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'[PIC]: Date Time application'
2000\12\11@094014 by Nuri ERGINER

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Hi,
I need your help for implementing a date time application via 16F84
Do I need extra hardware or is it possible to build a remainder only with
16F84.

The remainder will be triggered when the pre-defined date and time has come.

Regards,

Nuri Erginer

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2000\12\11@095430 by D Lloyd

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Hi,

I'm not sure what your precise requirements are.....are you wanting to
maintain a date/time for the duration that the 16f84 is off?
Either way, you could run the 84 from a battery and maintain the time in
software or use an external device with a battery & 32kHz oscillator.

An "easy" way of maintaning a software calendar is to use a four byte "UTC"
value [seconds since 1970] which is easier than performing a full calendar
compare. If you use the external RTC, however, you will have to perform
conversions to/from UTC so it might be easier to just use the format of the
RTC.

Regards,
Dan





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Hi,
I need your help for implementing a date time application via 16F84
Do I need extra hardware or is it possible to build a remainder only with
16F84.

The remainder will be triggered when the pre-defined date and time has
come.

Regards,

Nuri Erginer

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2000\12\11@112456 by M. Adam Davis

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There are clock/calendar chips with an alarm output which can go off
according to the date/time.  It should be easier and cheaper to use one of
those either by itself, or with a pic.

-Adam

Nuri ERGINER wrote:
{Quote hidden}

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2000\12\11@191833 by Byron A Jeff

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>
> Hi,
> I need your help for implementing a date time application via 16F84
> Do I need extra hardware or is it possible to build a remainder only with
> 16F84.

As pointed out elsewhere you'll probably need extra hardware.

If you want to self contain this in a PIC, consider stepping up to a
28/40 pin part with a timer1 onboard. This timer is outfitted with a
crystal osciallator designed for 32 Khz watch crystals. Also the timer
will continue to run and generate interrupts even when the PIC is SLEEPing.
This combination provides an effective date/time system without the
need for much additional hardware (a crystal and 2 caps). You'll need code
to track the clock, but little else.

>
> The remainder will be triggered when the pre-defined date and time has come.

Similar to an application I'm thinking about. I've put together a couple
of systems to trigger my outside lights at darkness and turn them off at
dawn. The first used an CDR to detect light. But it blew out a relay.

Currently I have the system connected to my Linux box's serial port driven
by an app that determines sunrise/sunset via a NOAA generated table.
However desktops are a bit too unstable for such applications.

What I'd like to do is embed the table along with a time/date clock in a
PIC16F877. The timer1 is perfect for the clock and the PIC can hold
a couple of years worth of tables easily.

The only stumbling block is retaining the correct time during power
outages or equipment transfers. It sucks to have to reset the time on a
clock especially on an important IMHO) application such as this one.

The logical answer is battery backup and I got some good ideas from

http://www.repairfaq.org/ELE/F_NiCd_Battery.html

which discusses the care and feeding of NiCads. The one issue that
concerns me is the suggestion that constant trickle charging will lead
to battery overcharge/voltage depression/loss of battery life.

So I'd be interested in suggestions on how to setup proper feeding of the
backup battery. The right way according to the document is recharge only
after use then only have a 1-2 ma topoff charge to prevent self-discharge.

Well this is fairly easily done with relays. However it seems like overkill
to me. I plan on using a constant current driver using a LM317. The resistor
between the ADJ and Vout pins determines the available current. Simply by
switching in a lower valued resistor in parallel to the 1ma resistor, the
amount of current provided to the battery can be changed.

But how can I switch in the resistor with a semiconductor device? For example
with a relay a circuit like this (excuse the poor ASCII art):


 o--------------/\/\/\/\/\/---------------o
 |                                        |
 |    /                                   |
 o---/  --------/\/\/\/\/\/---------------o

    COIL

is easily done. How can I replace the relay with a semiconductor knowing
nothing about the charactaristics (voltage differential, current, resistance
of the circuit above.

I'll probably fold and just use a 5V reed relay right in the spot indicated
because I can be sure that it works. But I'm real curious about semiconductor
alternatives.

BAJ

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2000\12\12@055712 by Michael Rigby-Jones

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{Quote hidden}

A bipolar PNP would probably work here, emitter to the LM317 output,
collector to the adjust pin via the resistor. Pulling the base to ground via
a current limiting resistor, would switch the transistor on.

A P type Mosfet would also be ok, with the source going to the output, drain
going to the adjust pin via the resistor and the gate pulled low to turn on
the transistor.  One thing to watch for here is the Vgs of the Mosfet.
There must be enough voltage between the source and the gate to fully turn
on the mosfet.

Regards

Mike

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2000\12\12@083715 by kayode.ayandokun

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Nuri ERGINER <EraseMEnerginerspamSSM.GOV.TR> wrote:
> Hi,
> I need your help for implementing a date time application via 16F84
> Do I need extra hardware or is it possible to build a remainder only with
> 16F84.
>
> The remainder will be triggered when the pre-defined date and time has come.
>
> Regards,
>
> Nuri Erginer

It is quite  possible to build a real time clock based on a 16F84.
One solution is to use a 3.2768MHz crystal and the following clock
calculation to give you a one second timebase.
; Clock calculation
; Crystal speed 3.2768 MHz
; Cycle speed = 819200 cycles per second
; Prescaler set to 128.  128 * 256 = 32768
; 819200 / 32768 = 25
; Increment the number of seconds every 25 counts of the interrupt
counter.

To manager the clock / calendar there are a few examples around.

http://www.hut.fi/~jalapaav/Electronics/Pic/Clock/

The March 1999 issue of Everyday Practical Electronics contained a
design based around a 10MHz 16F84 for a video time and date generator.
This might be of interest as a starting point.  Unfortuneately I don't
have the article to hand but you can get details from
http://www.epemag.wimborne.co.uk/0399.htm and the source code is
available from http://www.epemag.com/library2.htm.  The whole back issue
can be ordered from the site above.

Hope this helps.

Kayode.
________________________________________________________________________
Visit http://www.stv5730a.co.uk and put your next PIC project on TV
with the 16F84 STV5730A on screen display project board.

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