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JAM2014
Joined: 24 Apr 2014 Posts: 138
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Daylight savings correction..... |
Posted: Wed Jan 03, 2018 4:26 pm |
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Hi All,
I'm looking for ideas on how best to implement a strategy to add daylight savings time correction to an existing clock project. This clock consists of a PIC, an RTC (DS3231), and an GLCD display. The time/date is set with three pushbuttons and a simple menu system. In concept this is an easy task, but some of the details might become a bit messy if not handled correctly. I'm trying to formulate the logic that will get this task done seamlessly. I've already got working code that allows me to calculate the day in March that daylight savings begins, and the day in November that it ends. Now, I need to use this information to intelligently maintain the correct time.
I want the user to be able to set the clock to the current local time whether it's standard time or daylight time. From this point forward, I then want the clock to automatically adjust the time as necessary to compensate for daylight savings.
My plan is to have the time/date menu system set the RTC with the current time/date & set a boolean flag, 'bIsDST', in EEPROM to keep track of whether the currently stored RTC time represents daylight time or not. Then, at each subsequent time transition, the code will add/subtract one
hour from the RTC time & set the bIsDST flag as appropriate.
Does this scheme sound like it will get the job done? It seems like it will, but I don't want to under/over-think the problem!
Thanks!
Jack |
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temtronic
Joined: 01 Jul 2010 Posts: 9177 Location: Greensville,Ontario
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Posted: Wed Jan 03, 2018 6:31 pm |
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It sounds good 'on paper', try it,.. but I'd use the battery backed RAM of the RTC module, at least for test purposes. It's faster, never wears out (in case of a code booboo) and simple to cut code for.
Jay |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Wed Jan 03, 2018 7:42 pm |
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Quote: |
From this point forward, I then want the clock to automatically adjust the
time as necessary to compensate for daylight savings.
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Why not do it the conventional way ? i.e., check continuously for the
DST start and end date/times, and do the adjustment once in March
and once in November ?
http://www.instructables.com/id/Adding-Daylight-Savings-Time-to-Your-RTC/ |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19382
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Posted: Thu Jan 04, 2018 1:35 am |
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The normal way to do this is to Have the clock itself not change, but work in UTC. Then as PCM_programmer says just turn on the DST flag in March, and off in November. Ideally store just two UTC offsets. The one for 'winter' time (so the local time zone), and the one for 'summer'. This way you can cope with situations where the DST is not just one hour. Historically some countries did try 2 hour DST. Don't know if any do at present, but it is worth having the system able to cope. If the flag is 'on', you add the winter offset, if 'off' the summer offset. |
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temtronic
Joined: 01 Jul 2010 Posts: 9177 Location: Greensville,Ontario
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Posted: Thu Jan 04, 2018 6:03 am |
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Yeah DST isn't 'standard' ( no pun ). Here in Canada, Newfoundland is 1/2hr off..and some areas don't observe DST.
The problem I've had over the years is 'they' changed the dates for DST 2 or 3 times since the late 70s. Original RTC chips like the DS1287 had the dates 'burned in' and could change automatically. Nowadays theres a huge Windows program in every PC that does what we can do in 4-5 lines of PIC code !
A benefit of using UTC is your code is easliy transportable to other countries or if you use a GPS sat to 'sync' your RTC to.
Jay |
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RF_Developer
Joined: 07 Feb 2011 Posts: 839
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Posted: Thu Jan 04, 2018 9:00 am |
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I have a nixie tube clock at home. Built from a kit. Took all of half an hour to put together. Nice little thing :-) I've got the optional GPS synchroniser.
The solution the clock designer adopted was simple: all these problems of different places changing by different amounts at different dates is a big problem. So it has a simple push button on the back that you press to manually select savings mode. Only need to press it twice a year (in most places). In countries that don't have daylight savings you don't have to press it at all. The rare places with half and two hours offsets just have to lump it, and in truth wouldn't be buying it anyway as the supplier is a UK one-man-band and so they are probably only going to the UK and a few European countries.
Simple is often better. Implementing the locallised change dates and offsets is simple but complex if "properly" implemented. PCs can do it because they have all that locallisation provided with the OS and have access to updates for any changes.
I do remember doing a radar remote sampling system which worked exclusively in UTC (no savings). It had a GPS card and a Windows clock sync application called Tardis (its still going apparently: http://www.mingham-smith.com/tardis.htm). It fell over for a week in March 2000 as it was using the DOS daylight savings API/code that turned out to give a different change date from the Windows (at that time was still using 95) daylight savings code. It was a real mess for a week, after which it came back into synchronisation. That went down as a Y2K bug of sorts. |
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temtronic
Joined: 01 Jul 2010 Posts: 9177 Location: Greensville,Ontario
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Posted: Thu Jan 04, 2018 10:02 am |
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NIXIEs !!! nice, I rememeber them...zappp.... and 'line of NE2' digital readouts and ,sigh fixin ASR-33s.
I still have some 5 volt, 7 segment INCANDESCENT DIP pkg displays around here ...'somewhere'.
I also 'upgraded' the monodigichron to a PIC18F46K22 and 8by12 LED display. kinda overkill but 46K22s are always nearby !
Jay |
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RF_Developer
Joined: 07 Feb 2011 Posts: 839
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temtronic
Joined: 01 Jul 2010 Posts: 9177 Location: Greensville,Ontario
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Posted: Thu Jan 04, 2018 11:09 am |
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7442 I think is the high voltage BDC> 0-9 pin output.
guess I should have saved all my NIXIE tubes.....$$$
Jay |
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JAM2014
Joined: 24 Apr 2014 Posts: 138
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Posted: Fri Jan 05, 2018 12:32 pm |
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Hi All,
Thanks for the replies!
PCM, thanks for the link to that code! That basic method is essentially what I have been thinking about implementing, and that which I tried to describe in my first post!
I'll report back in a week or so how things worked out!
Jack |
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