Significance of AVcc pin in ATMEGA 16

[varunag]

I'm using a ATMEGA16 microcontroller. I was having problem getting it to work. I found somewhere on the internet that even if I'm not using the analog signals, I need to short the AVcc pin to the Vcc. Can anyone give any reason as to why we need to do this?

 

[deakie]

from the datasheet...

AVCC AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally connected
to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC
through a low-pass filter.

VCC is the standard power in for the digital circuitry and AVCC is for the analog.
The separation is for when you have a mixed mode circuit or board so that you can run your power lines seperately to avoid crosstalk noise.
Digital switching noise may affect your analog signals and vice versa. So the best design separates them on the board.
You will notice that they suggest a low pass filter when using the AVCC power from the digital power and this allows your dc through while blocking the high frequency digital surges as demanded from the switching circuits.

 

[varunag]

thanks deakie for the reply.

well this information is what I get from most of the places. I suppose that I did not frame my question properly. What I meant in my question is "why do we have to connect the AVcc externally to the Vcc, even if the ADC is not used?"

I suppose my question will be more clear to you now...

thanks,
varun

 

[deakie]

Otherise port A won't function along with the interconnecting circuits inside the chip.

To expand...its a choice for when you use the ADC only...the AVCC port allows an independent voltage line for the ADC for use under analog conditions but must always be connected nevertheless. you just get the choice to filter it.

 

[varunag]

well thanks. but there must be some reason for it. I hope someone might be knowing the reason for this.
Or I'm asking a too trivial thing, and I shouldn't be bothering about it?

 

[deakie]

I just said...you just get the choice to filter it.
How else would you get that choice if it wasnt connected externally?
if it was internal, you couldn't do it.

 

[varunag]

oh okay. got it.
so you mean, that we always need to power the AVcc. If we don't need to use the ADC feature then we can short AVcc and Vcc.
But if we want to use ADC, then we can use a filter at AVcc.

thanks deakie.

 

[deakie]

Yeah...thats it exactly.
No worries and you're welcome.