RM0319 datasheet(72/368 Pages) STMICROELECTRONICS

72 / 368 page

Multiport DDR controller (MPMC)

RM0319

72/368

Doc ID 022640 Rev 3

If every port ordering parameters are set to the same value, the scan order will default to the

numbered port order.

Weighted round-robin arbitration summary

The MPMC weighted round-robin arbitration system merges the concepts of round-robin

operation, priority, relative priority and port ordering. The incoming commands are

separated into priority groups based on the priority of the associated port for that type of

command. Inside each priority group, the relative priority values are examined to settle the

arbitration winner. If relative priority values are the same and no individual command can be

selected, the scan order is used to select among the requests.

Finally the highest priority command incoming from the highest relative priority port, having

the highest location in the scan order, will be selected and sent to the core.

For instance, let us consider the system described in Table 22. The counters refer to the

ones inside each port priority group to guarantee that relative priorities are maintained. To

simplify, on assumes the command queue never be full and commands are only received at

priority level 0.

The behaviour is shown inTable 23. The highest requesting port in the scan order always

wins arbitration and the scan order is dynamically modified whenever any port counter

reaches its allocated relative priority value.

Note:

If the command queue was considered, cycles where the command queue was full would

not have any arbitration winner and therefore the counter values and scan order would not

change on that cycle.

Table 21.

Port ordering example

Parameter

System B

System C

ahb0_port_ordering

ahb1_port_ordering

ahb2_port_ordering

ahb3_port_ordering

ahb4_port_ordering

ahb5_port_ordering

ahb6_port_ordering

ahb7_port_ordering

Port Scan Order

P5-P7-P6-P0-P1-P2-P3-P4

P1-P6-P7-P5-P0-P2-P3-P4

Table 22.

System D specifications

Parameter

Port 0

Port 1

Port 2

Port 3

ahbX_priority0_relative_priority

ahbX_port_ordering

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