I am going to purchase an arduino Mega2560 and I would
like to know whether it uses a 16 Mhz crystal or resonator
I know a crystal is used for serial communication, but what about
the clock for the atmega2560, is it a crystal or resonator(like on the UNO),
also in here --> link
it appears that the have replaced the the crystal for the USB with a
resonator, is this true,
I know that a resonator is good enough for most jobs and the answer to the above questions will not affect my decision in the least,
It is purely for my information only because the I tried searching
and I could find an answer.
The USB Specification frequency tolerance for full speed devices is 0.25%. We have no way of knowing what is the actual tolerance of devices used to build the boards, and, in fact we have no way of knowing whether the devices were even specified with the correct load capacitance for the ATmega. "Typical" inexpensive resonators might have tolerances of 0.5% or even more, although tighter tolerances may be available.
Regards,
Dave
Footnote:
We also have no way of knowing whether the crystals used on some of the Arduino boards that we buy are specified at the correct load capacitance, but even with rather large errors in load capacitance, frequencies are typically within 100 ppm or so.. Atmel's data sheets tend not to be very specific about crystal load capacitance. (I checked the ATmega8U2 data sheet some time ago and it said the load capacitor values should be somewhere around 12-22 pF. That's the same range given for ATmega328, '1280 and '2560 devices. Digital IC circuitry used for oscillators with external crystals or resonators tend not to be particularly tightly controlled by manufacturers, and values vary from batch to batch. As a matter of fact my observation of several '328 boards that I have built is that the frequency is actually some few tens of parts per million high (or even a little more) when using 22 pF capacitors. Other batches of chips may have slightly different characteristics.
The purpose of the USB frequency specification is this:
If actual frequency of controllers at both ends of the USB cable are within the USB tolerance specification, USB should be OK.
However...
If the frequency tolerance specification of the crystal or resonator in the controller at one end of the USB cable is looser than the USB specification, the actual frequency very well may be within spec.
And...
Even if the actual frequency is (slightly) out of tolerance, it still might work.
Note that directly measuring the in-circuit operating frequency of a crystal or resonator with a 'scope or a counter requires an extremely low-capacitance test probe (kind of expensive), not the one that I would expect to come with a general-purpose instrument.
Bottom line:
I hate to repeat myself, but we have no way of knowing what is the specification (let alone the actual operating frequency) of the resonator or crystal installed on boards that we buy. If it works, it works.
Regards,
Dave
Footnote:
If I were designing a USB circuit with an ATmega8U interface I would probably specify a crystal. That doesn't mean that a UNO that you buy is automatically defective just because it has a resonator.
I mean, if it is for a few circuits for my own use, the cost of a crystal versus cost of a resonator is a non-issue. I would use the low-risk approach of making sure my end of the circuit is designed within specification. If, for some reason, I absolutely had to use a resonator that was at the edge of (or beyond) the USB tolerance specification, I would have a test plan that let me verify with absolute certitude that that part of the circuit was operating properly before spending any time debugging software or other hardware issues.
On the other hand if I were going to produce large quantities of these, the risk of having a certain percentage of failures (in the Manufacturing Test Department or in the field) due to marginal circuit design would weigh very heavily on my sense of self-worth as a designer, such as it is.
I don't own either a Uno or mega2560. But from following the boards introduction (via their published pictures) if appears early one used crystals for both chips and later they changed to resonators for both chips. That may not be an accurate statement but it's what I'm going with.
So which one you get if you order would depend on how long it's been in stock with the seller? And yes I would prefer crystals if I was buying, even though resonators might be 'good as needed' or not.
Again, thanks for the replies,
Well as you said, if it works, it works
Although most people would prefer having a
crystal simply because its a crystal, a resonator seems
to be good enough.
is a bit misleading because it says it uses a 16 MHz crystal and
not a resonator.
When will the next iteration or the arduino arrive,
If it is during the next 2-3 months I will feel disappointed
because I just bought my Mega, Should I be concerned about this??
Will it arrive during the next 2-3 months since after all, it is 2011
:o
because the I tried searching
