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trouble_DS1307.php   5912 Bytes    02-02-2017 19:22:46

Troubleshooting a DS1307Z

I2C RTC, displaying 165.165.2165

The Setup : A DS1307 on a shield, mated with an Arduino/Genuino

✈ The (unhappy) Circuit

The circuit demanded for a Real Time Clock with I2C. A DS1307 seemed a smart choice. It is powered with +5 V. As the datasheet specifies a battery voltage from 2 - 4 V, the initial design used 4 x LL4148 (because of the low leakeage current), in series, to get rid of approx 2 V and therfore land in the middle of the specified range. The charge is stored in a "gold cap".

The original circuit

✈ The (unhappy) Behaviour

The circuit worked nicely after switching on. But, after some days the clock seemed unhappy.

without oooops :-)

✈ Measurements • Discoveries

The reason is the following : The voltage over the storage capacitor will keep on climbing (up to 5 V), as the current drawn from the gold cap is very low - and for very low currents, the voltage drop across the diodes is also very low. Zero volts at zero current.

The voltage over the gold cap climbs ...

✈ The (happy) Circuit

As the mechanism - which leads to malfunction - was clear, the solution was also clear. With only one diode, but this time connected to the +3.3 V, the voltage above the gold cap can never go above the +3.3 V - with a 1N5711 (Schottky Diode) it settles at about 3.0 V - just the value we aimed for ...

The modified circuit

✈ Share your thoughts

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t1 = 3913 d

t2 = 629 ms

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