All the info in relation to the caps to use with crystals says 22pF and this is what I use but I am putting together a board the size of a 40pin DIL but containing USB port, ICSP port crystal, heart beat LED etc etc for breadboarding.
In order to get the component count down and keep the device within size constraints I have been looking for an alternative to the typical crystal and caps way of doing things and found few resonators which run at the right speed and are SMD so perfect BUT they have different capacitor values.
What is the range of values that will work correctly?
Can anyone point me at suitable SMD resonators for 16MHz & 8MHz operation.
Is that a board like this cowasaki?
Thru-hole DIP ATMega1284 board, with Uno shield headers.
I also did up a wirewrap version, used 15pF caps, seems work. Generally I use 22pF tho.
You can get SMD xtals too, I used one here on the ATMega32U4 Leonardino (little Leonardo with a uSD card on the bottom)
Both PCBs are available for purchase.
A search at Digikey for 8 MHz SMD resonators with built in capacitors yields 2 pages of results, maybe 9 different package types.
Similarly for 16 MHz SMD, a page of results with 5 different package types.
I designed one for the MEGA8U with heart beat LED, reset button, USB and a ICSP socket. Just to allow me to place it on a breadboard program the bootloader and have a play with it really. I ordered 10 of them including postage for £10 ($15 ish). I found a resonator on Eagle which was smaller than a normal crystal and with it being a resonator rather than a crystal and caps it saved 2 components.
Bob, if the Mega32u4 is pin compatible how much is a bare board with UK postage. I have a complete PCB manufacturing setup with bubble etching and everything but it's so much easier with a commercial board and I only want maybe 3 of them.
cowasaki,
Leonardino is designed to look like a Leonardo to the IDE, so you would download the same bootloader and install sketches the same way.
Boards are $3 each, can send to the UK for $1.50. Digikey.com is a US distributor, am sure they can send to the UK as well, not sure how much.
mmcp42 is in the UK as well, you could check with him as to UK sources.
the button you are using is large 4pin through hole, there are smaller 2pin SMD buttons.
I am getting a bulk bag of them hopefully in the next few days,
10ct/each or 5ct @10 pcs
Resonators are only good for low speed USB, if you want full speed usually a crystal is required (at least for PICs). If you don't want to use USB then a coil would be sufficient, also gets rid of the extra capacitors. I never tried SMD coils but should be possible.
I use three-pin ceramic resonators. The 22pF cap value seems to be more or less standard, but the datasheet will usually have specifications, or a formula, for which you can derive the proper values. Always follow the datasheet if there's any conflict between that and common values.
That said, the exact value is only critical in terms of accuracy. If your chip is standalone and interfacing with other devices that aren't ultra precise timing critical, a resonator is more than adequate. For an ATmega acting as a USB client, get a crystal and follow the recommendations on caps (or buy a crystal with integrated caps) as the spec will be much tighter than your typical use case. I have 12MHz crystals for this, and 8 & 16MHz 3-pin resonators for everything else. Mine are all through-hole, but I'm sure the same applies to SMD.
