DS1307 RTC and Power supply problem

[ex4]

I got strange problem with my DS1307 RTC interfacing. I'm using AVR ATMEGA8535 and power supply with Full wave rectifier and CT type transformer (12V 500mA)
Everytime i plugged in an out the circuits from the AC line my DS1307 gone wild and perform wrong reading
Any body got any suggestion to fix it? my arrangement is like this

AC Line -> Transformer 500mA 12V CT -> Full wave rectifier -> RTC+AVR

 

[throwaway18]

The ds1307 needs a power supply between 4.5v and 5.5v
Are you giving it a correct supply?

Have you got a battery connected to the ds1307?
Have you got decoupling capacitors close to the power poings of the AVR and the ds1307?

 

[Mr_Programmer]

I expect that you have a appropriate line filter build in.
The crystal for the 32kHz should be as close as possible to the IC pins and the body should be connected to GND (if it is a T/H crystal). A GND area round about the DS1337 is also not a bad idea.

Regards...

 

[neelam29]

and if u dont wanna connect the battery then connect condensor of a big value between those pins.

 

[x_zoli]

Hi,

The DS1307 cannot operate while it's not supplied correctly, so if only the battery remains connected the RTC still keeps the time correctly, but you cannot read the time.

"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."

You should redesign your schematic to use the battery as in the schematic attached, so the RTC will see the battery as the primary power supply.

Best Regards

 

[ex4]

I have follow the suggested schematic from the datsheet, and also add batterey back up CR2032. My decoupling capacitor is big 1000uF/25V
I've been try the circuit with my Non CT PSU and it's doesn't behave like this, since I notice that the rising time of output voltage from my Non CT PSU is slow.

 

[BeeBop]

Hi ex,
This sounds like a filter capacitor, not a decoupling cap!

My decoupling capacitor is big 1000uF/25V

Try a small ceramic, or a mono, marked 104, which is 0.1 uF.
Although, I don't think this is the problem. I think it has to do more with your power supply. RE:

I notice that the rising time of output voltage from my Non CT PSU is slow.

Best wishes,
Robert

 

[ex4]

oh yes sorry for the mistakes
I thought so, so any suggestion how to make my PSU voltage rising slowly???
maybe soft on off using transistor???

 

[BeeBop]

Hey Ex,
You don't need to appologize, for anything! I think we all make mistakes, and when others point them out, that is how we learn. Heaven knows I make my share.:|

As for the PSU, I wonder how it is built. Transformer -> bridge rectifier -> filter cap -> regulator -> decoupling cap? It is hard to say why you have a slow rise time without knowing your circuit. I think you should be able to get it without a soft start, etc.

Regards,
Robert

PS, speaking of mistakes, I just re-read your post, and I made a mistake!

The slow rise time works, you say. Mmmm, how big is the filter cap in your PSU? I think a larger cap would give you what you want.
R

 

[ex4]

Well this is what makes all strange
when i tested it with my non CT PSU and the PSU only using 470uF/25V filter cap, it's ok. I pull out and in the adapter and my circuits works OK from the beginning
but when i change it to my CT PSU even with 1000uF/25V filter cap the circuits is running wild, and the rising voltage time also much smaller than the one with non CT PSU :?::?::?::?:

 

[Mr_Programmer]

Can you show us your circuit, maybe we can find out what is going on there..

 

[ex4]

finally, after a day looking at the program, i fixed it
it seems there is bug in my code considering with eeprom acces time, but now it's under control
thank you for the suggestion