has anyone replaced the crystal on the Mega 2560? I've seen some documents on how to change the crystal on an Uno R3 but the Uno R3 uses a different resonator and has the pinouts and through-holes for an HC49 crystal.
I really don't understand why the design cheaped out and used a ceramic resonator for the CPU clock. How much money did they save, really?
The 1000ppm "accuracy" (or lack thereof) of the ceramic resonator and its immense temperature drift are driving me crazy. My project requires much better resolution...
orly_andico:
I really don't understand why the design cheaped out and used a ceramic resonator for the CPU clock. How much money did they save, really?
Per board, not a lot. Per 10,000 boards, quite a sizeable amount.
I browsed some pricing at Mouser.com, both can be had for ~20 cents each when buying by the reel.
They could have left holes/pads for crystal/caps for those that wanted to change.
orly_andico:
I really don't understand why the design cheaped out and used a ceramic resonator for the CPU clock. How much money did they save, really?
Per board, not a lot. Per 10,000 boards, quite a sizeable amount.
I don't buy this rationalization, given how much the boards are being sold for. They are
really very expensive for what you get, and the poor timing accuracy of the ceramic
resonators is a huge demerit.
Maybe they got a huge truckload of resonators for free ("just bring your shovel!")
It would be nice if some third party made a Mega 2560 with a crystal for the clock. There are tons of China clones on ebay (in fact my Mega 2560 is a China clone) and they all slavishly copied the Arduino design. Maybe there really are compelling economic reasons to use ceramic resonators.
It's especially a salt in the wound type of injury in that they did use a proper crystal for the USB converter 16U2 micro, probably because the USB timing requirements wouldn't work reliably with a ceramic resonator. But for our users sketches they seemed to feel tight timing requirements would never be required, NOT.
Frankly that is one of the reasons I choose to not buy Uno and mega2560 type boards after they were released, instead staying with boards still using crystal resonators and FTDI USB chips, they have been faultless and reliable for me and are still available as clone supplied (well maybe not in a 2560 type?)
orly_andico:
Maybe there really are compelling economic reasons to use ceramic resonators.
Compelling? As lefty indicates, the downsides far outweigh anything else.
With 0.5% tolerance on ceramic resonators, that clocks out to as much as 0.3 sec
every minute in timing error. That's horrendous for an embedded system.
Do you think there'd be much interest?
I priced up a board at small qty pricing from digikey,
No autopower select, no SCK buffering. 16U2/2560, 2A 5V regularor, 1A 3.3V regulator with reverse bias protect diodes.
All 0805 size components, 1812 for PTC fuse
Comes out to ~$75.20 assembled for 10, dropping with qty, down to $48 for 100, then way less drastically after that. Mailing cost ~$5.25 in flat rate box (not risk getting squished in padded envelope).
(left out power select header & jumper, reset enable select header & jumper, so will be a touch more).
I'd be willing to design this up, & arrange having them built up via CBAS-USA, would need upfront orders from folks to cover material costs.
Any takers?
Do you think there'd be much interest? ...... 16U2/2560
I think it's an orphan. From my perspective, I don't see why anyone would want to solder up
their own 100-pin smt chip. For D-I-Y stuff, the ATmega1284 is a real step up from the 328,
and DIP40 is easy enough to build with. And even 2/off 1284 boards for about the same price
as one Mega board gives you as many I/O pins and 2X the processing power.