is there a real time clock on MDBT42Q bare module? If so, how accurate is it? If not, how can I add one?
Yes, there is! Someone asked this a week ago about the Puck (which has the same chip) so the answer there might help: http://forum.espruino.com/conversations/326756/#comment14462460
In terms of accuracy it's about 20 parts per million I believe. Realistically most on-chip RTCs even with separate crystals end up around that as that's what a lot of the crystals are rated at.
The DS3231 that you can add on is significantly better though at around 2ppm though.
Sorry, I don't understand what 20 parts per million means, is it like a few seconds a month?
20 (parts) per Hundred (parts) = 20 per 100 = 20% = 0.20
20 parts per Million (parts) = 20 per 1,000,000 = 0.000020.
Since an average month has 30 x 24 x 60 x 60 = 2,592,000 seconds, about 2,600,000 seconds = 2.6 M seconds , 20ppm means 20x2.6 = 52 seconds +- accuracy per month, about 1 Minute per Month or roughly 2 Seconds per Day.
1 Minute per Month
2 Seconds per Day
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