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Daylight savings correction.....

 
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JAM2014



Joined: 24 Apr 2014
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Daylight savings correction.....
PostPosted: Wed Jan 03, 2018 4:26 pm     Reply with quote

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
temtronic



Joined: 01 Jul 2010
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PostPosted: Wed Jan 03, 2018 6:31 pm     Reply with quote

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 [spam]) and simple to cut code for.
Jay
PCM programmer



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PostPosted: Wed Jan 03, 2018 7:42 pm     Reply with quote

Quote:

From this point forward, I then want the clock to automatically adjust the
time as necessary to compensate for daylight savings.

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/
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Thu Jan 04, 2018 1:35 am     Reply with quote

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.
temtronic



Joined: 01 Jul 2010
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PostPosted: Thu Jan 04, 2018 6:03 am     Reply with quote

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
RF_Developer



Joined: 07 Feb 2011
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PostPosted: Thu Jan 04, 2018 9:00 am     Reply with quote

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.
temtronic



Joined: 01 Jul 2010
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PostPosted: Thu Jan 04, 2018 10:02 am     Reply with quote

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
RF_Developer



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PostPosted: Thu Jan 04, 2018 10:56 am     Reply with quote

temtronic wrote:
NIXIEs !!! nice, I rememeber them...zappp.... and 'line of NE2' digital readouts and ,sigh fixin ASR-33s.


Its a retro thing :-) and the nixies are russian new old(ish) stock. It's got a PIC in there though, plus some high voltage driver thingy. https://www.pvelectronics.co.uk/index.php?main_page=product_info&cPath=1&products_id=105 In truth I didn't have to do much!
temtronic



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PostPosted: Thu Jan 04, 2018 11:09 am     Reply with quote

7442 I think is the high voltage BDC> 0-9 pin output.
guess I should have saved all my NIXIE tubes.....$$$
Jay
JAM2014



Joined: 24 Apr 2014
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PostPosted: Fri Jan 05, 2018 12:32 pm     Reply with quote

PCM programmer wrote:
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/


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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