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IDT Bus Arbitration
Memory and Peripheral Bus Arbitration
79RC32438 User Reference Manual
5 - 15
November 4, 2002
Notes
When the external bus master observes BGN asserted, it owns the memory and peripheral bus and may
drive the above signals. The external bus master maintains BRN asserted during the entire time it owns the
memory and peripheral bus. It relinquishes ownership by negating BRN
1
and tri-stating the above memory
and peripheral bus signals. The RC32438 acknowledges that external ownership of the bus has been relin-
quished by negating BGN
2
, and it begins driving the memory and peripheral bus signals. This process is
illustrated in Figure 5.17.
The RC32438 may request that an external bus master relinquish ownership of the memory and periph-
eral bus early, for example due to a higher priority internal pending transaction, by negating BGN while BRN
is asserted. When the external bus master observes this, it relinquishes ownership by negating BRN and tri-
stating the memory and peripheral bus signals. The RC32438 regains ownership and may drive the
memory and peripheral bus signals when it observes BRN negated. This process is illustrated in Figure
7.17. Since BRN is an asynchronous input to the RC32438, which is double sampled, it must be negated for
at least three EXTCLK clock cycles before being asserted.
When the RC32438 owns the memory and peripheral bus and there are no device controller transac-
tions in progress, the RC32438 may drive or tri-state the data bus (MDATA[15:0]).
Figure 5.17 External Bus Arbitration
Figure 5.18 External Bus Arbitration with RC32438 Requesting that Ownership Be Relinquished
1.
Once an external bus master has relinquished ownership of the bus by negating BRN, it should not assert BRN
until the RC32438 acknowledges the negation of BRN by negating BGN.
2.
It is guaranteed that the RC32438 will assert BGN for no less than three EXTCLK clock cycles.
EXTCLK
BRN
BGN
ownership
1
4
5
2
3
RC32438 Owns Bus
RC32438 Owns Bus
External Bus Master Owns Bus
1.
2.
3.
4.
5.
External bus master requests ownership of memory and peripheral bus by asserting BRN.
The RC32438 tri-states memory and peripheral bus signals and asserts BGN to indicate that it has relinquished ownership of the bus.
When the external bus master observes BGN asserted, it drives memory and peripheral bus signals.
External bus master relinquishes ownership of memory and peripheral bus by negating BRN.
The RC32438 acknowledges ownership and begins driving memory and peripheral bus signals.
EXTCLK
BRN
BGN
ownership
1
5
6
2
3
RC32438 Owns Bus
RC32438 Owns Bus
External Bus Master Owns Bus
4
1.
2.
3.
4.
5.
6.
External bus master requests ownership of memory and peripheral bus by asserting BRN.
The RC32438 tri-states memory and peripheral bus signals and asserts BGN to indicate that it has relinquished ownership of the bus.
When the external bus master observes BGN asserted, it drives memory and peripheral bus signals.
The RC32438 requests that the external bus master relinquish ownership of the memory and peripheral bus by negating BGN
The external bus master relinquishes ownership of the memory and peripheral bus by negating BRN.
The RC32438 observes that ownership of the bus has been relinquished and begins driving memory and peripheral bus
signals.