RM0319 datasheet(68/368 Pages) STMICROELECTRONICS

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Multiport DDR controller (MPMC)

RM0319

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Doc ID 022640 Rev 3

The weighted round-robin arbitration scheme is a three-step arbitration system. All

commands are routed into priority groups based on the priority of the requests. Inside each

priority group, requests are processed according to the “weight” (relative priority) of each

port. Finally, each priority group sends a single command to the priority select module,

which passes the highest priority command on to the memory controller core.

This arbitration scheme also supports two additional features. First, for situations where the

priority (port and relative) for multiple commands are identical, a port ordering system

whereby the user may adjust the order in which the ports are considered is provided.

Beside, for situations where two ports may be linked, a mechanism allowing the pair of ports

to share arbitration bandwidth for a better bandwidth efficiency, is also supplied.

Weighted round-robin arbitration is a complex arbitration scheme. To understand the

operation, each concept must be first understood individually. This will be matter of the

following Sections.

Understanding round-robin operation

Round-robin operation is the simplest form of arbitration and is the best one for systems not

requiring requests to be treated preferentially to maintain bandwidth or minimize latency.

This scheme uses a counter that rotates through the port numbers, incrementing every time

a port request is granted.

If the port that the counter is checking has an active request and the core command queue

is not full, the request will be sent to the core.

If there is not an active request for that port, the port itself will be skipped and the next port

will be checked.

In any case, the counter is incremented one-by-one as any request has been processed,

regardless of which port's request was arbitrated.

Round-robin arbitration guarantees that each port's requests can be successfully arbitrated

into the core every N cycles, where N is the number of ports in the component itself.

No port will ever be locked out, and any port can have its requests processed on every cycle

as long as all other ports are idle and the command queue is not full.

An example of the round-robin scheme is shown in Table 19.

Cycles 0, 2 and 6 show the system behaviour when the command queue is full. Cycle 8

shows the system behaviour when the port addressed by the arbitration counter does not

have an active request.

All other cycles show normal behaviour.

Table 19.

Round-Robin operation example

Cycle

Port addressed

by an arbitration

counter

Port requesting

Command

queue full?

Winner of

arbitration

Value of counter

at the next cycle

YYYYYes

None

YYYYNo

YYYYes

None

YYYYNo

YYNo

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