AVR-Chat

Hello

  you can add 3v3 ldo linear regulator to power SD and justify logic singal levels (phillips appnote AN97055) or apply integral levels shift IC and use your atmega at 16MHz with 5V power supply.
  If you need faster solution, you could use arm7 or cortex for example with great performance and 3.3v power supply.

Best regards,
Eugene

--- In AVR-Chat@yahoogroups.com, farhan ahmad <a_decent22@...> wrote:

>
> Hello All,
> 
> I just want to have feedback about some tests made by me. I have been working with ATMEGA64L. ATMEGA64L-8AU (Vdd=2.7V to 5.5V) works with 8MHz external crystal oscillator as stated in the specification. 
> 
> Since I am using SD card and I am extracting data from SD card and send it to the TFT display. I am facing the problem of speed, as with 8MHz crystal oscillator, I found that out the data to be displayed on the TFT screen is very slow. So, I decided to use ATMEGA64-16AU with 16MHz crystal oscillator but after looking at the datasheet found out that ATMEGA64-16AU micro-controller works with the 4.5V to 5.5V Vdd range and for this reason I cant use the controller because SD card works with 3.3V Vdd. 
> 
> Then I use the same micro-controller ATMEGA64L-8AU and use the 16MHz external oscillator crystal. I found out that the data to be displayed on the TFT is pretty fast but the problem is that it is beyond the specification because ATMEGA64L-8AU (Vdd=2.7V to 5.5V) can only work with 8Mhz external crystal oscillator.
> 
> Could somebody comment what are the pros and cons of using ATMEGA64L-8AU with 16MHz oscillator crystal. 
> 
> Best Regards,
> Farhan Ahmad
>

Hello All,

I just want to have feedback about some tests made by me. I have been working with ATMEGA64L. ATMEGA64L-8AU (Vdd=2.7V to 5.5V) works with 8MHz external crystal oscillator as stated in the specification. 

Since I am using SD card and I am extracting data from SD card and send it to the TFT display. I am facing the problem of speed, as with 8MHz crystal oscillator, I found that out the data to be displayed on the TFT screen is very slow. So, I decided to use ATMEGA64-16AU with 16MHz crystal oscillator but after looking at the datasheet found out that ATMEGA64-16AU micro-controller works with the 4.5V to 5.5V Vdd range and for this reason I cant use the controller because SD card works with 3.3V Vdd. 

Then I use the same micro-controller ATMEGA64L-8AU and use the 16MHz external oscillator crystal. I found out that the data to be displayed on the TFT is pretty fast but the problem is that it is beyond the specification because ATMEGA64L-8AU (Vdd=2.7V to 5.5V) can only work with 8Mhz external crystal oscillator.

Could somebody comment what are the pros and cons of using ATMEGA64L-8AU with 16MHz oscillator crystal. 

Best Regards,
Farhan Ahmad

What about buffering the SD from 5V to 3.3V. I have done this and it works
fine. Allowed me to run the Mega at 16MHz at 5V and still use the SD at
3.3V.
 
Might be easier than trying to find a faster CPU at 3.3V.
 
Dave...
---
Very funny Scotty, now beam down my clothes!!!
---

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of farhan ahmad
Sent: 29 January 2011 18:10
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] ATMEGA64L, ATMEGA64
 
  
Hello All,

I just want to have feedback about some tests made by me. I have been
working with ATMEGA64L. ATMEGA64L-8AU (Vdd=2.7V to 5.5V) works with 8MHz
external crystal oscillator as stated in the specification. 

Since I am using SD card and I am extracting data from SD card and send it
to the TFT display. I am facing the problem of speed, as with 8MHz crystal
oscillator, I found that out the data to be displayed on the TFT screen is
very slow. So, I decided to use ATMEGA64-16AU with 16MHz crystal oscillator
but after looking at the datasheet found out that ATMEGA64-16AU
micro-controller works with the 4.5V to 5.5V Vdd range and for this reason I
cant use the controller because SD card works with 3.3V Vdd. 

Then I use the same micro-controller ATMEGA64L-8AU and use the 16MHz
external oscillator crystal. I found out that the data to be displayed on
the TFT is pretty fast but the problem is that it is beyond the
specification because ATMEGA64L-8AU (Vdd=2.7V to 5.5V) can only work with
8Mhz external crystal oscillator.

Could somebody comment what are the pros and cons of using ATMEGA64L-8AU
with 16MHz oscillator crystal. 

Best Regards,
Farhan Ahmad 


[Non-text portions of this message have been removed]

--- In AVR-Chat@yahoogroups.com, farhan ahmad <a_decent22@...> wrote:

> Could somebody comment what are the
> pros and cons of using ATMEGA64L-8AU
> with 16MHz oscillator crystal. 

Why even think of this?  Just use an ATmega64A-AU, which is rated for 16 MHz operation from 2.7 to 5.5 volts.

Graham.

--- In AVR-Chat@yahoogroups.com, Mike Payson <mike@...> wrote:

> ... tidbit is usually hidden fairly
> deep into the datasheet ... on the
> ATMega64A, if you check page 344 of
> the datasheet you see that it is
> only rated to 10MHz at 3.3V ...

Well, thank you very much for pointing that out.  From the front page of the data sheet, I had the impression that the new "A" parts would run at up to 16 MHz down to 2.7 volts.  This seems reasonable from a die shrink or process improvement, but, you're right, all that has happened is that the "A" part offers all the combinations of speeds and supply voltages of the old 64, 64L and 64V parts.  This is explained much better on the ATmega164A/PA/etc. data sheet (8272A).  It's really not clear in the Features list of the ATmega64A data sheet.  Thanks again.

Graham.

On Sat, Jan 29, 2011 at 4:54 AM, ecros_technology
<Yahoo37849@ecrostech.com>wrote:

> --- In AVR-Chat@yahoogroups.com, farhan ahmad <a_decent22@...> wrote:
>
> > Could somebody comment what are the
> > pros and cons of using ATMEGA64L-8AU
> > with 16MHz oscillator crystal.
>
> Why even think of this?  Just use an ATmega64A-AU, which is rated for 16
> MHz operation from 2.7 to 5.5 volts.
>
> Graham.


Most ATMega chips are only rated for 16MHz+ speeds above 4.5V. If you run
below that you are limited to a lower maximum speed. Unfortunately that
tidbit is usually hidden fairly deep into the datasheet, so you really need
to dig to find it. For example, on the ATMega64A, if you check page 344 of
the datasheet you see that it is only rated to 10MHz at 3.3V:
http://www.atmel.com/dyn/resources/prod_documents/doc8160.pdf. That is not
to say that it would not work with a 16MHz crystal... It might work fine,
but you may get intermittent bugs or other problems. It is usually better to
play it safe and run at 5V and level shift to talk to the 3v3 components.


[Non-text portions of this message have been removed]

On 30-Jan-11 20:38, Mike Payson wrote:
>
> Most ATMega chips are only rated for 16MHz+ speeds above 4.5V. If you run
> below that you are limited to a lower maximum speed. Unfortunately that
> tidbit is usually hidden fairly deep into the datasheet, so you really need
> to dig to find it. For example, on the ATMega64A, if you check page 344 of
> the datasheet you see that it is only rated to 10MHz at 3.3V:
> http://www.atmel.com/dyn/resources/prod_documents/doc8160.pdf. That is not
> to say that it would not work with a 16MHz crystal... It might work fine,
> but you may get intermittent bugs or other problems. It is usually better to
> play it safe and run at 5V and level shift to talk to the 3v3 components.



     What could happen if the clock prescaler is set to 1/2 when using 
the 16MHz crystal @ 3.3V?

     Mark Jordan

On Sun, Jan 30, 2011 at 3:32 PM, <enkitec@gmail.com> wrote:

>     What could happen if the clock prescaler is set to 1/2 when using
> the 16MHz crystal @ 3.3V?
>
>
No clue... At a guess that would work fine, but I could be wrong.


[Non-text portions of this message have been removed]

On 30-Jan-11 21:44, Mike Payson wrote:
> On Sun, Jan 30, 2011 at 3:32 PM,<enkitec@gmail.com>  wrote:
>
>>      What could happen if the clock prescaler is set to 1/2 when using
>> the 16MHz crystal @ 3.3V?
>>
>>
> No clue... At a guess that would work fine, but I could be wrong.
>
>
     I asked that because I have some ~500 ATmega48/168 in the field
running at 15MHz @ 3.6V with the prescaler set to 1/2.
     No problems at all for three years now.

     Mark Jordan