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Rabbit 6710 version B ethernet not coming up reliably

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The custom interface board design we've used for 4 years (with Rev A1 6710 Rabbits) boots and connects via DHCP (or fixed IP) within seconds, using either Dynamic C 10.64, or 10.72 (with patched serial libs), as it does on the Digi RCM7600 board as well of course.

The new Rev B 6710 Rabbit also starts and connects within seconds on the same RCM6700 Interface Board, but fails to bring up the network interface at all, 90% of the time when plugged into our interface board - no Ethernet LEDs, no DHCP assigned, no ping response etc.

Given that there have been no software or other hardware changes, the only remaining difference is the Rev B of the 6710, which is not specifically documented other than the serial lib patch note.

We did numerous tests swapping hardware and looking at scope traces:
The only common factor is Rev B, which does have circuit/trace difference(s):

So, does anyone have new, reliable code fix (special settings?) or documented changes?
asked Nov 9, 2015 in Rabbit by rjsdotorg New to the Community (12 points)
Hello, Did you inform this issue to Digi?
Yes, the last response was a quote for paid support. If we could at least get updated schematic and BOM it would probably be resolved.

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1 Answer

+1 vote
The issue seems to be resolved by adding a 47uF cap to the 3.3V supply. The regulator is borderline spec for the 6700 and power-on surge appears to send it into a rest loop.
The new AT45DB081E flash has only ~6mA extra draw over the previous part
but that might have been enough to droop below 2.93V when needed, and trigger a reset cycle.
answered Nov 19, 2015 by rjsdotorg New to the Community (12 points)
Is that issue present when using the RCM6710 in Digi's development board?  I'm wondering if it's a design problem with your custom interface, or something on the RCM6710 itself.
Yes, it is only on our interface board; the chopper used has a spec for 300mA (which worked with Rev A) but the new flash/code on Rev B bumped it just enough such that boot was unreliable much of the time, but not always.
A 47uF cap smoothed the startup surge, and our next rev will have a 1A regulator.