Hi !
For me, I'd want the quarz, because I want to synchronize watches across multiple arduinos and to my nas etc..
Just switch clock sources from the internal to quartz for the 32kHz clock source, and your are done. Only after such a switchover you will recognize if the quarz is there, of course. Since I have not found many boards that have the quartz soldered in, it may well be a cool strategy to implement a check upon setting up if the external quartz is there, e.g. by temporarily useing a timer with the main clock source to determine the frequency of the 32kHz internal or external signal. When you can't measure a signal (0 pulses), switch back to internal 32kHz signal. After doing these checks it is 1 line of code to modify the RTC calibration to have low error for the current conditions like temperature and core voltage. But the library would have to repeat this, to be stable over some days, using up valuable timer resources.
Bottom line is: since so many boards don't have it, it lets us draw the conclusion that no one hat low-power applications in mind, that the sam3x is well capaple of.a
Still I will try to solder some quartz in.
Low drift is always preferred. Quatz has 20ppm, so 20sec after 20*10^6 sec. passed max. error. Nice.
It uses very low power, still, but this means: put in just the right components, otherwise it may not resonate.
So far I only read hints as to how to calculate the correct capacitance of quarz and the 2 capacitors to ground.
Intermittent question:
Couldn't you just send it to me ;)) ?
Really, I was kind a disappointed that my sanguino Due china clone has neither 1.27mm Pin headers for JTAG debugging, nor the real time clock "populated", emptyness on board!. And some 10kOhm between Pin18 ATmega16u2 output and main reset line, doing very instable reset. Soldered a 1kOhm on top, fine now.
So now it takes me quite some time to understand that I probably can _not_ just rip some 32kHz quarz from the next peice of electronic junk like old motherboard.
You have to adapt the capacitors to ground according to
on page 11:
C_L_ex32 = 2 * ( C_crystal - C_pcb - C_para_in_µC )
Thanks to the pdf with DUE circuitry we now, that C_L_ext32 = 22pF.
Right now after a lot of googling I thing that 9pF is the capacitance of the quartz one should use.
Since C_para in datasheet is given with 1,4pF,
22/2 - 1,4 - 9 = 0.6pF is the value that the designers of the PCB assumed for the PCB stripes to the quartz. I think this value is assumed a bit low, but well. If it is not working, I will have to buy the 12pF version of the quartz.
So I have found an affordable source at some electronic distributor for hobbyists that sends out in small numbers in germany:
Data is
Article-Nr.: | V705871 |
manufacturer-Nr.: | CM7V-T1A32.768kHz9pF+/-20ppmTAQC |
EAN.: | 2050001862013
|
Datasheet of quarz is
I have some problems getting affordable 22pF 0402 capacitors, either 3EUR _per piece_ or 8EUR per 1000. Both is not what I want. 37EUR for kit of 3500 will be my way if I intend to fiddle araount, with shaking hands an pincer and soldering iron, freshly sharpened tip ;)
(SMD 0402: 1 x 0,5 x 0,4 mm ^3 !!! : Dual USB Microscope is on the wish list, X-Mas perhaps... This displayed on a smartphone, hanging in a 3D cardbox in front of my eyes. Kinda cyborg, but could be cool and affordable.)
Steady hands and a good soldering iron
Andi