October 18, 2011

First look: EtherMega production samples

It's taken longer than we would have liked, but the EtherMega has finally made the leap from bits to atoms!

It's got "the lot": everything we could cram into the most versatile board possible. It has the Mega 2560 microcontroller with lots of code space, ram and I/O; on board Ethernet; micro SD card slot; USB; and a switchmode power supply.

Some things of note:

PCB Colour. The picture above shows a design-validation sample so the PCB colours aren't correct: the production units will be in the usual Freetronics colours with yellow markings.

Power supply. The reason for the big delay in getting to this point has been swapping out the linear reg for a switchmode supply. You'd think it would be simple, but no, it turned out to have all sorts of side-effects! The supply we've used is rated up to 28V input, which means you can connect it to any handy power supply in the 7-28Vdc range and it'll just work without causing overheating problems. Current model boards with a linear regulator and an Ethernet shield run a tight-rope between getting enough power to run the Ethernet chip (which requires a good, solid 5V) and overheating the reg. Because I tend to mount Arduinos in odd places (inside walls, etc) it's important to me to have a board that runs cold, so I was determined to go with the switchmode supply even though it caused delays.

Power source selection. Near the upper left of the board you'll see a 3-way male header with a jumper fitted. That's to select the power source between USB and DC IN. Yes, we dropped the power auto-select for this one, in favour of a reliable high current power selection jumper. One of the big complications with the switchmode supply is that the chip can't handle a back-voltage being applied to its output, so with the traditional supply auto-switching circuit the chip gets fried the moment you plug in USB power. Oops. In the end a simple jumper was the most robust solution. We looked at switches to use instead, and found that tiny switches able to fit on the board just aren't rated to the 300mA+ that would be required. In fact most of the tiny surface-mount switches you see are only rated to about 200-300mA maximum! So, a jumper it is.

Soooo close. The first batch of production units will begin any day now.

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