DP83848C: Preamble SFD Length is shortage

Hi Daisuke,

Can you please share any scope plots or data captured when 6.5 octet preamble is noticed in 10Base-T ? We do not expect this behavior, the preamble will be 7 octects as defined by IEEE standard.

Are you forcing the PHY in 10Mbps ? Can you please elaborate on how you were able to reproduce this issue ?

Regards,
Rahul

Hi,

Yes. I use 10BASE-T.
I attached mii bus waveform I captured.

When I send ping packet from PC, this issue is reproduced about 50%.

FYI: Part number : DP83848IVV

Regards,
Daisuke

Hi Daisuke,

Thank you for sharing the info, please let me have a look at it and respond to you.

Regards,
Rahul

Hi Rahul,

Do you have any updates?
Please let me know If you need more information.

Regards,
Daisuke

Hi Daisuke,

I am trying to find an evaluation board for DP83848 to see if I can reproduce this issue, looks like we do not have an EVM in house. I will update you soon.

Sorry for the delay.

Regards,

Rahul

Hi Rahul,

OK.

I would like to know the logical conditions under which this issue occurs.
For example, if the RX signal is electrically unstable, etc.

I'll be waiting for an update from you.

Regards,
Daisuke

Hi Daisuke,

Can you force the speed of the PHY in 100mbps and check if this issue is being noticed in 100M speed as well ? You can disable ANEG and write Reg 0h, bit 13 for 100 Mbps speed.

Is preamble suppression enabled ? (Reg 1h, bit 6) - can you read this ?

Is the SQUELCH configuration ON ? (Reg 1Ah, [11:9]) - can you read this ?

Regards,
Rahul

Hi Rahul,

When PHY Link Speed is 100M, the issue is not reproduced.
We also tested 10M with Auto Negotiation disabled  and the issue was reproduced.

Here is PHY register values. (10M)

01h : 786D
=> bit 6 is "1"

1Ah : 0804
=> [11:9] is "100"

Regards,
Daisuke

Hi Daisuke,

Can you try disabling Preamble Suppression and check if the issue is being reproduced in 10Mbps ?

Can you also try to change the link partner and check if you are noticing this with a different link partner as well ?

Regards,
Rahul

Hi Rahul,

Thank you for your reply.

I tried disabling peramble suppression but the issue is reproduced.
I also changed link partner but not solved.

What other reasons could be considered if DP83848 were to drop the preamble intentionally?

Hi Daisuke,

There is a smart squelch feature in the PHY, which is a part of 10Base-T standard. The signal at the start of the packet is checked by smart squelch and any pulses not exceeding the squelch level will be rejected. This checking procedure results in the loss of typically three preamble bits at the beginning of each packet.

More details in the datasheet, section 5.5.1.3.2:

https://www.ti.com/lit/ds/symlink/dp83848-ep.pdf?ts=1686765097539&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FDP83848-EP

Regards,
Rahul

Sorry! I made a mistake pushing "report as abusive".

Thank you for your reply.
I'll check it.

Hi Rahul,

Does Smart Squelch sometimes cause a 3-bit loss of the preamble even when the input signal quality is correct?
Or is the preamble being loss due to issues with the input signal quality?

Regards,
Daisuke

Hi Daisuke,

My understanding is that Smart Squelch will begin and only after all these conditions have been satisfied will a control signal be generated to indicate to the remainder of the circuitry that valid data is present. At this time, the smart squelch circuitry is reset.

Once good data has been detected, the squelch levels are reduced to minimize the effect of noise causing premature End of Packet detection.

Did you see continuous loss of preamble or is it getting fixed after a point ?

Regards,
Rahul