Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 11 Jul 2023 19:05:19 +0300 | Subject | Re: [PATCH 0/2] net: phy: dp83822: Add support for line class driver configuration | From | Vesa Jääskeläinen <> |
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On 11.7.2023 18.35, Andrew Lunn wrote: > >>> So before accepting any patches, we need a better understanding of >>> that reduced MLT-3 is and why you would want to use it. >> OK. >> >> My understanding is that as we have PHY<->PHY link it needs to handle itself >> in standard way. Thus the MLT-3 full mode is required for communicating with >> Ethernet switch. >> >> It seems that Texas Instruments has figured out additional power saving >> mechanism by carefully selecting used magnetics (they have guidelines for >> that and list of supported ones). Now the thinking might have continued that >> let's make the power saving mode the default for all. > Do there guidelines for magnetic says anything about what to do when > using unsupported ones. Like turn reduced MLT-3 off? > >> With carefully selected magnetics one most likely gets correct looking >> signal when measured from the cable and thus the other party then gets >> I tried to look up what does this class A and class B mean but I am unable >> to find the reasoning for that. > If you look at the oscilloscope screenshots in the support forum, it > looks like in reduced MLT-3 mode, The TX- and TX+ pins only have two > states, not three. It relies on the magnetics to combine the two > signals to produce a three state signal, and handle the bias in each > signal. > > When in MLT-3 mode, i expect the TX- and TX+ pins do real MLT-3. > > With real MLT-3, you can then do capacitor coupling to other devices > which conform to 802.3.
From the datasheet (https://www.ti.com/lit/ds/symlink/dp83822i.pdf?ts=1689018777543):
"8.4.3.1.4 Binary to MLT-3 Converter The Binary to MLT-3 conversion is accomplished by converting the serial binary data stream output from the NRZI encoder into two binary data streams with alternately phased logic one events. These two binary streams are then fed to the twisted pair output driver which converts the voltage to current and alternately drives either side of the transmit transformer primary winding, resulting in a minimal current MLT-3 signal.
The 100BASE-TX MLT-3 signal sourced by the PMD Output Pair common driver is slew rate controlled. This should be considered when selecting AC coupling magnetics to ensure TP-PMD Standard compliant transition times (3 ns < Trise/fall < 5 ns).
The 100BASE-TX transmit TP-PMD function within the DP83822 is capable of sourcing only MLT-3 encoded data. Binary output from the PMD Output Pair is not possible in 100 Mbps mode. Fully encoded MLT-3 on both Tx+ and Tx- and can be configured by configuring Register 0x0404h (for example, in transformer-less designs)."
and then about transformer:
"11.1.3.1 Transformer Recommendations The following magnetics have been tested with the DP83822 using the DP83822EVM. Table 11-1. Recommended Transformers
[table here]
Table 11-2. Transformer Electrical Specifications
[table here]
"
They have also application note (https://www.ti.com/lit/an/snla079d/snla079d.pdf?ts=1689063082391&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FDP83822I%253FkeyMatch%253D%2526tisearch%253Dsearch-everything%2526usecase%253Dpartmatches):
"2 MDI (TP/CAT-V) Connections"
and
"10.2 Magnetics"
Which has more or less same info as the datasheet.
To me that just says follow these guidelines and please select one form already tested lists. And if you must then look for similar products and make sure that these limits are matched.
Perhaps you are able to decode that better :). I trust our electrical engineers to do the right job ;)
> >> In a way this could even be: >> >> ti,force-standard-mlt-3-signaling; > Maybe. Or ti,disable-proprietary-line-coding > > Lets give TI a couple of days to comment. > > Andrew
Sure.
I am OK with any of the three variants proposed so far.
Perhaps leaning towards more to new ones above.
Thanks, Vesa Jääskeläinen
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