RTC chip or PIC?

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  1. - 27th July 2009, 15:04 #1
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    Default RTC chip or PIC?

    Hello,

    I was asking myself why one would prefer to use an particular RTC chip instead of using a PIC running a 32,768kHz crystal?

    Unfortunately, I don't have such a crystal in my drawer and can't make any tests but is there anything "obvious" making a big difference I do not see?
    Roger
  2. - 27th July 2009, 15:18 #2
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    I like RTC because of all of the built in stuff. Date, time, alarms etc. Save alot of programming and code space. Always running, check it when needed.
    Dave
    Always wear safety glasses while programming.
  3. - 27th July 2009, 16:12 #3
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    Quote Originally Posted by flotulopex View Post
    ... why one would prefer to use an particular RTC chip ...
    I prefer the ST M41T81. In addition to macrackit's points, it survives power cycles (even recording the time power was lost), can generate a squarewave of several frequencies and battery life is >10 years. I suppose one could duplicate all that with a small PIC but there's no cost saving.
    Last edited by dhouston; - 27th July 2009 at 16:14.
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  4. - 27th July 2009, 19:15 #4
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    I prefer the ST M41T81
    Good product, unfortunatly sold only in 8-pin SOIC package.

    Al.
    All progress began with an idea
  5. - 28th July 2009, 10:15 #5
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    Default How to trigger the "second" increment?

    I have started some trials with a DS1307.

    Since the smallest "common" change in RTC are seconds, I assume there is no use for the PIC to query constantly the RTC; every second should be enough.

    How do I achieve this? I couldn't find a clue in either datasheets
    http://www.datasheetcatalog.org/data...xim/DS1307.pdf
    http://www.datasheetcatalog.org/data...onics/7529.pdf

    Maybe, I should rather use a RTC chip with alarm such as a DS1678
    http://datasheets.maxim-ic.com/en/ds/DS1678.pdf
    Last edited by flotulopex; - 28th July 2009 at 10:49. Reason: adds
    Roger
  6. - 28th July 2009, 11:18 #6
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    Here is what I do with the DS1337. I have not used the one you have.
    Setup:
    Code:
    SecReg CON $00
DS_SCL	VAR	PORTD.1
DS_SDA	VAR	PORTD.0
RTC	CON	%11010000
sec VAR BYTE ' seconds
MINs VAR BYTE ' minutes
hr VAR BYTE ' hours
day VAR BYTE ' day
date VAR BYTE ' date
mon VAR BYTE ' month
yr VAR BYTE ' year
    Then when I want to read the RTC:
    Code:
    I2CRead DS_SDA, DS_SCL, RTC, SecReg, [sec,MINs,hr,day,date,mon,yr] 
PAUSE 100
Serout2 PORTC.6, 16572, [ "TIME ", HEX2 hr, ":", HEX2 MINs, ":", HEX2 sec,$d, $a]
Serout2 PORTC.6, 16572, [ "DATE ", HEX2 mon,"-",HEX2 date,"-",HEX2 yr,$d, $a]
    Maybe this can be adapted to yours.
    Dave
    Always wear safety glasses while programming.
  7. - 28th July 2009, 13:18 #7
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    Here are two types:

    RV-3029-C2 : Extremely Accurate RTC Module with embedded Xtal.

    * Offers temperature compensated time (very high time-accuracy of ± 6ppm from -40°C to +85°C and ± 8ppm from -40°C to +125°C.)

    * Integrated Temperature Sensor with digital-output and offers 8 Bytes
    RAM and 2 Bytes EEPROM for customer's application.


    Here is another one:
    RTC-4553
    • Built-in crystal unit.
    • Builtin 30 x 4-bit S-RAM.
    • Reference pulse output. (1024 Hz, 1/10 Hz)
    etc.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte
  8. - 28th July 2009, 19:30 #8
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    Quote Originally Posted by aratti View Post
    Good product, unfortunatly sold only in 8-pin SOIC package.
    What package would you prefer?
    www.davehouston.net
  9. - 28th July 2009, 19:38 #9
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    I think the DS1337's alarm can be set for one second. Probably doing the same thing as Dave mentioned with his.
    Dave
    Always wear safety glasses while programming.
  10. - 28th July 2009, 21:40 #10
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    Default Voilà

    Okay.

    So I modified the code as this:
    Code:
    <html><head></head><body><!--StartFragment--><pre><code><font color="#000080"><i>' PIC16F630 Fuses
</i></font><font color="#008000">@ DEVICE WDT_ON

</font><font color="#000080"><i>'-------------------------------------------------------------------------------
' Registers   76543210
</i></font>WPUA       = %00110100 <font color="#000080"><i>'Weak Pull-up of PORTA for SCL and SDA on DS1307
</i></font>TRISA      = %00000100 <font color="#000080"><i>'Set Input/Output

'-------------------------------------------------------------------------------
' Variables
</i></font>DSOut    <b>VAR </b>PORTA.2 <font color="#000080"><i>'DS1307 SQW pin #7

'-------------------------------------------------------------------------------
</i></font>MAIN:
<b>IF </b>DSOut = 0 <b>THEN
   GOSUB </b>Read_1307  <font color="#000080"><i>'Read the time &amp; date
   </i></font><b>GOSUB </b>lcd
   <font color="#000080"><i>'put PIC in low-power mode for 576+288+72=936ms
   </i></font><b>NAP </b>5
   <b>NAP </b>4
   <b>NAP </b>2
<b>ENDIF
GOTO </b>MAIN:
<b>END
</b></code></pre><!--EndFragment--></body>
</html>
    Maybe it doesn't make a lot of sense to put the PIC in low-power mode. How is it done in the "industrial" life?
    Roger
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