Pullups and pulldows

[Rob]

Hi.

Could somone tell me what should be the values for External weak pullups.
I'm using 10K but I thing it is too much. I read somewere I should use 4.7K.
So what value should I use to be safe?

I have used both values without problems although I normally use 10k.

Do I need to use external pulldown resistors on pins that are default LOW.
Let's say I want RA5 on PIC16F684 to be low by default oce I disable internalm wpu, should I use an external weak pull down resistor to tie it to ground? If yes, what value?

It really does depend on what the input to the PIC is coming from although, generally, it is best to use a pull-down resistor otherwise it mght be left 'floating'.

Should use a RC networks (0.1 capa with an external resistor for weak pull ups or just a resistor?

again, depends on what it is being connected to. Some times it is necessary to use a capacitor to reduce noise etc

I've seen that in some circuit, the designer used a 47 UF capacitor on his 7.2V power supply for supplying a circuit with a PIC16F684. Why would he use this capacitor as the current is DC?? Is it needed to filter something or to prevent some voltage loss or something?

I'm assuming the capacitor was placed across the power supply of a voltage regulator. This is common practice and helps "smooth" the voltage. If I'm using a 5V regulator, I normally have a 47uF or 100uF across the input to the regulator and 10uF on the output of the regulator. Normally, I have 100nF capacitors across both input and output of the regulator in addition to the 47/100uF and 10uF caps. Some may say that this is unneccessary with todays voltage regs. Have a look at the specific voltage regulator data sheet; it may suggest different values.

Hope this helps

Rob

[xnihilo]

I have used both values without problems although I normally use 10k.



It really does depend on what the input to the PIC is coming from although, generally, it is best to use a pull-down resistor otherwise it mght be left 'floating'.

The signal is 5V output from another PIC16F684 (on the same power supply).

again, depends on what it is being connected to. Some times it is necessary to use a capacitor to reduce noise etc



I'm assuming the capacitor was placed across the power supply of a voltage regulator. This is common practice and helps "smooth" the voltage. If I'm using a 5V regulator, I normally have a 47uF or 100uF across the input to the regulator and 10uF on the output of the regulator. Normally, I have 100nF capacitors across both input and output of the regulator in addition to the 47/100uF and 10uF caps. Some may say that this is unneccessary with todays voltage regs. Have a look at the specific voltage regulator data sheet; it may suggest different values.

It is between the anode and the cathode of the battery...

Hope this helps

Rob

[mackrackit]

If you use 10K you will have 0.0005 amps across the resistor.
If you use 4.7K you will have 0.001 amps across the resistor.
That is why 10K is normally used. It saves power if you have this condition several times.

Power supplies:
If you are running off of batteries a capacitor may not be necessary. If there is some thing in the circuit though that creates some noise then a 22uf or larger might be needed. It is difficult to figure before hand, you will find what works.

The regulators used now days are pretty good, but there is still some noise getting through from the rectifier. So caps and some times inductors are needed. If you do not have a scope but a good multi-meter foe AC, read the voltage in AC and see what you have. For example , I some times use a power supply from a PC for a bench supply. The AC voltage on the 12 volt side will be 0.004 volts. When a 4700uF cap is placed across the positive and negative (0) rails the AC voltage drops to .000. Now it is good enough for a PIC.

The larger the capacitor

It is between the anode and the cathode of the battery...

in this case will also help in brown out conditions.

[xnihilo]

If you use 10K you will have 0.0005 amps across the resistor.
If you use 4.7K you will have 0.001 amps across the resistor.
That is why 10K is normally used. It saves power if you have this condition several times.

Power supplies:
If you are running off of batteries a capacitor may not be necessary. If there is some thing in the circuit though that creates some noise then a 22uf or larger might be needed. It is difficult to figure before hand, you will find what works.

The regulators used now days are pretty good, but there is still some noise getting through from the rectifier. So caps and some times inductors are needed. If you do not have a scope but a good multi-meter foe AC, read the voltage in AC and see what you have. For example , I some times use a power supply from a PC for a bench supply. The AC voltage on the 12 volt side will be 0.004 volts. When a 4700uF cap is placed across the positive and negative (0) rails the AC voltage drops to .000. Now it is good enough for a PIC.

The larger the capacitor in this case will also help in brown out conditions.

What is a brown out? A decrease in the supply? I don't use BOD in my config byte.

What would make noise in my circuit?? voltage regulator? LCD display?

Okay for 10K. I will use them as pullups and pulldown. In my circuit I have strange behaviors on a portA.5 pin set as input (internal WPU disabled), default low and with no external Weak pulldown. When I touch any pin (no matter which one), there is an INT trigger as if +5V was getting to this specific pin (this pin is int enabled when getting from LOW to HIGH). I guess I should use a weak pull down...???

[mackrackit]

What is a brown out? A decrease in the supply? I don't use BOD in my config byte.

Even if you do not configure BOD the PIC will only operate within a certain voltage range for the OSC speed it is running at. So if something causes a voltage draw for a short period of time that the regulator can not handle the large capacitor will help. (maybe a LCD back light coming on )

What would make noise in my circuit?? voltage regulator? LCD display?

A switch used as an input that changes position rapidly, or an output to a high speed relay, etc...

Okay for 10K. I will use them as pullups and pulldown. In my circuit I have strange behaviors on a portA.5 pin set as input (internal WPU disabled), default low and with no external Weak pulldown. When I touch any pin (no matter which one), there is an INT trigger as if +5V was getting to this specific pin (this pin is int enabled when getting from LOW to HIGH). I guess I should use a weak pull down...???

Personally I never have like the internal pull-ups, I always use an external pull up/down. And never let a pin float, always have control.

[xnihilo]

Even if you do not configure BOD the PIC will only operate within a certain voltage range for the OSC speed it is running at. So if something causes a voltage draw for a short period of time that the regulator can not handle the large capacitor will help. (maybe a LCD back light coming on )

A switch used as an input that changes position rapidly, or an output to a high speed relay, etc...

Personally I never have like the internal pull-ups, I always use an external pull up/down. And never let a pin float, always have control.

Thank you very much for the information.
I need internal wpu because for my project i need to enable and disable them on the fly in software. Is there another easy alternative?

a

[mackrackit]

I need internal wpu because for my project i need to enable and disable them on the fly in software. Is there another easy alternative?

Can you tell us a little about your project? At this point I would just be guessing more than usual.