Part Number: DP83826I
Hello,
We had problems with DP83826 when using "high-quality" PCB routing of Ethernet signals (i.e. controlled impedance, shielding, etc) AND short Ethernet cables (below a few meters). When replacing the cable with long ones (above some tens of meters), the problem disappears.
An (almost) identical schematic, but implemented on a different system, with Ethernet signals travelling through multiple PCBs and connectors, consequently with "lower-quality" signal routing (I am speaking of the Ethernet signal pairs) - on this system we can work OK with both short and long cables.
The problem consist in the fact that autonegotiation is accomplished successfully, but after ~100 miliseconds the link is dropped by the DP83826. Attached is a scope view showing one of the Ethernet siganls, and the link_good (~led) signal, active high. So, connection is never established, and link is blinking.
By register inspection we found that the two counters 014 and 015 stay on zero, so there are no RX errors neither false carrier events.
Trying to solve the problem, I found out that register 416, bits 11-8 (described in datasheet as "Force control value for HPF") have a definite effect: with default 0xF it behaves as described above, same with 0xE,oxD, partially also with 0xC (longer periods between link drops), and starting with 0xC down to 0x0 the link stays on even with short cables on a well-routed, "high-frequency-capable" system.
My question is: can you help me understand the content of this bitfield / register? I assume an NDA may be required, our company already has such an NDA (for other components related to functional safety).
Alecsandru
P.S. Ethernet signal is different after each autonegotiation attemp, due to auto-MDIX. This can be seen also as a slight DC shift. The red portion is signal coming from link partner (PC), which has a (very) slow link drop reaction time. The yellow+red portion of the signal is when DP83826 pin is an output, during this portion is visible that signal is cut as soon as link signal drops. A zoomed detail of this event is shown below.
