master of the external bus. In other
words, the EBU drives the external bus as required in order to access devices located
on the external bus. While the EBU is in Owner Mode it is not possible for any other
master to perform any accesses on the external bus.
During Hold Mode, the EBU tri-states the appropriate signal on the external bus in order
to allow another external bus master to perform accesses on the external bus (i.e. to
allow another master to drive the various external bus signals without contention with
EBU).
13.3.2
Arbitration Signals and Parameters
The arbitration scheme consists of an external bus master that is responsible for
controlling the allocation of the external bus. This master is referred to as the “Arbiter”
within this document. The other external bus master (termed Participant within this
document) requests ownership of the bus, and when necessary, from the Arbiter. The
EBU can be programmed to operate either as an Arbiter or as a Participant (see
). The following three lines are used by the EBU to arbitrate the external bus.