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Seiko S-35390 RTC battery back-up with PIC uC

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Hi Everyone,
I am hoping someone can offer some advice with using the Seiko S-35390 Realtime Clock IC. The chip has an onboard feature for monitoring the 3V lithium back-up battery, but unfortunately only flags an error when the battery is too low to guarantee the memory contents of the device! I want to be able to flag an error when the battery is low, but before this happens, so must add some external circuitry. It is connected via I2C bus to a PIC micro (see schematic snip), therefore the first idea was to connect the battery to my only spare pin (analogue input) and then just measure its voltage externally (red line on schematic snip). However the problem is that when the main 5V supply fails (which powers the PIC) the internal clamp diode inside the PIC (marked D1) will become forward-biased and drain the battery! So the battery needs to be isolated from this input when the supply is missing. I think a FET can be added with its gate connected to the main system power supply to provide this function, but so far my attempts have failed. (Error is when the battery is low and/or missing the analogue input still reads a few volts.) Can anyone see a solution to this? Maybe there is another way around this? Thanks for reading...s35390.png


Well-Known Member
Most Helpful Member
How about a voltage supervisor chip like the Micrel MIC2775? It asserts an output when the monitored voltage falls too low. It draws 3.5uA.

Micrel MIC2775
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