I have made a nixie clock with 4 digits (shows only hour & and minute) but is seems like it doesnt keep the time accurate as it should. Clock diviated +2 minutes after 3 days. In the datasheet it is stated:
Accuracy ±2ppm from 0°C to +40°C
Accuracy ±3.5ppm from -40°C to +85°C
Which to my calculation was about 1-2 min diviation in a year. (0°C to +40°C)
I have few things on my mind that might cause this problem:
1)I had two ds3231 chip and I desolder one from "DS3231 for pi" module, another I did bought from local electronics shop however I am not certain which one I did solder to the board. Maybe I did some damage when I desolder/solder (I dont exceed 320-330
oC unless it isn't an IC ) and use that one in the board.
2) In the code I read the time every time SQW pin changes the state (I set SQW pin to 1hz output) maybe reading time that often might cause same problem. However in this case I would except a decrease in time not increase in time. I considered this because these modules usualy have an eeprom on them. I did looked in web but I didnt see anything that mentions this kind of situation so I dont think reading time this often from ds3231 might cause problem...
3) I have knock-off or a faulty ds3231 chip (which I am not even sure if it's possible or there is such thing) and it is simply way out of accuracy as it should be.
4) I am using 5V adapter (wall plug adapter) to power the circuit it outputs 5.10V (acording to my Multimeter) but I am not sure about any ripple or noise, but I have a input cap (470uF) which should be getting rid of some noise from the adapter which I am sure it has some but not much (I hope
)
Other than these I dont have any idea what might cause a problem fell free to point them...
First and third one can be solved by switching the ds3231 with the other one but I might need to wait couple of weeks to be sure so this one I dont want to try if there is another problem
For the 2. possible cause I will write another code that will read time from ds3231 every min, so that might solve my issue but as I said I am not hopefull about that one
Last one can be solved by 7805 regulator but I have to manually solder it some where in board and make changes and again confirm the accuracy by waiting few days or few weeks... (before sending to a pcb warehouse for some sample boards)
Part of the schematic is in the attachments...
Thanks in advance