RTC chip or PIC? [Archive] - MEL PICBASIC Forum

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

... 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/datasheet/maxim/DS1307.pdf
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/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:

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:

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:
<html><head></head><body><pre><code>' PIC16F630 Fuses
@ DEVICE BOD_OFF
@ DEVICE CPD_OFF
@ DEVICE PROTECT_OFF
@ DEVICE MCLR_OFF
@ DEVICE PWRT_OFF
@ DEVICE WDT_OFF
@ DEVICE INTRC_OSC_NOCLKOUT

'-------------------------------------------------------------------------------
' Registers 76543210
WPUA = %00110000 'Weak Pull-up of PORTA for SCL and SDA on DS1307
INTCON = %00000000 'Interrupts
OSCCAL = %01100000 'Oscillator Calibration when Internal RC set to 4MHz
OPTION_REG = %10000000 'PORTA Pull-Ups/TMR0/Prescaler
CMCON = %00000111 'Comparator Module is OFF
TRISA = %00000000 'Set Input/Output
PORTA = %00000000 'Ports High/Low
TRISC = %00000000 'Set Input/Output
PORTC = %00000000 '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 -> to Vss
'PORTC.4 ( 6) E
'PORTC.0 (11) DB4
'PORTC.1 (12) DB5
'PORTC.2 (13) DB6
'PORTC.3 (14) DB7

DEFINE LCD_DREG PORTC 'LCD data port
DEFINE LCD_DBIT 0 'LCD data starting PORT.bit (0 or 4)
DEFINE LCD_RSREG PORTC 'LCD register select port
DEFINE LCD_RSBIT 5 'LCD register select bit
DEFINE LCD_EREG PORTC 'LCD enable port
DEFINE LCD_EBIT 4 'LCD enable bit

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

'-------------------------------------------------------------------------------
PAUSE 1000 'time for LCD to settle
LCDOUT $FE, 1 'Clear LCD
GOSUB WRITE_1307 'set the time or don't

MAIN:
GOSUB READ_1307 'Read the time & date
GOSUB LCD
PAUSE 1000
GOTO MAIN:

'-------------------------------------------------------------------------------
LCD:

'Hour in "hh:mm:ss"
LCDOUT $FE, 2, DEC (RTCHour>>4 & $0F), DEC(RTCHour & $0F),":", _
DEC((RTCMin>>4)& $0F), DEC(RTCMin & $0F),":", _
DEC((RTCSec>>4)& $0F), DEC(RTCSec & $0F)
'Day
B_Ctr = 0
FOR A_Ctr = (((RTCDay & $07)-1)*3) TO (((RTCDay & $07)-1)*3)+2
LOOKUP A_Ctr,["SunMonTueWedThuFriSat"],Day_T
Day[B_Ctr] = Day_T
B_Ctr = B_Ctr + 1
NEXT A_Ctr
LCDOUT $FE, 2, $FE, $C0, Day[0], Day[1], Day[2]

'Date in "mm-dd-yyyy"
LCDOUT $FE, $C4, DEC((RTCDate>>4)& $03), DEC(RTCDate & $0F),"-", _
DEC((RTCMonth>>4)& $01), DEC(RTCMonth & $0F),"-", _
DEC(((RTCYear>>4)& $0F)*100),DEC(RTCYear & $0F)

RETURN

'-------------------------------------------------------------------------------
READ_1307:
' Read order is in Secs,Mins,Hours,Day,Date,Month,Year,Control
I2CREAD SDA,SCL,$D0,$00,[STR DB\8] ' Read string of 8 bytes from DS1307
RETURN

'-------------------------------------------------------------------------------
WRITE_1307:
'Set time & date to 23:59:55, Day 2, Date:Month:Year 27:08:2009
I2CWRITE SDA,SCL,$D0,$00,[$55,$59,$23,$02,$27,$08,$29,$90] ' Write to DS1307

RETURN ' Sec Min Hr Day D M Y Control

END
</code></pre></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

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:
<html><head></head><body><pre><code>' PIC16F630 Fuses
@ DEVICE WDT_ON

'-------------------------------------------------------------------------------
' Registers 76543210
WPUA = %00110100 'Weak Pull-up of PORTA for SCL and SDA on DS1307
TRISA = %00000100 'Set Input/Output

'-------------------------------------------------------------------------------
' Variables
DSOut VAR PORTA.2 'DS1307 SQW pin #7

'-------------------------------------------------------------------------------
MAIN:
IF DSOut = 0 THEN
GOSUB Read_1307 'Read the time & date
GOSUB lcd
'put PIC in low-power mode for 576+288+72=936ms
NAP 5
NAP 4
NAP 2
ENDIF
GOTO MAIN:
END
</code></pre></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. :)