See how active this thread is, and how many options you have! My two cents (sorry - pence)....
Longer term (days, weeks), a mains referenced clock will be your best bet - your utility suppliers see to that in the longer term, for the benefit of the many MILLIONS of legacy mechancial devices that use the AC mains as their only reference. However, such a clock will gain and loose a few seconds throughout different times of the day, when the grid slows down a little under heavey load (think car going up hill), then speeds up again to make good the "lost time" during the night and late mornings. If you want good absoute time, all the time, even with the actual of day time itself, then GPS or WWVB/MSF time code receivers are the way to go. But beware - radio signals can be unreliable, you will still need a way of maintaining time locally in the event that your external source fails for some reason. Also, you are looking at more cost, more wires, more software. Simple Crystal oscillators that are designed just to clock your CPU can be PANTS (quaint English expression meaning "not particulay good"). They will let you down in three ways; absolute frequency, long term drift and Temperature. Sure, some clever software can correct for absolute frequency errors once you know what the error is, but no simple solutions that I have seen will accomodate temperature drift, which can be significant. Which brings us to Temperature Controlled Crystal Oscillators (TXCO). These can be bought now as IC's that run cold (no heating requirements). The on-board precision crystal is already laser tuned to be pretty damn near spot on (and is also still controllable in absolute terms using internal software settings), AND an onboard temperature sensor adjusts the capacitance across the crystal to compensate for temp changes. Check out the DS3231 from Maxim. It is only available as an SMD part, I beleive, but I have hand soldered these without issue. This little device is quite capable of offering better than 15 seconds a year accuracy - more if you have the time to fine tune it in the first place. Maxim call it "Extremely Accurate", which for a general clock, it is. It leaves similar simpler crystal controlled RTCs in the dust for accuracy. And you get battery backup, of course. I have built clocks around ALL of the above options. They all work, within the limits mentioned. My favourite is now the DS3231. For all practical purposes (of mine), it makes the GPS or WWVB route somewhat redundant. That was probably THREE cents' worth :) On Jun 2, 3:39 am, threeneurons <threeneur...@yahoo.com> wrote: > > in the US, line frequencies are its adjusted > > whenever it hits 20s/10s of error. > > :http://en.wikipedia.org/wiki/Utility_frequency. > > > - Robert > > I've used line frequency for decades, and I only touch those clocks > twice a year. And usually that's only to adjust the hour (Daylight > Saving). Rarely touch the minutes button. The correction scheme > described on Wiki should be satisfactory for long term accuracy. Short > term, who cares. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to neonixie-l@googlegroups.com. To unsubscribe from this group, send email to neonixie-l+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
