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Author Topic: External clock and AREF connection for my augmented board ?  (Read 2278 times)

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Offline Robo73Topic starter

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Hi all,

This is my first time to write in this forum. First of all, I'd like to thank those who are developing this website
for their great contributions. It's one of the nicest robotics websites I've seen.

Now back to my question. I actually have two questions. Basically, I'm building my own development board
using ATmega32 chip. Since I'm experementing at this time, I just want to have one digital port, and 4
analog pins, besides of course the ISP header for programming. Very basic.

[1] Now my first question is regarding the external crystal. I believe the chip comes with internal clock which I
     think is 1 Mhz. I have a 4 Mhz clock that I'd like to connect to the chip, and here are my questions:

     a. How to connect the crystal to my chip ? It has two leads.
     b. If I connect it will there be an interference between the external and internal clocks?
     c. I've seen crystals that come in different shapes and numbers of leads. For example, I've seen one
        that comes in a DIP chip. How would the connection be different?

[2] My second question is regarding the AREF pin. PORTA is an analog port and I only need 4 pins. Now, I
     remember I've read in the tutorial that AREF should be connected to AVCC. Should that always be
    the case? When I need Not connect AREF to anything? here are the descriptions I found in the datasheet:


" 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."

" AREF is the analog reference pin for the A/D Converter."

For the low-pass filter they're referring to, is is just a capacitor and resistor? what should their values be?
 
That's all I have for now. Thnx in advance.

Robo




Offline Robo73Topic starter

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Re: External clock and AREF connection for my augmented board ?
« Reply #1 on: August 03, 2007, 02:15:05 PM »
It's ok guys   I can wait  :).
Take your time.

Robo

Offline Soeren

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Re: External clock and AREF connection for my augmented board ?
« Reply #2 on: August 03, 2007, 09:43:34 PM »
Hi,

 a. How to connect the crystal to my chip ? It has two leads.
Like the datasheet shows of course. Between pins 7 and 8 and then a small cap (like 15..20pF) from each pin to gnd.


b. If I connect it will there be an interference between the external and internal clocks?
No. if it's driven from an x-tal, it isn't using the internal oscillator.

 c. I've seen crystals that come in different shapes and numbers of leads. For example, I've seen one
        that comes in a DIP chip. How would the connection be different?
Sounds like you're talking about x-tal oscillators, which is an oscillator with x-tal and buffer - for connecting it, consult the datasheet, you're expected to do a little of your own footwork.

If the ADC is used, it should be connected to VCC through a low-pass filter."

For the low-pass filter they're referring to, is is just a capacitor and resistor? what should their values be?
Why do you care? You claim you won't be using the A/D-C.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: External clock and AREF connection for my augmented board ?
« Reply #3 on: August 03, 2007, 09:52:34 PM »
It's ok guys   I can wait  :).
Take your time.
You bloody well have to. This is people helping you in their spare time, so pipe down, we take our time without anybodys approval.

Want the right to such a rude post, then hire an engineering consultant - I'll charge you $225 an hour for consultancy work, so perhaps a more polite attitude is in its place?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Robo73Topic starter

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Re: External clock and AREF connection for my augmented board ?
« Reply #4 on: August 04, 2007, 03:06:14 PM »
Hello Soeren,

First of all thanks for the info you provided. Actually I'll be using A/D converter. That's why I said in my
original messege, and I quote:

" .... I just want to have one digital port, and 4 analog pins .... "

and also,

"PORTA is an analog port and I only need 4 pins."

So, if I want to use the analoge port (Port A) or actually some pins of it, then I must use the A/D converter.

Now, about the comment I made, it was suppposed to be a joke (i.e. for fun). That's why I put a smily face.
So, cheer up  ;D.

Thnx again.
 


Offline Soeren

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Re: External clock and AREF connection for my augmented board ?
« Reply #5 on: August 04, 2007, 09:35:52 PM »
Hi,

Perhaps I misinterpreted the "When I need Not connect AREF to anything"?

I'm still not sure whether you want to use an analog input or just the pins, that can be used for both analog and digital I/O's.
If you wan't analog inputs, you need to connect the supply as well as the reference of course.


Btw. I'm allways cheerfull, but much too often, I see people coming with a gimme-gimme and can't-be-soon-enough attitude, perhaps not realizing how bratty it makes them look, but still it P's me off.

A few things that would help people get serious answers (and more than once), would be, to remember that they're getting freebies and remember to read their post with fresh eyes before posting (however strange the preview function is on this board).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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