On 10/21/15 8:30 AM, Grant Likely wrote:
On Tue, Oct 20, 2015 at 9:11 PM, David Mandala firstname.lastname@example.org wrote:
- The board has a solder bridge jumper (QS1) for selecting between
running the IO at 3.3V and 5V. 5V is the default for compatibility, but someone who knows what they are doing can switch the solder bridge to run at 3.3V for everything. (Most sensor devices appear to be 3.3V these days. The ATMEGA will happily run at either 3.3V or 5V)
Q: Is a solder jumper the best way to select the voltage level? Or should I put a physical switch on the board?
Personally I'd use a 3 pin jumper, pin 1-2 is 3.3VDC operations, pin 2-3 5VDC operations. My second choice would be a switch. Last choice would be a solder bridge, you might be surprised how many people shy away from a soldering iron.
I have a concern here. A solder jumper I'm not too worried about, but I don't like the idea of routing all of the supply current through a shunt. I figured I'm not the first person to deal with this, so I went and took a look at what others have done. This is what I found on the Seeeduino design which can be run at both 3.3V and 5V:
The 3.3/5V selection switch is merely a signal routed to a couple of mosfets which gate the power. That removes the power supply trace from the jumper or switch entirely. I assume it is done that way to reduce noise on the supply rails. I'm wondering if I should do the same here.
If you have space for the parts, that makes the most sense to me. I too was worried about a small circuit board switch carrying power load, Generally jumpers have bigger chunks of copper bridging the posts, but not shunting the power makes the most sense. That makes the idea of a small circuit board switch much more attractive.