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Issues with the PCB
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Please be positive and constructive with your questions and comments.

Issues with the PCB

by BigMike on Thu Mar 13, 2008 12:44 pm

I received my new eShield a week or so ago and last night glued it all together.

I found three issues with the PCB that you may want to look at before you have another batch made:

1) Lack of heat sinking for the XPort module.

From the XPort manual:

For proper heat dissipation, the PCB should have approximately 1 square inch of copper attached to the shield tabs. The shield tabs are an important source of heat sinking for the device.


2) One of the plastic mounting lugs fouls the USB socket on the Diecimila.

This means that the shield doesn't lay flat.

3) The metal case could touch one or two of the vias.

Towards the rear of the XPort there are a couple of vias which could touch the wrapped-under part of the XPort's metal case. I think the vias carry RX/TX, but I'm not sure ATM.

I got round this by putting a some card underneath before soldering - not ideal, though.

One other thing which is a niggle / personal preference - I don't like having resistors sticking up in the air. The distance between centres could be more to allow the resistors to lay flat.

I powered the board and there was no smoke or noise, so at least the electrolytics are in correctly.

Now looking forward to getting this thing working.

Mike
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by cairn on Thu Mar 13, 2008 2:30 pm

2) One of the plastic mounting lugs fouls the USB socket on the Diecimila.

This means that the shield doesn't lay flat.


Agreed. I don't see any particular need for the XPort footprint to be centered within the XPort Direct/+ footprint. Maybe move it over a few mm in the next revision?

As for the shield tabs, I originally snipped mine off because I was using an RJ-45 breakout board from Sparkfun per Tom Igoe's directions in "Making Things Talk". My XPort does get kind of warm and I regret clipping them off. I might pick up a pack of "Memory Heatsinks" at the local Radio Shack to glue on to the XPort module. I think heat sinks are pretty.

All in all, I really like the XPort shield; thanks ladyada!

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by schill on Thu Mar 13, 2008 3:57 pm

I also agree that those would be useful changes. I've avoided some of those issues by using a Direct module instead.

I was also wondering about the vertical resistors. There's plenty of room on the board to have them flat. Maybe people get used to trying to cram as much on a board as possible that they forget there are other options :) .
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Re: Issues with the PCB

by adafruit on Fri Mar 14, 2008 5:01 pm

thanks, ill be putting in most of these sorts of things into the next revision! none of the beta testers brought this up so they were not fixed in the current rev. :(

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by cairn on Sun Mar 16, 2008 12:05 am

May I also suggest putting a larger 3.3V regulator on the next rev? The MCP1700 puts out a max of 200mA at 3.3V and the XPort pulls between 190mA and 267mA when active. This is all according to the datasheets; I haven't had any problems while using my XPort with the shield, but I'd be a lot more comfortable with a higher capacity regulator.

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by adafruit on Tue Mar 18, 2008 12:39 am

hmm, the mcp1700 should be able to easily supply 250mA according to the datasheet.
the direct takes 220ma max according to the datasheet (integration guide) but i dont see the xport (non-direct) power reqs. do you have a reference for that? thx!

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by cairn on Tue Mar 18, 2008 3:08 am

ladyada wrote:hmm, the mcp1700 should be able to easily supply 250mA according to the datasheet.


Oops! You're right, I was reading my < and > symbols backwards. The MCP1700 has a max output of 200mA at less than 2.5V and a max of 250mA at or above 2.5V; so the MCP1700 at 3.3V can indeed source up to 250mA.

ladyada wrote:the direct takes 220ma max according to the datasheet (integration guide) but i dont see the xport (non-direct) power reqs. do you have a reference for that? thx!


The XPort Data Sheet (PDF) on page 4 has a "DC Characteristics" chart where it says "Supply Current (idle)@ 48 MHz: 119mA" all the way up to a max current of 267mA for "Supply Current (10BASE-T activity)@ 88 MHz".

The XPort Direct and Direct+ Product Briefs say "200mA Typical (240mA max)" on page 2, but don't give more detailed information. Theintegration guide for the XPort Direct gives a "Supply Current (100BASE-T activity)@ 48 MHz: 224mA (nominal)" on page 18, but I couldn't find a maximum anywhere.

The integration guide for the XPort Direct+ gives similar info (on page 16) as the XPort datasheet above; 119mA up to 262mA.

I hope that helps!

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by adafruit on Tue Mar 18, 2008 2:28 pm

cairn wrote:The XPort Direct and Direct+ Product Briefs say "200mA Typical (240mA max)" on page 2, but don't give more detailed information. Theintegration guide for the XPort Direct gives a "Supply Current (100BASE-T activity)@ 48 MHz: 224mA (nominal)" on page 18, but I couldn't find a maximum anywhere.


huh i wonder how you know if its at 48mhz or 88mhz?

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by cairn on Tue Mar 18, 2008 3:13 pm

In the XPort user guide, page 24, it shows the screen for "Server Settings" under server configuration. You can see that there is a option to change the CPU performance mode to Low, Medium, or High. The next page details, "Select the XPort’s performance mode. Higher performance settings require more energy. Regular is 48 Mhz; High is 88 Mhz. The default is Regular." It doesn't say what "Low" performance mode equates to.

I don't have an XPort Direct or Direct+, but I assume it's the same.

EDIT: I found the same info in the XPort Direct+ user guide on page 22. I couldn't find any info about regular vs. high performance mode in the XPort Direct user guide.

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by cairn on Tue Mar 18, 2008 5:08 pm

OK, another update after digging through the datasheets a bit more. The "High Performance" mode is only available on the XPort and XPort Direct+; it is not available on the XPort Direct. Enabling High Performance mode sets the XPort to 88MHz which allows the use of the higher serial data speeds (460Kbps and 920Kbps).

All three devices (XPort, Direct and Direct+) run at 48MHz by default which limits the max serial data speed to 230.4Kbps (the datasheet actually says "The standard CPU performance mode supports up to 230400 Kbps." but I think that "K" is a typo). All the above info is from page 62 of the XPort user guide.

Finally, I found some relevant and useful info about both the shield tabs and the high performance mode in the Lantronix Tech FAQs:

If you are planning to run an XPort-03 in high performance mode to allow 920k baud rates a heat sink of at least 1 square inch of copper is highly recommended. This mode lowers the maximum operating temperature from +85°C to +75°C (with a 1 inch square heat sink) because the EX chip generates more heat when running at 88MHz than at 48MHz.


There's more information about the necessity of heat-sinking the XPort at the above link. I hope this is all useful in the next rev!

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Please be positive and constructive with your questions and comments.