What to do with MCLR pin if its not used?

[sanddune008]

Hi All,


1) What to do with MCLR pin if its not used?

I have connect the MCLR pin to Vdd throught a
330 ohm Pull up resistor is this okay?

2) Is it necessary to connect the Vdd pin through a capacitor to GND
to prevent noise?.........correct me if i am wrong.

Thanks in advance

[PCM programmer]

[Ttelmah]

And, for the capacitor, yes.
You need to start with a smooth supply. Depends how this is being generated, but if (for instance), you are using a voltage regulator, _this_, will have it's own specifications for the capacitors it needs. You will often find that regulators like the 7805, _must_ have a capacitor really close to the regulator on it's output to stop it oscillating, and a much larger capacitor on the input, that provides the 'reservoir'. Then with this supply, you then need capacitors close to things that will introduce switching transients. This includes most logic gates, and things like the processor itself. These want to be optimised to block high frequencies. So for these, it is not a case of 'bigger is better', but a case of chosing capacitor technologies that are good at handling high frequencies, rather than low frequencies. Typically something like a 0.1uF ceramic capacitor. If you look at things like PC motherboards, and video cards, you will find that there are perhaps a dozen large capacitors close to the supply components, and then many dozens of small capacitors across the board. Often at about every second IC.

Then there are lots of other 'small' design things that are often forgotten. For instance, if you have a lot of capacitance on the regulated supply, and several loads on the incoming supply, that are permanently present, it may well become necessary to add a reverse biased diode across the regulator. The problem here is that if the incoming supply can drop quickly, while the regulated supply stops dropping, when internal circuitry turns off, you can end up with the regulated supply having a higher voltage than the incoming supply, when the rail is turned off. This is an remarkably 'easy' way to kill some regulators....

When laying out supply rails, think of them like water courses. If you have large loads present, have thick tracks running directly 'to' them, from the supply. Then connect smaller rails for low power circuitry back to a single connection point at the supply. Avoid ever tapping rails off at two points 'along' a track.

Best Wishes

[sanddune008]

Thank you very much guys ......