Go Down

Topic: SMD crystals for Arduino? (Read 1 time) previous topic - next topic

dtokez

Thanks for all the useful info guys!

The resonators sound like a good option, I don't need a great level of accuracy, and I'm using a RTC for the longer term time management anyhow :)

Thanks for the info Jack! It's one thing answering a question for someone but telling them how to answer it themselves is great :) one thing I'm not sure about is how you calculate the 'stray resistance'?

Thanks again

dhenry

Quote
The resonators sound like a good option, I don't need a great level of accuracy,


Let me flip that coin for you. If you are OK with the (in)accuracy of a resonator, you are going to be more than OK with the (in)accuracy caused by no load capacitance or out-of-spec load capacitance. A crystal without no capacitance will beat out a resonator and provides you the option to obtain accurate timing in the future if you decide to solder in the capacitor(s).

With that, it is very difficult, in my view, to justify a resonator over a crystal, and that's why you rarely see them used in well designed products.

dc42


Quote
The resonators sound like a good option, I don't need a great level of accuracy,


Let me flip that coin for you. If you are OK with the (in)accuracy of a resonator, you are going to be more than OK with the (in)accuracy caused by no load capacitance or out-of-spec load capacitance. A crystal without no capacitance will beat out a resonator and provides you the option to obtain accurate timing in the future if you decide to solder in the capacitor(s).

With that, it is very difficult, in my view, to justify a resonator over a crystal, and that's why you rarely see them used in well designed products.


There is nothing intrinsically wrong with a the design that uses a resonator, if the frequency tolerance and drift of the resonator are within the required parameters. You might just as well say that well-designed products don't use 1% tolerance resistors, because 0.1% tolerance resistors are available.

On the other hand, a circuit using an atmega328p and crystal with no capacitors is definitely not well-designed, because it's running the mcu outside its recommended operating conditions (C1, C2 = 12 to 22pF, see sections 9.3 - 9.4 of the datasheet).
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Jack Christensen

#8
Nov 13, 2012, 08:25 pm Last Edit: Nov 13, 2012, 08:27 pm by Jack Christensen Reason: 1

Thanks for the info Jack! It's one thing answering a question for someone but telling them how to answer it themselves is great :) one thing I'm not sure about is how you calculate the 'stray resistance capacitance'?


Sure thing! As for stray capacitance, that might take some doing to actually measure, and test equipment that I don't have, so I don't calculate it, I just assume 5pF. Note that the Fox document says

Quote
As a rule of thumb, Cstray may be assumed to equal 5.0 pF.


so I just go with that in absence of any better estimate.

For most purposes, resonators are fine too, as noted. The Uno actually uses a resonator I believe.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

dhenry

There is also the pin capacitance.

Go Up