Do I need a CAP on VIN to ground

I am developing a small board with a ATMEGA328, 2 line serial LCD and a BMP085 breakout to be used as a vario altimeter. I need some guidance on the power supply.

I have 5v regulator to supply the atmega and lcd and a 3.3v regulator to supply the BMP085 and LV side of a sparkfun logic level convertor (I intend to solder the convertor and bmp085 breakout to pin headers on the board.

I have a 10uF cap on the out side of both regulators going to ground my question is should I include a cap on the VIN to ground and what would the suggested value be?

Yes. Now what is Vin on your board, is it the arduino 5V line? Anyway when in doubt use a0.1uF.

No there is no arduino board as such, I am using a ATMega328 on my own board. VIN will be in the range of 6-12v

Maybe this schematic will help show what I am doing.

Ok, Yes you need a capacitor on the input to the regulators. A 10uF would be fine.

You also need a 0.1uF ceramic on pin 7 to ground and one from pin 21 to ground.

Ok thanks, what is the purpose of the caps between pin 7 and ground and 21 and ground?

Thanks, Mark

Pin 7 - supply decoupling Pin 21 - Aref decoupling - that data sheet says you must have this even if you are not using the A/D.


Ok great thanks for the guidance.