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'[PIC] cheapest RTC solution..?'
2009\06\24@082053 by Mike Harrison

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I have a slightly odd requirement for what can be considered to be a comms logger - needs to wake a
few times a sec, look for some data, and if received, record a couple of bytes & timestamp it. Only
need 2 IO's, must be very low avarage power (2032  coin cell )
I can't run everything on a 32K xtal as I need higher speed to do the comms - a T1 oscillator would
be usaeble but the lower-end 8/14 pin PICs I've looked at so far don't do T1 oscillators. The
ability to turn off the 32K osc for storage would be an advantage
Unfortunately Microchip's parametric tables don't allow a search for T1 osc capability....

I'm thinking the solution will either be :
PIC12 (e.g. 635) + external RTC chip.
PIC16 with T1 osc
May need a few K of  external eeprom. so a comined ee+rtc device  may be an option if it works out
cheaper

So the question boils down to
1) what is the cheapest RTC chip ( qty 1K, readily available, sensible package - no BGA/CSP).
and
2)  what is the cheapest PIC (or AVR) with nanowatt (low power WDT wake - for use when RTC disabled)
and T1 oscillator.

Any suggestions welcome.
Not really interested in going outside PIC & AVR as learning yet another toolset isn't worth it for
this project, unless there's a super-cheap/good solution!

2009\06\24@091245 by Bob Ammerman

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You might consider using a 32Khz crystal for timekeeping and the internal RC
oscillator for operation. You can calibrate your baud rate divisor using the
known speed of the 32Khz oscillator.

-- Bob Ammerman
RAm Systems

{Original Message removed}

2009\06\24@112245 by Dwayne Reid

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At 06:21 AM 6/24/2009, Mike Harrison wrote:

>I have a slightly odd requirement for what can be considered to be a
>comms logger - needs to wake a
>few times a sec, look for some data, and if received, record a
>couple of bytes & timestamp it. Only
>need 2 IO's, must be very low avarage power (2032  coin cell )

I know that you aren't really interested in other processor families,
but this project just screams out "MSP430" to me.

The TI MSP430 has its own share of problems but is just excellent for
low power applications such as what you just mentioned.  Toolset was
surprisingly easy to use - even for assembler programmers like my
co-worker and myself.

One of the little dev boards given away at TI's annual "430 day" has
a real-time clock driving a LCD display and has been running for at
least 3 years on its original 2032 coin cell.  It has to wake up at
least often enough to refresh the LCD.

TI does have some small pin-count chips available now.

dwayne

--
Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2009\06\24@120912 by Alan B. Pearce

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You may want to look at http://www.qwikandlow.com/ where the author
discusses using a pair of 18LF series devices. In the downloadable book he
appears to use a 32kHz crystal for a clock using the standard timers.

2009\06\25@000147 by Justin Richards

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I cant help with the original post but I am working an what I think is an
application that incorporates RTC.

What i have been curious about is a definition of RTC.

The app I have updates a 32 bit seconds counter via timer interrupt and the
seconds counter is synced every 24 hours to a local NTP server.

The civil time (GMT or WST) is then calculated every time it is needed ie
when the time display is updated (every second).

So I dont keep track of min,hour, day, month, year so to speak but I do keep
track of the secs since the epoch.

Is this considered an RTC.  It also forgets the time when the power is lost
(something I want to fix) and needs to resync to NTP.

I cant decided if it is best to simply track the secs and calculate civil
time every display update, or track civil time and update the 32 bit secs
counter every 24 hours and then sync civil time from this.

Is there a standard approach here.

All suggestions comment welcome.

Cheers Justin





2009/6/24 Alan B. Pearce <.....Alan.B.PearceKILLspamspam@spam@stfc.ac.uk>

> You may want to look at http://www.qwikandlow.com/ where the author
> discusses using a pair of 18LF series devices. In the downloadable book he
> appears to use a 32kHz crystal for a clock using the standard timers.
>
> -

2009\06\25@064908 by Jan-Erik Soderholm

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Not specificaly answering your queestions maybe,
but note that many of the newer PIC18's has
an RTC builtin. See e.g. PIC18F24J11.

Justin Richards wrote:
{Quote hidden}

>> --

2009\06\25@083523 by Gerhard Fiedler

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Justin Richards wrote:

> What i have been curious about is a definition of RTC.
>
> The app I have updates a 32 bit seconds counter via timer interrupt
> and the seconds counter is synced every 24 hours to a local NTP
> server.
>
> The civil time (GMT or WST) is then calculated every time it is
> needed ie when the time display is updated (every second).
>
> So I dont keep track of min,hour, day, month, year so to speak but I
> do keep track of the secs since the epoch.
>
> Is this considered an RTC.  

IMO yes.

> It also forgets the time when the power is lost (something I want to
> fix) and needs to resync to NTP.

It's an RTC without a battery backup :)

I think typically an RTC is a module that does the calculation
(especially the date calcs) for you. When you do it in firmware (and it
doesn't matter whether you do it on demand or whenever you increment),
you got an RTC in firmware...

I don't know for sure, but I guess it costs less cycles to keep the time
in secs, mins etc. than to do the calculation on demand, especially when
you need to calculate date.

Gerhard

2009\06\25@095440 by olin piclist

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Gerhard Fiedler wrote:
> I think typically an RTC is a module that does the calculation
> (especially the date calcs) for you.

I think a RTC is anything that tracks real time.  Commonly available
separate RTC modules do usually provide expanded date format, but I don't
see that as a requirement.  Sometimes a absolute seconds since a reference
time is more convenient for doing internal calculations.  In fact, a
expanded date is usually only good for displaying to a user.  Caclculating
time differences with such a format can be a pain.

If a internal absolute seconds value and user-visible expanded date are both
required, it's probably easier to keep both live than trying to convert from
one to the other on demand.  In other words, it's a lot easier to increment
year/month/day/hour/minute/second every second than to convert that from
seconds since the start of year 2000, or whatever you use as your 0
reference time.


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Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\06\25@181208 by Jinx

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> In fact, a expanded date is usually only good for displaying to a
> user. Calculating time differences with such a format can be a pain

I found that. I have a couple of projects where a calendar was
abandoned in favour of seconds. A ticked calendar is running but,
for me anyway, it's tidier to calculate intervals in seconds and then
convert back to calendar (division / table), which is the user input
and PIC display



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