So, having read the docs through for the first time, I’m left with two question categories. I’m sure there will be others, but for the purpose of roughing the Notecard into a hardware design, I need to start here. This post will be about the USB question, since the other is only marginally related I’ll make it a second topic.
In one place, you guys put this: “In order to take advantage of various Notecard features pertaining to “dynamic line voltage detection” (both now and in the future), you must connect the VUSB power pin and support the USB power design aspects of the Notecard.” In another, you state that I can just leave the USB pins unconnected if I don’t plan to use the USB function.
So, there’s an issue - do I have to hook up VUSB or not? The design has 3.3 and 12V, but no 5V at all. I recall a statement that VUSB has to be nominally 5V, so I suspect just hooking it to 3.3V is right out. You also don’t document the current draw I could expect to see from the VUSB pin, at least not that I could find. I would assume it’s less than 500mA, but if I use a linear from 12V that’s still a lot of waste power dissipation if I don’t need to power it at all.
I also can’t find any reference to a definition of “dynamic line voltage detection,” so I don’t really know what I might be losing if I just let it float.
Also want to confirm I don’t need to pull the USB data lines in either direction, just let them waft in the breeze, if they’re not being used.
Next USB question, and it sort of overlaps the second topic in the Venn diagram of things, and that’s “what if I only want the USB…occasionally?” The micro I’m using won’t care and can’t attach to it. But I could see an edge case where I might want to jack in a laptop to see diagnostic logging if the unit’s running but not doing quite what I expected. Assuming I don’t need to provide 5V to VUSB most of the time, is it kosher to power up VUSB and connect to the USB data pins after the unit’s been powered up, or will this cause issues?