Method to keep track of dates without adding a DS1302 RTC?

T&E Engineer

I have a 6432 display board that I want to be able to display a message based on the day to set of dates. For example, if it was Valentines Day, it would display 1 or 2 Valentines Day messages. If it was the Holiday Season (e.g. a weeks worth of dates lets say), it would display Holiday type messages. I already know how to display messages but I want to find an algorythm that can keep track of a date or set of dates that I can use on an SX28 (or probably more likely an SX48). The messages will just be in DATA statements or in an array string or something.

Does anyone have any ideas on how this can be easily done vice having to add a DS1302 (Real Time Clock) to check the dates. Can the SX chips keep track of dates and how? I don't need to get down to the second or hour - just day or group of days.

Thanks.

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Zoot

Why the resistance to an RTC? With a crystal it's only several dollars. If you were just doing day of the week and time it wouldn't matter perhaps, but once you want to track the actual date things get tricky pretty fast.

That said, you could put the length of each month into data statements for reading (e.g. 31, 28, 31, 30, 31, 30, etc) so that as you count days upward you know when to roll the month over. You would need:

- secs count
- mins count
- hours count (to 23)
- days count
- months count
- year (even one digit would be OK, just so you could track if it's a leap year or not)

Don't forget code for changing all those values so you can reset time in the event of a power off or reset (another advantage of an RTC - easy battery backup).

Psuedo-code:

IF secs >= 60 THEN
    secs = 0
    INC mins
    IF mins >= 60
       mins = 0
       INC hours
       IF hours >= 24 THEN
           hours = 0
           INC dayOfWeek
           IF dayOfWeek > 7 THEN
               dayOfWeek = 1
           ENDIF
           INC date
           READ AddrLabel + month - 1, totalDays
           IF month = 2 THEN
               IF year & $3 = 0 THEN
                   totalDays = 29
               ENDIF
           ENDIF
           IF date > totalDays THEN
               date = 1
               INC month
               IF month > 12 THEN
                   month = 1
                    INC years
                ENDIF
           ENDIF
       ENDIF
    ENDIF
 ENDIF    

I think that covers it.

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Post Edited (Zoot) : 3/25/2009 2:39:54 PM GMT

Zoot

P.S. -- holidays can be a bear -- example figure out Thanksgiving -- it's not so easy......... stuff like Christmas and New Year's is easy obviously. I would still use table lookups for most holidays.

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Post Edited (Zoot) : 3/25/2009 2:40:36 PM GMT

T&E Engineer

So would using an RTC be easier? I could but just wanted to keep it simple. But it doesn't sound like it will.

Peter Verkaik

Quite some math will be involved, either way.

http://www.cpearson.com/excel/holidays.htm

regards peter

Old_Lady

RTC would be a *much* simpler solution!
I don't have a big SW background, but I've bumped up against the limits of the SX48 already... and had to cut out stuff in order to make it fit. I'm assuming that a big look up table would take a lot of room, especially since (depending on the year) the holidays can switch weeks, like Zoots said earlier.

Beau Schwabe

T&E Engineer,

Agreed, an RTC would be a much simpler solution, but if you were doing this as a learning exercise you could spend the time coding.

There is a "tricky" way to determine the number of days in each month without a lookup table by looking at the month value as a binary nibble and comparing Bit0 with Bit4.
Excluding February, if the Bits disagree, then the month has 31 days, otherwise the month has 30 days.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Zoot

RTCs are so cheap and easy to use and will probably be more accurate over the long-term than the SX (esp. if using a resonator on the SX). Digikey does USPS first-class shipping these days, so you can probably have them send you a chip and crystal for < $5 including shipping.

Remember though, that the RTC will not be a holiday Calendar -- you'll still have to figure out holidays from the date or day of week or week of the month, etc. As I wrote above, for dated holidays it's easy, but for "drifting" holidays (e.g. Thanksgiving, Labor Day, etc.) it can be much trickier.

There are some bonuses to an RTC also, like maintaining time even when shut-down or during reset, a bit of extra (battery backed-up) RAM, etc.

Old_Lady: the lookup table for days in a month would only be 12 bytes -- so it's not really big at all:

AddrLabel:
DATA 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31

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T&amp;E Engineer

OK - I will go with the DS1302 since I already have working time/date code for this and the 6432 board.

Thanks again for all the great input and thought into this!

pjv

Hi All;

In regard to time keeping, I need to convert in both directions between regular (Julian?) Years, Months, Days, Hours etc. and "UNIX" time, and find it quite a bear. I use Phil Pilgrim's clever algorithms for that, but in SX/B it consumes tons of precious code space that I have compacted quite a bit. Leap years really screw things up.

Does anyone have clever shorthand tricks to share?

Cheers,

Peter (pjv)

Zoot

pjv -- can you post your code (maybe in a new thread). I haven't seen PhiPi's algorithms. Presume you are talking about UNIX epochal time since 1 Jan 1970. I presume leap seconds need to be handled manually as they come up unless the device is online and has access to a timeserver (in which case you could obtain epochal time w/o calculation anyway). en.wikipedia.org/wiki/Leap_second

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StarMan

I don't know if this helps anyone but it's kind of interesting.· I learned it in my very first programming class at UCSD in 1981.· We were learning the engineering progamming language of the future, Fortran IV... with punchcards on a Burroughs.

For any given date it calculates the day of the week.

Here's the wiki: http://en.wikipedia.org/wiki/Zeller's_congruence

Chris I.