On 10.7.2023 21.38, Andrew Lunn wrote:
On Mon, Jul 10, 2023 at 08:56:18PM +0300, Vesa Jääskeläinen wrote:
Add support to specify either class A or class B (default) for line driver.
Class A: full MLT-3 on both Tx+ and Tx–
Class B: reduced MLT-3
By default the PHY is in Class B mode.
Hi Vesa
Do you have a reference to 802.3 or some other document which
describes these. How does reduced differ from full? Is this really a
hardware property of the board?
Thanks
Andrew
Hi Andrew,
This is needed for configuration in link between DP83822 and Ethernet
Switch chip. In the connection there there is no Ethernet cable at all
but routes within the circuit boards but instead has capacitive coupling
on routes.
The other Ethernet port with the same phy is connected to normal
Ethernet cable and in there this configuration is not needed.
So the setting itself is related to specific circuit board design.
MLT-3 is related to encoding used in the signals -- I suppose wiki page
is good introduction reference:
https://en.wikipedia.org/wiki/MLT-3_encoding
I suppose this question in TI's forum gives indication what is the
effect of the setting:
https://e2e.ti.com/support/interface-group/interface/f/interface-forum/572998/dp83822hf-transformerless-operation
TI's datasheet is not too verbose about the effect:
https://www.ti.com/lit/ds/symlink/dp83822hf.pdf?ts=1688188848392&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FDP83822HF
I cannot say I am expert on the Ethernet line encoding -- my
understanding is that full MLT-3 is "normal" signaling mode and reduced
MLT-3 mode is extension on the chip to perhaps reduce energy consumption
with real Ethernet cables or something like that but in chip-to-chip
link the reduced mode does not work and full MLT-3 is needed in order
for link to come up.
I hope this helps?
Thanks,
Vesa Jääskeläinen
