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SYSTEM CONTROL MODULE
System bus arbiter
140 Hardware Reference NS9215
If the bus is granted to a default master and continues to be in the IDLE state
longer than a specified period of time, an AHB bus arbiter timeout is
generated. An AHB bus arbiter timeout can be configured to interrupt the CPU
or to reset the chip.
SPLIT transfers A SPLIT transfer occurs when a slave is not ready to perform the transfer. The slave
splits, or masks, its master, taking away the master’s bus ownership and allowing
other masters to perform transactions until the slave has the appropriate resources
to perform its master’s transaction.
The bus arbiter supports SPLIT transfers. When a SPLIT response is issued by a slave,
the current master is masked for further bus requesting until a corresponding
hsplit_x[15:0] signal is issued by the slave indicating that the slave is ready to
complete the transfer. The arbiter uses the
hsplit_x[15:0] signals to unmask the
corresponding master, and treats the master as the highest-priority requester for
the immediate next round of arbitration. The master eventually is granted access to
the bus to try the transfer again.
Note:
The arbiter automatically blocks bus requests with addresses directed at a
“SPLITting” slave until that SPLIT transaction is completed.
Arbiter
configuration
example
This example shows how to configure the AHB arbiter to guarantee bandwidth to a
given master. These are the conditions in this example:
5 AHB masters — CPU, Ethernet Rx, Ethernet Tx, IO hub, and external DMA
AHB clock frequency — 75 MHz
Average access time per 16-byte memory access — 4 clock cycles
The ARM926EJ-S is guaranteed one-half the total memory bandwidth
In this example, the bandwidth for each master can be calculated using this
formula:
Bandwidth per master:
= [(75MHz/2) / (4 clock cycles per access x 5 masters)] x 16 bytes
= 60MB/master
Note:
The worst case scenario is that there are 90 Mbps total to be split by all 5
masters.
if this meets the requirements of all the masters, the AHB arbiter is programmed
like this:
BRC0[31:24] = 8’b1_0_00_0000 channel enabled, 100%, ARM7EJ-S
BRC0[23:16] = 8’b1_0_00_0001 channel enabled, 100%, Ethernet Rx
BRC0[15:8] = 8’b1_0_00_0000 channel enabled, 100%, Ethernet TX
BRC0[7:0] = 8’b1_0_00_0101 channel enabled, 100%, IO hub
BRC1[31:24] = 8’b1_0_00_0011 channel enabled, 100%, Ext DMA