Clocking and Resets - 2.7 English

1G/10G/25G Switching Ethernet Subsystem Product Guide (PG292)

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2.7 English

Refer to the Clocking and Resets for these requirements.

Ensure that the clock frequencies for both the 1G/10G/25G Ethernet Subsystem as well as the Xilinx Transceiver reference clock match the configuration requested when the subsystem was ordered. The core clock has a minimum frequency associated with it. The maximum core clock frequency is determined by timing constraints. The minimum core clock frequency is derived from the required Ethernet bandwidth plus the margin reserved for clock tolerance, wander and jitter.

The first thing to verify during debugging is to ensure that resets remain asserted until the clock is stable. It must be frequency-stable as well as free from glitches before the 1G/10G/25G Ethernet Subsystem is taken out of reset. This applies to both the SerDes clock as well as the core clock.

If any subsequent instability is detected in a clock, the 1G/10G/25G Ethernet Subsystem must be reset. One example of such instability is a loss of CDR lock. The user logic should determine all external conditions which would require a reset (e.g. clock glitches, loss of CDR lock, power supply glitches, etc.).

The GT requires a GTRXRESET after the serial data becomes valid to ensure correct CDR lock to the data. This is required after powering on, resetting or reconnecting the link partner. At the core level to avoid interruption on the TX side of the link, the reset can be triggered using gtwiz_reset_rx_datapath. If available, signal detect or inversion of loss of signal from the optics can be used to trigger the reset. If signal detect or loss of signal are not available, timeout logic can be added to monitor if alignment has not completed and issue the gtwiz_reset_rx_datapath reset.

Configuration changes cannot be made unless the subsystem is reset. An example of a configuration change would be setting a different maximum packet length. Check the description for the particular signal on the port list to determine if this requirement applies to the parameter that is being changed.