v4 at the fab

So, I changed my mind again. I was about 2/3 of the way through breadboarding the fourth version of the watch circuitry, and I decided I didn’t want to build a SPI bus with multiple slaves. That, and 8mhz was really too slow. So, I went back, and re-designed with the NRF51822. It’s a fine chip, and somewhat reduces my part count.

I’ve sent the board version off to the fab, but not much else has changed. I’m going to desolder the NRF from one of my RFduinos, to skip building a bootloader for now. It’s a pretty simple matter, but there aren’t any open source examples that I can find, and so I’m taking the easy route. If I ever produce more than one of these at a time, I’ll build a bootloader from scratch, and try to make it as open as possible. The softdevice that runs the radio is a blob, which is unfortunate, but there’s nothing I can really do about that, unless I end up making tens of thousands of these things, and can hire someone to build an open version. I’ve looked around for alternative ARM M0 chips with bluetooth, and there aren’t that many. NXP makes this one , but it also has a blob for the BT stack. There is an open-ish bootloader for it, in active development, but there isn’t really any reason to switch at the moment. I’m sort of thinking the next version will have wifi, but the power requirements for that are nuts.

Things I need to write about: Using KiCad, from schematic to board. It’s actually not that bad, once you figure out a few things. Also, design considerations for manufacture, which I’ve mostly been ignoring. Through hole components are bad, and dense packing is bad. I don’t have any ground planes in the current board, which means that I’ll probably have weird intermittent faults, if it works at all.

Posted by Matt on 2015-05-12 04:49:27 +0000