Stephen

Thanks for the reminder re constantly improving the documentation.

Perhaps some of the FAQs posted here will answer your questions located on the docwiki:

http://docwiki.gumstix.org/index.php/Frequently_asked_questions/Heat_and_power#Can_just_one_power_supply_be_used_for_two_or_three_board_configurations.3F


Don
+++++++++++++

On Tue, May 27, 2008 at 9:49 AM, Stephen Hersey <n1xnx@rcn.com> wrote:
OK, color me annoyed.
I've purchased a Connex motherboard and Robostix and NetMMC expansion modules, and I want to make sure not to fry'em when I power them up. Sooo, I've been scouring the Web site, the newsgroup archives, and the schematics. Alas, while Gumstix circuit design is cool and creative, their documentation is sadly deficient.

Trouble is, I cannot find any information telling me how to power the aforementioned stack properly. Both the Robostix and NetMMC have DC input jacks; presumably, the processor is powered from the voltage regulator on an expansion card.

Now, I can see two possible ways to power this stack, and there's NO CLUE ANYWHERE on the Gumstix site telling me how to determine the proper power system for a multiboard stack. Options seem to be:

A) Supply 5V power to the Robostix alone (since the ATMEGA128 is a 5V chip), and hope that its regulator can handle the 3.3V bus load from all three boards; this seems chancy, as I read somewhere that the NetMMC should get 4V instead of 5V, presumably to limit the power dissipation on the 3.3V regulator; consequently, the Robostix 3.3V reg will take a thermal hammering from this treatment and may cook.

B) Supply 5V to the Robostix, and 4V to the NetMMC; also chancy, as it's generally a Bad Idea to connect the outputs of two independent voltage regulators, which would appear to be the probable result of doing this; the Micrel datasheet for the MIC5219 regulator doesn't suggest this as a potential option, either.

C) Perform surgery on the boards to disconnect their internal regulators, and provide external 3.3V and 5V power; this is  unappealing, since these boards are supposed to work together without microsurgery.

Can anyone provide a rational approach to powering this stack?
Is there a document somewhere on powering a stack that I've overlooked?

Thanks, Steve H
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