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
