internal clock accuracy?
pico
Posts: 29
in Propeller 1
Hey guys,
I'm planning to build a digital clock with the propeller. It's gonna have a real-time clock and everything.
I've read in the forums that an external crystal provides greater accuracy. However, I just want to use the internal clock if I can.
Will it be okay if I just use the less-accurate internal crystal for my clock? Or is accuracy a huge deciding factor in my project?
Also, if I have to use an external crystal, do I need capacitors? I've played with arduinos a lot and they all require 22pf caps for the external crystal. However, most of the schematics for the propeller I've seen on google don't have caps for the external clock.
TL;DR: Is the internal crystal okay for time-critical projects, like a digital clock? Do I need capacitors for an external crystal?
THANKS!!!
I'm planning to build a digital clock with the propeller. It's gonna have a real-time clock and everything.
I've read in the forums that an external crystal provides greater accuracy. However, I just want to use the internal clock if I can.
Will it be okay if I just use the less-accurate internal crystal for my clock? Or is accuracy a huge deciding factor in my project?
Also, if I have to use an external crystal, do I need capacitors? I've played with arduinos a lot and they all require 22pf caps for the external crystal. However, most of the schematics for the propeller I've seen on google don't have caps for the external clock.
TL;DR: Is the internal crystal okay for time-critical projects, like a digital clock? Do I need capacitors for an external crystal?
THANKS!!!
Comments
If you were powering the clock from the AC line rather than a battery you could always use that as the timekeeping signal.
Yeah, an RTC chip. I will use the DS1307 and a 32kHz crystal. Sorry, but what do you mean by "it will set the system accuracy?"
-Phil
The cheapest RTC's do not have temperature compensation, so with a Prop Xtal, you can use one crystal to check the other.
The RTC uses Cap/Battery to keep time across power outages, and the Prop Xtal is enabled frequently to re-calibrate.
If you calibrate after assembly using a GPS 1pps, you are then down to the aging drift and ambient temperature changes.
What display do you plan ?
So you have to decide, "How good is good enough?" For the best accuracy, eliminate the RTC and add a GPS module -- assuming you get reception where the system will operate.
-Phil
That leaves Crystal temperature curves, like
AT-CUT :
and 32kHz xtals, are much worse over full range, but possibly tolerable over a narrow temperature band...
and nice plots here
https://vectronblog.com/2017/03/31/when-is-a-crystal-not-a-crystal-tuning-forks/
Has anyone plotted Prop RC stability with temperature, to use that as a low cost temperature sense ?
-Phil
Just use a COG to do the RTC. For this, you will need a good xtal (no caps reqd) on the prop. Don't forget to connect all power and all ground pins, and bypass capacitors at each power/ground pair. That is 2 pairs for a DIP, 4 pairs for the QFP chip.
Possible reasons would be
* Time keeping over power interrupt / outages
* Lower total system power
but if neither of those matter to the OP, then yes, they could skip the RTC and use the Prop, with Xtal.
Addit: I see Mouser has ±10ppm Tolerance ±10ppm Frequency Stability Crystals, showing as new, in HC49/4H packages. Stock of 4MHz which would give lower Icc than 5MHz
Your replies all helped me a lot!
Except they get removed at night, the body core temperature is regulated, not the extremities, and watches are cooled by the surroundings as well as heated by the arm!
One that I've actually used as a reference for a crystal-less gadget operating from a CR2032 is the Abracon ASTMK0.001KHZ, also MEMs and small, but not microscopic, it has an frequency curve comparable to a tuning fork crystal and a convenient 1Hz output for referencing the Prop RC modes.