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Background:
Observed Issue:
Questions:
Related Posts I Found:
Hi Steven,
What are the pass/fail conditions for your loopback tests?
I am checking for more information for question (1) - regarding question (2) do you see a passing result when forcing signal detect with 0x16[9] = '1'?
Thank you,
Evan
For its loopback test, the processor sets the internal loopback bit and sends an Ethernet packet. It expects to receive the same packet back. The test is a pass when the data in the received packet is the same as what was sent. The test is a fail otherwise. With the FX_SD input low, there is no received packet back.
The datasheet does not describe how the 100BASE-TX signal detect function and the FX_SD input interact when in fiber mode. I assumed they were independent and hadn't fiddled with any of the SD related mode bits. I will test what happens when the SD_FORCE_PMA bit is set and let you know.
Regards,
Steven
Hi Steven,
Thanks for clarifying this - this looks to be expected behavior with Signal Detect in 100-FX mode. Please share results when able to test with forced SD.
Regards,
Evan
Setting the SD_FORCE_PMA did not change the outcome.
Based on the statements in the datasheet that "Due to the nature of the descrambler function, all 100BASE-TX Loopback modes will cause an initial “dead-time” of up to 550 µs during which time no data will be present at the receive MII outputs." (page 24) and "Setting [the LOOPBACK] bit may cause the descrambler to lose synchronization and produce a 500 µs “dead time” before any valid data will appear at the MII receive outputs." (page 70), we also tried adding a delay between setting the loopback bit and and sending the packet. With the delay, the state of the signal detect input doesn't matter and our loopback test passes.
Can you please explain why the dead time on scrambler would matter for fiber mode since the scrambler should be bypassed? Also is the discrepancy between 500 and 550 in the two dead time descriptions intentional? If not, which one is correct?
Are there any block diagrams like Figures 7-1 and 7-2 for fiber mode?
Hi Steven,
Which setting is being used on 0x16[8] "SD_OPTION" ? If it is default operation, I share your confusion for why this dead time is causing loopback in fiber mode to fail loopback test. I need to clarify with the team the possible causes for this.
Unfortunately we do not have figures available for Fiber TX/RX blocks, I will check if there is a comparable PHY that can be referenced for these blocks.
Thank you,
Evan