Saturday, October 26, 2013

3.3v it is, and ordered new batch of PCBs

I looked at what was needed to move the Arduino 'cpu' from 5v to 3.3v, and it looks doable.  the CPU certainly support 3.3v, a bit at a lower clock frequency, but there are some futzing that is needed in the boot-loader that I still need to research.   In any case - to get things moving along I revised the hardware to a pure 3.3v only design and orders PCBs today.   As soon as the Mouser BOM website starts cooperating,  I will place parts order as well.

There were a few considerations in moving the 3.3v, mainly software.  It seems the Arduino boot-loader enables a 'brownout' detection in the Amtel CPU and sets it trigger for something like 3.8v, that will need to be adjusted.  A bigger change was the need to reduce the CPU clock speed from 16Mhz to 8Mhz. - with the concern being will the firmware still work?   I did some timing tests.  It takes around 1-2mS for the current firmware to complete a 'cycle', and there is about 6mS per cycle of headroom.   Meaning I can add another 6mS of code (which I did via dummy delay(1); called) in the mainloop and things will still work well.   So, with a 6:1 buffer I feel comfortable cutting down to a 3:1 when running at 8Mhz.

While I was at it, I added a proper ISPC connector, in case I need to re-flash the boot-loader after soldering in the CPUs  (or worst case, in case I abandon the Arduino environment all together and go with a native Amtel development environment).  Needed to scrub all the resister dividers, as well as change up the Zeners.

I have already posted v0.1.1. of the schematics and PCB layout (See tabs above).  Will post a BOM in the next day or so, as soon as Mouser starts cooperating.  And I will go back in an update the Hardware Overview to reflect the new 3.3v only system.

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