PIC backup power supply: switchover ???

[xnihilo]

Here is the topic:

I am using two curcuits, based on PIC16F684.
Main circuit is powered by 5V (9V battery to 5V voltage regulator).
Auxiliary circuit is powered by 3V (one small battery) because it will be its backup supply.
Auxilliary circuit is supposed to be normaly connected to the power supply of main circuit (5V) but, if disconnected, it should use the 3V power supply.

Would someone know and be so kind to tell me how I could, electricaly design my circuits so I can implement such diagram?
I will need a system to switchover from 3V to the 5V of the main circuit and vice versa when separating auxiliary circuit from main circuit.

The fact is that this switchover must be immediate because the PIC CANNOT loose its power supply and reset.

Thanks for your help.

Regards.

[mister_e]

2 diodes and there you go. Low voltage drop please let's say 1N5817
<img SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2516&stc=1&d=120852848 5">

[xnihilo]

Thank you for your quick reply.
Can I ask why you are using diodes? To avoid peak currents? Wouldn't 1N4001 diodes be okay (I have a bunch of these...)
I see a capacitor, what would be the value? 100UF would be fine or do I need higher value? Do you use it to prevent a voltage drop at the time the voltage switching occurs?
I disabled BOD in the pics, should I expect a reset anyway if the voltage drops a little?

Thanks mister_e

[mister_e]

Diode are there to avoid conflict between your voltage source... with this, the higher voltage will win the race

Capacitor, 10 uF and more should be enough. For filtering purpose.

You could use 1N4xxx diode, bear in mind their voltage drop is around .6V so your 3V will be 2.4 volt when your battery is at full-charge.

The pic will work as long you haven reach the Brown-Out threshold voltage OR his voltage threshold depending on the osc speed you're using.

You could still use another much fancy system, this one work pretty well in many cases.

[xnihilo]

Diode are there to avoid conflict between your voltage source... with this, the higher voltage will win the race

Capacitor, 10 uF and more should be enough. For filtering purpose.

You could use 1N4xxx diode, bear in mind their voltage drop is around .6V so your 3V will be 2.4 volt when your battery is at full-charge.

The pic will work as long you haven reach the Brown-Out threshold voltage OR his voltage threshold depending on the osc speed you're using.

You could still use another much fancy system, this one work pretty well in many cases.

I didn't know that diodes had such use
What's wrong if both supplies are connected at the same time in parallel???

Filtering capacitors? What do they filter?

Voltage drop? Right... 2.4v is a little too low, knowing the PIC will be fine down to 2V but I can't use 8MHz anymore if using 2V... In this case I can use 3*1.5v batteries instead and use 1N4001 diodes, right?

I disabled Brown Oout Detect in the config words, do I still need to care about this feature?

What fancy system are you talkning about?

[skimask]

...................whoops...............

[skimask]

You could still use another much fancy system, this one work pretty well in many cases.

What fancy system are you talkning about?

Yes, tell me more about Fancy System v1.0...............

[mister_e]

What's wrong if both supplies are connected at the same time in parallel???

what happen if you connect a 5 volt directly to a 3v battery?

Filtering capacitors? What do they filter?

voltage regulator output, battery... yeah you may think battery don't produce noise.. well yes and no, they have internal resistor, everything that need juice will make the voltage drop a little bit, knowing there's a clock source somewhere and you will also drive something, the voltage will swing accordingly.

Do a test, remove all capacitors, do a led blink (10mSec or so) then use your scope to monitor your Vdd rails... cool huh? now add the mandatory filtering caps... how better now?

Voltage drop? Right... 2.4v is a little too low, knowing the PIC will be fine down to 2V but I can't use 8MHz anymore if using 2V... In this case I can use 3*1.5v batteries instead and use 1N4001 diodes, right?

Sounds right

I disabled Brown Oout Detect in the config words, do I still need to care about this feature?

No, it will be disabled. It's not a bad idea to have it enabled so far. Just look the datasheet to know it's threshold and what advantage it may give you.

What fancy system are you talkning about?

Don't worry, way to complicated....