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TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-12
V2.0, 2007-07
EBU, V2.0
1. The external master wants to perform an external bus access by asserting a low
signal on the HOLD input.
2. When the EBU is able to release bus ownership, it enters Hold Mode by tri-stating its
bus interface lines and drives HLDA = 0 to indicate that it has released the bus. At
this point, the external master ia allowed to drive the bus.
3. Two clock (LMBCLK) cycles after issuing HLDA low, the EBU drives BREQ low in
order to regain bus ownership. This bus request is issued whether or not the EBU has
a pending external bus access. However, the external master will ignore this signal
until it has finished its bus access. This scheme assures that the external master can
perform at least one complete external bus access.
4. When the external master has completed its access, it tri-states its bus interface and
sets HOLD to inactive (high) level to signal that it has released the bus back to the
EBU.
5. When the EBU detects that the bus has been released, it returns HLDA to high level
and returns to Owner Mode by actively driving the bus interface lines. There is always
at least one clock (LMBCLK) cycle delay from the release of the HOLD input to the
EBU driving the bus.
6. Finally, the EBU deactivates the BREQ signal one clock (LMBCLK) cycle after
deactivation of HLDA. Now (and not earlier) the external master can generate a new
hold request to the EBU.
This sequence assures that the EBU can perform at least one complete bus cycle before
it re-enters Hold Mode as a result of a request from the external master.
The conditions that cause change of bus ownership when the EBU is operating in Arbiter
Mode are shown in