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PIC battery backup

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Mosaic

Well-Known Member
Hi ,
I am running a 16F886 @ 5v and I want to place a pair of 1.5V dry cells as a 3V battery backup with a low Vf schottky blocker. I don't have any pins free, but I was wondering if I place a 39K resistor inline with the primary supply (12V DC) to the TX RS232 pin and use that pin as a digital input to sample the voltage if that would be a reasonable way to monitor the main supply. The built in PIC clamping diodes could handle the 0.3mA overvoltage load.

I need the PIC to detect the loss of main supply and enter a current saving mode to just maintain a clock.

When RS232 ops are called for I can momentarily stop the voltage monitoring to execute comms.
Would that 0.3mA pull up affect RS232 comms?
 
in my RS232 dual driver i have a few transistors attached to the RS lines both side to run leds, so personaly i cant seem your 0.3mA making any difference to any but the very high rates, but i dont know for sure just that they run leds via transistors fine and mine ive had running pretty fast.
Also dosnt RS458 use a pullup or am i getting confused?
 
I think I would be looking towards using one of Maxim's or Linear Technology's excellent devices for this. Virtually no switching noise, very low dropout etc etc :)
 
If its the tx pin 300ua probably isnt going to affect it much.
You can use a pair of fets as a diode for the battery supply with very low vdrop.
 
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