56.6.23.3.3 Transmit Interspersing Capability

On the transmit side, the interspersing capability is achieved by preempting M-frames from the pMAC in order to give the priority to Express frames from the eMAC.

If the pMAC is in the process of transmitting a frame and the eMAC requests to transmit, then the M-frame from the pMAC is halted with the M-frame transmission stopped and appended with a 32-bit MCRC before transmission of the express frame commences.

The additional support for IEEE 802.1Qbv also provides the capability to send time-scheduled express frames. This is supported by a set of programmable registers to define when the frame should be transmitted.

There are certain circumstances where a pMAC transmission will not be pre-empted, this is handled in accordance to the IEEE 802.3br standard as defined below:

• There are less than 64 M-frame bytes left to transmit
• The M-frame transmission started at the moment when an Express packet requested the channel (the minimum number of bytes to be transmitted before preemption can occur is programmable using the register add_frag_size register). In this case preemption will start after the minimum number of bytes of the M-frame have been transmitted.
• If the preemption function is not enabled
• If the preemption function is enabled, the verification function is not disabled and the link partner couldn’t complete the handshake protocol indicating that it cannot support the IEEE 802.3br standard.