RTC problem

**RobertD** · 08-01-2011

Hi Friends, I'm using a real time clock chip (DS1307) which has back up battery when power goes out. My problem is the chip doesn't keep up time when on B/U power, it just stops there. When power is back on, it keep time from there OK. Problem is power is not always on, but I need accurate time stamps. Chip communicates via I2C serial. Just wondering if there is any line of code I have to add to initialize the chip start up...

_______________

Quote from Data sheet:

CLOCK AND CALENDAR
The time and calendar information is obtained by reading the appropriate register bytes. The RTC
registers are illustrated in Figure 3. The time and calendar are set or initialized by writing the appropriate
register bytes. The contents of the time and calendar registers are in the BCD format. Bit 7 of register 0
is the clock halt (CH) bit. When this bit is set to a 1, the oscillator is disabled. When cleared to a 0, the
oscillator is enabled.
Please note that the initial power-on state of all registers is not defined. Therefore, it is important
to enable the oscillator (CH bit = 0) during initial configuration.
The DS1307 can be run in either 12-hour or 24-hour mode. Bit 6 of the hours register is defined as the
12- or 24-hour mode select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5 is
the AM/PM bit with logic high being PM. In the 24-hour mode, bit 5 is the second 10 hour bit (20-
23 hours).
_____________________

DS1307 PDF Datasheet

**tabstop** · 08-01-2011

If the chip does not keep time when the power is off, then you need to assume every time the power comes on that the clock is wrong and find a way to acquire the correct time (perhaps you can ask the thing that the chip is communicating with what time it is?)

**anduril462** · 08-01-2011

Well, considering we don't have a schematic or single line of code from you, it's hard to say whether you need to add a line of code. The chip is supposed to have an automatic fail-over to battery power, and it should keep time if configured properly. Maybe you send it all the right commands, but have a design/wiring problem. Maybe your battery is bad. Maybe the fail-over circuitry in your RTC is bad. We need more info from you to actually provide any useful help. Also note that this is a C programming forum not an embedded or electronics specific forum. Nevertheless, we have some embedded- and electronics-savvy people here and we'll do our best to help you, if we can get some useful info from you.

**Mario F.** · 08-01-2011

The DS1307 is a well known SRTC and it operates with a backup battery. If yours is not, this must be because either the battery is dead or it's too low voltage (get a new 3V one), excluding any defect to the chip.

>> Just wondering if there is any line of code I have to add to initialize the chip start up...

No. If it is not keeping time between Vbat and Vcc swaps there's nothing you can do other than setting the time as tabstop suggests above. This is how you will do it (taken a long time ago from some PIC blog I don't really remember anymore):

Code:

void set_rtc(unsigned short addr, unsigned short data) {

    I2C_Start(); // Start signal. Obligatory!

    I2C_Wr(0xD0); // I2C send byte: device address(0xD) + Write (0))
    I2C_Wr(addr); // send byte
    I2C_Wr(data); // send data

    I2C_Stop(); // Stop signal. Obligatory!

}

// Set the NOW time (my local time)
set_rtc(0,0x80); // 0 second reset and stop oscillator
set_rtc(1,0x15); // 15 minutes
set_rtc(2,0x18); // 18 hours
set_rtc(3,0x02); // Monday
set_rtc(4,0x01); // Day 1
set_rtc(5,0x08); // Month 8th
set_rtc(6,0x11); // Year 2011
set_rtc(7,0x10); // Control. Always the same.
set_rtc(0,0x00); // Start oscillator

**~~CommonTater~~** · 08-01-2011

Originally Posted by RobertD

Hi Friends, I'm using a real time clock chip (DS1307) which has back up battery when power goes out. My problem is the chip doesn't keep up time when on B/U power, it just stops there.

According to the datasheet only the memory is battery backed. You should probably look into using a larger battery and keeping the chip itself powered during power off...

**anduril462** · 08-01-2011

Originally Posted by CommonTater

According to the datasheet only the memory is battery backed. You should probably look into using a larger battery and keeping the chip itself powered during power off...

From the data sheet, p6, Pin Description table, pin 8:

Primary Power Supply. When voltage is applied within normal limits, the device is fully
accessible and data can be written and read. When a backup supply is connected to the
device and VCC is below VTP, read and writes are inhibited. However, the timekeeping
function continues unaffected by the lower input voltage.

It should continue to keep time just fine. I'm with Mario on battery issues.

@OP: Can you hook up a steady 3V to the battery terminal and see if that works?

**Mario F.** · 08-01-2011

Indeed. The backup battery will power timekeeping too. I would have known by now if it didn't.
I believe the problem here is that his oscillator is stopping when changing to battery backup mode while the SRAM retains the last time. This is indicative of a defective/failing battery or a low power battery because the oscillator will stop at a higher DC than that required by SRAM to persist data. Essentially the OP's SRTC is operating in data retention-only mode when switching to Vbat. A mode actually supported by the DS1307.

And you are absolutely right. The OP needs to provide a schematic or let us know the exact board model. Other than the possibility above, it could be this is by design, or if he built it himself he fumbled somewhere.

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