RM0319 datasheet(81/368 Pages) STMICROELECTRONICS

81 / 368 page

RM0319

Multiport DDR controller (MPMC)

Doc ID 022640 Rev 3

81/368

some drawbacks. For this reason, MPMC offers two options that affect commands once they

have been placed in the command queue.

High-Priority command swapping

Commands are assigned priority values to ensure that critical commands are executed

before less important commands. Therefore, it is desirable that high-priority commands

cross the core as sooner as possible. The placement algorithm sorts commands by priority

though it cannot avoid a scenario such as a high-priority command waiting at the top of the

command queue while another command, possibly of a lower priority, is in process.

The high-priority command swapping feature allows this new high-priority command to be

executed quite sooner. If the user has enabled the swapping function by the swap_en

parameter, the entry at the top of the command queue will be compared with the current

command in progress. If the command queue's top entry is of a higher priority (not the same

priority), and it does not have an address, source ID or WRITE buffer conflict with the

current command being executed, the original command will be interrupted.

If the command has to be interrupted, it will be halted after completing the current burst,

stacked and placed at the top of the queue, while the new command will be executed. As

long as the command queue is not full, new commands may continue to be inserted into the

command queue based on the placement rules, even at the head of the queue ahead of the

interrupted command. The top entry in the command queue will be executed next.

Whenever the interrupted command is resumed, it will start from the point at which it was

interrupted.

Note:

Priority 0 commands could never be interrupted, so the user should set any commands that

should not be interrupted to priority 0.

Command ageing

Since commands can be inserted ahead of existing commands in the command queue, it

could happen a low priority command remains at the bottom of the queue indefinitely. To

avoid such a lockout condition, aging counters have been included in the placement logic

that measure the number of cycles that each command has been waiting. If command aging

is enabled through the active_ageing parameter and an aging counter hits its maximum, the

priority of the associated command will be decremented by one (lower priority commands

are executed first). This increases the likelihood that this command will move to the top of

the command queue and be executed.

Note:

This command does not move relative positions in the command queue when it ages; the

new priority will be considered when placing new commands into the command queue.

Aging is controlled by a master aging-rate counter and command aging counters associated

with each command in the command queue. The age_count and command_age_count

parameters hold the initial values for each of these counters, respectively. When the master

counter counts down the age_count value, a signal is sent to the command aging counters

to decrement. When the command aging counters have completely decremented, the

priority of the associated command is decremented by one and the counter is reset.

Therefore, a command does not age by a priority level until the total elapsed cycles has

reached the product of the age_count and command_age_count values. The maximum

number of cycles that any command can wait in the command queue until reaching the top

priority level is the product of the age_count value, the command_age_count value, and the

number of priority levels in the system.

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