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8 bit PIC with RTC chip.

asanyi

Hi, my circuit is composed from a PIC16F15345 microcontroller and a MCP79411 RTC chip, connected with i2c, it has an own 32.768 crystal. The 16F15345 also drives deprez meters and LEDs with PWM signals. While the power of the main circuit is switched off, the RTC runs correctly (precise timing) from its backup 3V battery. But activating the power of the PIC and the PWM driving, the RTC has lost its correct operation, delaying 30 minutes per 12 hour! Any idea to fix is accepted...

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  • It sounds like the solenoids are messing with the power supply. Do they have snubber diodes? Can you isolate the power to the RTC? - supply power through a small resistor so you can put a capacitor on the power pins at the RTC. 

  • in reply to dougw

    Now we see the circuit we can see there are no diodes.

    First off I would add small diodes from the junction of solenoids and 3k3 resistors to 5V supply. (Cathode to 5V). Because the current in the solenoids is very low this may not be the problem.

    Next step is to look around the circuit with a scope, looking for dips in supply or spikes.

    If no scope available I would try 22uF across 5V supply.

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  • in reply to dougw

    Now we see the circuit we can see there are no diodes.

    First off I would add small diodes from the junction of solenoids and 3k3 resistors to 5V supply. (Cathode to 5V). Because the current in the solenoids is very low this may not be the problem.

    Next step is to look around the circuit with a scope, looking for dips in supply or spikes.

    If no scope available I would try 22uF across 5V supply.

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  • in reply to michaelkellett

    It seems to be the problem is solved, thanks for your ideas. The next modification were applied: an additional 100uF capacitor on the output side of the 5V regulator, a serial 100 Ohm resistor between the 5V and the VDD pin of the RTC, and paraller Schottky diodes on the coils of the meters, no any change in the oscillator layout of the RTC. So right know the timing is correct no any delay in normal operation. Thanks a lot guys, it was pleasure to discuss with you. Live long and prosper.