|DIY4X USB power tested, and more photos|
Message #6 Posted by Eric Smith on 5 Jan 2012, 1:39 a.m.,
in response to message #1 by Eric Smith
Designing the DIY4X such that it can be powered from either 3V (lithium coin cell, or two alkaline AAA cells), or from 5V from USB, while not drawing more current from the battery than we previously had without the USB circuitry, was slightly tricky. We weren't 100% certain that our method would work properly, but we tested it this evening and it seems to work OK.
When powered from USB, we still have about 15uA of drain from the battery, regardless of what mode the calculator is in, off, idle, or running. Part of that is the quiescent power consumption of the boost regulator used to power the calculator from the battery, but that doesn't explain the full 15uA. However, we feel that 15uA is low enough that it is not an issue. That's about the same current that the DIY4X draws from the battery when "off" and with no USB power. (We expect to do several things to reduce the "off" current in the future, but aren't sure how it will affect the battery drain when USB power is applied.)
The USB port will not recharge any kind of batteries, and supports calculator operation as a USB device only, not host or OTG.
We don't yet have the new processor with USB support (and bigger memory), so at the moment the actual USB data lines are not wired.
The MicroSD socket will eventually be jumpered over to the SD socket footprint on the DIY4 main board.
Here are three more sets of photos:
I'm sorry that the pictures aren't better; I don't have a real digital camera or tripod here, and have been forced to rely on my cell phone.