PIC weak pull-ups on battery power with voltage divider

[cc85]

Hi, I have a pic16f1847a and I am running it on 4*aa batteries using a voltage divider. The pic drives a stepper motor driver L293DD. This works fine.

I also have 3 buttons which use weak internal pull-ups on the pic inputs. These work fine when the pic is powered from a pickit3. When powered from the batteries and voltage divider the weak pull-ups make the input high and when a button is pressed, the input is bought low. The problem is that the button presses are not recognised in the interrupt when battery powered but they are when pickit3 powered.

Any ideas on how to resolve this?

One soluthion would be to use a 3.3v voltage regulator as Im rather sure it is power related but to keep production costs down I would like to stick to the voltage divider if possible.

 

[shumifan50]

In my experience with PICs they need a stable, noise free supply and that will not be provided by a voltage divider, especially not when the supply also drives a stepper motor.

 

[Arouse1973]

Another option is to use a zener diode regulator for acceptable voltage regulation for low power circuits. It is a cheap alternative to a regulator but depending on the current demands of the circuit which maybe consist of a duty cycle of increased current on and off.

The zener diode in the quiet periods would be drawing most of the maximum load current and that's wasteful, but probably no more that your current design. For this reason I would only use them in a circuit which had non critical voltage regulation requirements and run off a power supply. My personal preference would be a non zener regulator (LDO Regulator) but some of these can be inefficient and this really shows at high currents but they do have better load and line responses than the zener version.

The ultimate for efficiency is the switching regulator but I guess that might be overkill for what you want.

Example of zener diode regulator circuit showing relationship between load current and zener current in a varying load circuit with a approx. 100 mV drop at the largest load current. Notice how the zener current increases when the circuit is drawing a smaller load current. You could reduce the zener current if you could accept a lower voltage at maximum load.

 

[BobK]

Two diodes between battery + and Vdd might provide a more stable voltage than a resistive divider.

Bob

 

[Arouse1973]

Good idea Bob.
Adam

 

[cc85]

Thank you for your replies!
I think I will go with a voltage regulator, some should arrive today to try. I just think it's the easiest and safest option.
I do like the diode option but I need to go from 6v to 3.3v as its a surface mounted pic. That would require 5 diodes.

Chris

 

[Arouse1973]

Good Call, it is the best option really.
Adam