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Ethernet AVB Endpoint User Guide www.xilinx.com 63
UG492 July 23, 2010
Tx Arbiter
• During AV traffic transmission, credit is removed at a rate defined by the sendSlope.
• The hiLimit and loLimit settings impose a fixed range on the possible values of
credit. If the available credit hits one of these limits, it will not exceed, but saturate at
the magnitude of that limit. These limits are fixed in the netlist to ensure that the
interface is not used incorrectly.
The overall intention of the two settings idleSlope and sendSlope is to spread out the AV
traffic transmission as evenly as possible over time, preventing periods of bursty AV
transmission surrounded by idle AV transmission periods. No further background
information is provided in this document with regard to the credit-based algorithm.
The remainder of this section describes the idleSlope, and sendSlope variables from the
perspective of the Ethernet AVB Endpoint core.
Tx Arbiter Bandwidth Control
The Ethernet AVB Endpoint core contains four configuration registers, used for setting the
cores local definitions of “idleSlope,” and “sendSlope.”
The configuration register settings are described in general, and then from the point of
view of a single example which describes the calculations made to set the register default
values. This example dedicates up to 75% of the overall bandwidth to be reserved for the
AV traffic (leaving at least 25% for the Legacy Traffic).
The calculations described are independent of Ethernet operating speed (no re-calculation
is required when changing between Ethernet speeds of 1 Gbps and 100 Mbps).
idleSlope
The general equation is:
idleSlopeValue=(AV percentage / 100) x 8192
In this example, dedicating up to 75% of the total bandwidth to the AV traffic, we obtain:
idleSlopeValue=(75 / 100) x 8192 = 6144
The calculated value for the idleSlopeValue should be written directly to the “Tx
Arbiter Idle Slope Control Register.” This provides a per-byte increment value when
relating this to Legacy Ethernet frame transmission.
sendSlope
The general equation is:
sendSlopeValue=((100 - AV percentage) / 100) x 8192
In this example, dedicating up to 75% of the total bandwidth to the AV traffic, we obtain:
sendSlopeValue=((100 - 75) / 100) x 8192 = 2048
The calculated value for the sendSlopeValue should be written directly to the “Tx
Arbiter Send Slope Control Register.” This provides a per-byte decrement value when
relating this to AV Ethernet frame transmission.