VSC7440 datasheet(293/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

276

To enable bandwidth limiting down to very low data rates, the calculated IPG can be scaled. In other

words, multiplied by a power of 2 in the range 2 to 1024. By this it is possible to limit the data rate down

to 2.3% (for 1536 byte frames).

3.27.6.1 Setting Up Frame Overhead

The following table lists the frame overhead configuration registers.

To increase the minimum inter-packet gap for all packets, set

DSM::TX_RATE_LIMIT_MODE.TX_RATE_LIMIT_FRAME_OVERHEAD_ENA to 1, and set

DSM::RATE_CTRL.FRM_GAP_COMP to a value between 13 to 255.

For more information about the scaling feature if the GAP must be above 255, see Setting Up Transmit

Data Rate Limiting, page 275.

3.27.6.2 Setting Up Payload Data Rate

The following table lists the payload data rate configuration registers.

To limit the data rate relative to the transmitted payload, set

DSM::TX_RATE_LIMIT_MODE.TX_RATE_LIMIT_PAYLOAD_RATE_ENA to 1.

The inter-packet gap increase can be expressed as:

ΔIPG = 2S × (256/R) × L

Where:

S: Scaling factor DSM::TX_RATE_LIMIT_MODE.IPG_SCALE_VAL

R: Stretch ratio DSM::TX_IPG_STRETCH_RATIO_CFG.TX_FINE_IPG_STRETCH_RATIO

L: Payload length, possibly modified according to

DSM::TX_RATE_LIMIT_MODE.PAYLOAD_PREAM_CFG and

DSM::TX_RATE_LIMIT_MODE.PAYLOAD_CFG

Notes

Total IPG is 12 byte standard IPG plus IPG.

Values for R must not be below 1152 or above 518143.

Values for S must not be above 10.

Fractional parts of the resulting IPG value are carried forward to the next IPG.

Higher values for S allow for finer target accuracy/granularity on the cost of an increased IPG jitter.

The utilization can be expressed as:

U = L / (L + IPG) or

Table 202 •

Rate Limiting Frame Overhead Configuration Registers Overview

Target::Register.Field

Description

Replication

DSM::TX_RATE_LIMIT_MODE.

TX_RATE_LIMIT_FRAME_OVERHEAD_ENA

Enable txRateLimitFrameOverhead mode.

Per port

DSM::RATE_CTRL.FRM_GAP_COMP

Inter-packet gap to be used.

Per port

Table 203 •

Rate Limiting Payload Data Rate Configuration Registers Overview

Target::Register.Field

Description

Replication

DSM::TX_RATE_LIMIT_MODE.

TX_RATE_LIMIT_PAYLOAD_RATE_ENA

Enable txRateLimitPayloadRate mode.

Per port

DSM::TX_IPG_STRETCH_RATIO_CFG.

TX_FINE_IPG_STRETCH_RATIO

Stretch ratio.

Per port

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