When enabled and configured, EST allows express queue traffic to be scheduled (placed) on the wire at specific repeatable time intervals.
EST operates on a repeating time interval generated by the CPTS EST function generator. For example, a 125us repeating time interval can be configured.
Each Ethernet port has 128 EST fetch commands maximum in the global EST fetch RAM.
Each 22-bit fetch command consists of a 14-bit fetch count (14 MSB’s) and an 8-bit priority fetch allow (8 LSB’s) that will be applied for the fetch count time in wireside clocks.
The configured port fetch commands are executed in sequence, beginning at port address zero each time through the time interval beginning at cycle start.
EST allows non-scheduled express and prempt queue traffic to be cleared from the wire to ensure that the scheduled traffic is transmitted at the proper time (zero allow).
EST can be used with or without premption. The
CPSW_PN_IET_CONTROL_REG[23-16] MAC_PREMPT value determines whether the
priority is enabled on the express or prempt queue. Whether a priority is on
the express or prempt queue only effects the wire clear time from an EST
operation perspective.
Software should not move priorities to the prempt queue unless preemption is configured, enabled, and verified allowing preemption to occur.
Express packet time stamp events can be enabled to assist software in configuring and timing EST operations.