RTC chip or PIC?

**flotulopex** · - 27th July 2009, 15:04

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?

**mackrackit** · - 27th July 2009, 15:18

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.

**dhouston** · - 27th July 2009, 16:12

Originally Posted by flotulopex

... 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.

**aratti** · - 27th July 2009, 19:15

I prefer the ST M41T81

Good product, unfortunatly sold only in 8-pin SOIC package.

Al.

**flotulopex** · - 28th July 2009, 10:15

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

**mackrackit** · - 28th July 2009, 11:18

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.

**sayzer** · - 28th July 2009, 13:18

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.

**flotulopex** · - 28th July 2009, 19:24

Thanks mackrackit,

My question was more about to understand how to trigger the time (seconds) change to start a display update or refresh.

I was thinking about putting the PIC in low-power mode until the next second change happens; how to achieve this the best way?

So is there any particular specification of RTC chips that will allow one to find a modell having, for example, a functionality setting a bit or a flag telling the second has changed?

BTW, here is the code (multiple sources - thanks to All of them) I work on now for my tests:

Code:

<html><head></head><body><!--StartFragment--><pre><code><font color="#000080"><i>' PIC16F630 Fuses
</i></font><font color="#008000">@ DEVICE BOD_OFF
@ DEVICE CPD_OFF
@ DEVICE PROTECT_OFF
@ DEVICE MCLR_OFF
@ DEVICE PWRT_OFF
@ DEVICE WDT_OFF
@ DEVICE INTRC_OSC_NOCLKOUT

</font><font color="#000080"><i>'-------------------------------------------------------------------------------
' Registers   76543210
</i></font>WPUA       = %00110000 <font color="#000080"><i>'Weak Pull-up of PORTA for SCL and SDA on DS1307
</i></font>INTCON     = %00000000 <font color="#000080"><i>'Interrupts
</i></font>OSCCAL     = %01100000 <font color="#000080"><i>'Oscillator Calibration when Internal RC set to 4MHz
</i></font>OPTION_REG = %10000000 <font color="#000080"><i>'PORTA Pull-Ups/TMR0/Prescaler
</i></font>CMCON      = %00000111 <font color="#000080"><i>'Comparator Module is OFF
</i></font>TRISA      = %00000000 <font color="#000080"><i>'Set Input/Output
</i></font>PORTA      = %00000000 <font color="#000080"><i>'Ports High/Low
</i></font>TRISC      = %00000000 <font color="#000080"><i>'Set Input/Output
</i></font>PORTC      = %00000000 <font color="#000080"><i>'Ports High/Low

'-------------------------------------------------------------------------------
'PORTA.0 = ICSP Data
'PORTA.1 = ICSP Clock
'PORTA.2
'PORTA.3 = MCLR
'PORTA.4 = SDA
'PORTA.5 = SCL
'PORTC.0 = LCD-DB4
'PORTC.1 = LCD-DB5
'PORTC.2 = LCD-DB6
'PORTC.3 = LCD-DB7
'PORTC.4 = LCD-E
'PORTC.5 = LCD-RS

'-------------------------------------------------------------------------------
' LCD cabling
'-       ( 1) Vss - GND
'-	 ( 2) Vdd / Vcc +5V
'-	 ( 3) Vee / Vo / Contrast adjust / Pot 4k7(1-2-W3) or R2k2 (GND-3)
'PORTC.5 ( 4) RS
'-	 ( 5) R/W -&gt; to Vss
'PORTC.4 ( 6) E
'PORTC.0 (11) DB4
'PORTC.1 (12) DB5
'PORTC.2 (13) DB6
'PORTC.3 (14) DB7

</i></font><b>DEFINE </b>LCD_DREG PORTC   <font color="#000080"><i>'LCD data port 
</i></font><b>DEFINE </b>LCD_DBIT 0       <font color="#000080"><i>'LCD data starting PORT.bit (0 or 4)
</i></font><b>DEFINE </b>LCD_RSREG PORTC  <font color="#000080"><i>'LCD register select port 
</i></font><b>DEFINE </b>LCD_RSBIT 5      <font color="#000080"><i>'LCD register select bit 
</i></font><b>DEFINE </b>LCD_EREG PORTC   <font color="#000080"><i>'LCD enable port 
</i></font><b>DEFINE </b>LCD_EBIT 4       <font color="#000080"><i>'LCD enable bit 

'-------------------------------------------------------------------------------
' Variables
</i></font>SDA      <b>VAR </b>PORTA.4 <font color="#000080"><i>'DS1307 SDA pin #5
</i></font>SCL      <b>VAR </b>PORTA.5 <font color="#000080"><i>'DS1307 SCL pin #6
'SDOut    var PORTC.2
</i></font>DB       <b>VAR BYTE</b>[8] <font color="#000080"><i>'Data byte array
</i></font>RTCSec   <b>VAR </b>DB[0]   <font color="#000080"><i>'alias individual bytes in array
</i></font>RTCMin   <b>VAR </b>DB[1]
RTCHour  <b>VAR </b>DB[2]
RTCDay   <b>VAR </b>DB[3]
RTCDate  <b>VAR </b>DB[4]
RTCMonth <b>VAR </b>DB[5]
RTCYear  <b>VAR </b>DB[6]
RTCCtrl  <b>VAR </b>DB[7]
AMPM     <b>VAR BIT
</b>A_Ctr    <b>VAR BYTE    </b><font color="#000080"><i>'counter
</i></font>B_Ctr    <b>VAR BYTE    </b><font color="#000080"><i>'counter
</i></font>Day_T    <b>VAR BYTE
</b>Day      <b>VAR BYTE</b>[3]
Day[0] = 0
Day[1] = 0
Day[2] = 0

<font color="#000080"><i>'-------------------------------------------------------------------------------
</i></font><b>PAUSE </b>1000        <font color="#000080"><i>'time for LCD to settle
</i></font><b>LCDOUT </b>$FE, 1     <font color="#000080"><i>'Clear LCD
</i></font><b>GOSUB </b>WRITE_1307  <font color="#000080"><i>'set the time or don't

</i></font>MAIN:
   <b>GOSUB </b>READ_1307 <font color="#000080"><i>'Read the time &amp; date
   </i></font><b>GOSUB </b>LCD
   <b>PAUSE </b>1000
<b>GOTO </b>MAIN:

<font color="#000080"><i>'-------------------------------------------------------------------------------
</i></font>LCD:

<font color="#000080"><i>'Hour in &quot;hh:mm:ss&quot;
</i></font><b>LCDOUT </b>$FE, 2, <b>DEC </b>(RTCHour&gt;&gt;4 &amp; $0F), <b>DEC</b>(RTCHour &amp; $0F),<font color="#FF0000">&quot;:&quot;</font>, _
               <b>DEC</b>((RTCMin&gt;&gt;4)&amp; $0F), <b>DEC</b>(RTCMin &amp; $0F),<font color="#FF0000">&quot;:&quot;</font>, _
               <b>DEC</b>((RTCSec&gt;&gt;4)&amp; $0F), <b>DEC</b>(RTCSec &amp; $0F)
<font color="#000080"><i>'Day
</i></font>B_Ctr = 0
<b>FOR </b>A_Ctr = (((RTCDay &amp; $07)-1)*3) <b>TO </b>(((RTCDay &amp; $07)-1)*3)+2
  <b>LOOKUP </b>A_Ctr,[<font color="#FF0000">&quot;SunMonTueWedThuFriSat&quot;</font>],Day_T
  Day[B_Ctr] = Day_T
  B_Ctr = B_Ctr + 1
<b>NEXT </b>A_Ctr
<b>LCDOUT </b>$FE, 2, $FE, $C0, Day[0], Day[1], Day[2]

<font color="#000080"><i>'Date in &quot;mm-dd-yyyy&quot;
</i></font><b>LCDOUT </b>$FE, $C4, <b>DEC</b>((RTCDate&gt;&gt;4)&amp; $03), <b>DEC</b>(RTCDate &amp; $0F),<font color="#FF0000">&quot;-&quot;</font>, _
                 <b>DEC</b>((RTCMonth&gt;&gt;4)&amp; $01), <b>DEC</b>(RTCMonth &amp; $0F),<font color="#FF0000">&quot;-&quot;</font>, _
                 <b>DEC</b>(((RTCYear&gt;&gt;4)&amp; $0F)*100),<b>DEC</b>(RTCYear &amp; $0F)

<b>RETURN    
    
</b><font color="#000080"><i>'-------------------------------------------------------------------------------    
</i></font>READ_1307:
<font color="#000080"><i>' Read order is in Secs,Mins,Hours,Day,Date,Month,Year,Control
</i></font><b>I2CREAD </b>SDA,SCL,$D0,$00,[<b>STR </b>DB\8] <font color="#000080"><i>' Read string of 8 bytes from DS1307
</i></font><b>RETURN

</b><font color="#000080"><i>'-------------------------------------------------------------------------------
</i></font>WRITE_1307:
<font color="#000080"><i>'Set time &amp; date to 23:59:55, Day 2, Date:Month:Year 27:08:2009
</i></font><b>I2CWRITE </b>SDA,SCL,$D0,$00,[$55,$59,$23,$02,$27,$08,$29,$90] <font color="#000080"><i>' Write to DS1307

</i></font><b>RETURN                  </b><font color="#000080"><i>' Sec Min Hr  Day D   M   Y   Control

</i></font><b>END
</b></code></pre><!--EndFragment--></body>
</html>

**dhouston** · - 28th July 2009, 19:29

Many output a square wave from (1Hz to 32kHz for the M41T81). You can monitor it. At 1Hz, a rising edge comes once per second.

**dhouston** · - 28th July 2009, 19:30

Originally Posted by aratti

Good product, unfortunatly sold only in 8-pin SOIC package.

What package would you prefer?

**Melanie** · - 28th July 2009, 19:37

The DS1307 is good +/- a few minutes every year.

A CR2032 Battery lasts many years. We've been shipping these for about seven years and haven't needed to change a Battery yet.

Use I2CREAD and I2CWRITE... example...

I2CWrite SDA,SCL,I2CAddress,DataB,[DataA]

where...

I2CAddress=$D0 (the BYTE value address for the device)

DataB is the BYTE value address representing the Register you are writing (usually in the range $00-$07 - see DS1307 Datasheet for what each register contains)

and DataA is the BYTE value that you are putting into the Register.

4K7 pull-up's on SCL and SDA required... a lower value may be required if you have multiple devices on the Bus.

You I2CREAD any time you feel like it... once a second, ten times a second... how ever often you feel like it. Note that you can program the DS1307 to output a pulse every second... you can use that to synchronise your PIC to read and display, or display on interrupt...

**mackrackit** · - 28th July 2009, 19:38

I think the DS1337's alarm can be set for one second. Probably doing the same thing as Dave mentioned with his.

**flotulopex** · - 28th July 2009, 21:40

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?

**rmteo** · - 28th July 2009, 22:02

Also, you can now get PIC's with a built-in RTCC.

Thread: RTC chip or PIC?

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Trigger the time (seconds) change

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