RM0319 datasheet(76/368 Pages) STMICROELECTRONICS

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Multiport DDR controller (MPMC)

RM0319

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Doc ID 022640 Rev 3

Priority relaxing allows low priority commands to be able to move through the arbiter to the

core. This will ensure that the system can meet maximum latency requirements.

Port pairing

The memory controller arbiter embeds a feature which allows adjacent ports to be grouped

together and considered jointly for arbitration. The weighted_round_robin_weight_sharing

parameter controls this function, with one bit per pair of ports in the memory controller. Bit 0

handles ports 0 and 1, Bit 1 handles ports 2 and 3 and so on. Where memory controller

interfaces to an odd number of ports, the highest numbered port is excluded from the port

pairing system.

Since the ports are grouped together, their relative priorities are not considered separately.

Referring to Section , the general formula for port priority allocation is the ratio of that port's

relative priority (ahbX_priorityY_relative_priority) to the sum of all requesting port's relative

priority values. In this case, the relative priority value of only one of the paired ports is used

for the sum calculation. This means that the bandwidth will be divided differently among the

ports.

Let us consider the port pair at the top of the scan order: if one only port is requesting it will

win arbitration. If both are requesting, port ordering is used to determine which port wins

arbitration.

When the ports are paired, their scan order can never be altered and they will always remain

together in the scan order. Their counters increment together, so when they reach their

relative priority value, the port pair will dynamically be placed at the bottom of the scan order

for that priority group.

In order for port weight sharing to be used, the relative priority parameters for the port pair

must be programmed to the same value and the port order of the paired ports should be

sequential. If either condition is not followed, an error bit will be set to1'b1.

Let us consider System G as described inTable 28. Again, to simplify the command queue is

considered to never be full, commands from ports 0, 1, 2 and 3 are only received at priority

level 0 and commands from ports 4 and 5 are always at priority 1.

However, now ports P0 - P1, and ports P4 - P5 are paired.

110

010

P0-P1-P3-P2

P4-P5

Y P4

110

020

P0-P1-P3-P2

P5-P4

210

000

P0-P1-P3-P2

P5-P4

220

000

P0-P1-P3-P2

P5-P4

Y P2

221

000

P0-P1-P3-P2

P5-P4

Table 27.

System F operation with priority relaxing (continued)

Cycle

Ports Requesting

Relaxed

Ports

Arbitration

Winner

Next Counter

Next Scan Order

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