ATmega8 operating at lower supply voltage than recommended

Go To Last Post
9 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

Preamble:
according to datasheet we all know that ATMega8 operates at 4.5V to 5.5V and also the ATMega8L operates at 2.7V to 5.5V. OK.

General knowledge:
I have a circuit containing ATMega8 (internal osc @ 8MHz), 16x2 HD44780-based Character LCD and a slave coprocessor. The slave coprosessor operates at 3.3v, and connected to Mega8 using i2c communication protocol (SCL clock @ 20KHz).
Because of the coprosessor needs then I lower the whole circuit's supply voltage at 3.3V too (using some sort of regulator chip). LCD's power supply still using 5V.
Every time Mega8 should pick up data from slave coprosessor and display it on the LCD.

The problem is:
i2c communication didn't done in good performance.
the coprosessor often not acknowledge the communication initiated by Mega8. Sometime coprocessor works fine and i can pull out few data from it, but often not! The system works fine usually when i've just flashed the Mega8 or when I turned off power supply for a long time (about 5 minutes). When i reset the Mega8, it will be stuck because the coprocessor never acknowledged.

The question is:
is it fine using ATMega8 at supply as low as 3.3V since it is violate the power supply recommendation from datasheet? How low it can be? What is the effect if i do this? especially in i2c communications?
LCD controlling worked fine, i can write any characters on LCD at this condition. But communication with coprocessor didn't succesfully achieved. How can this be? Should i use Mega8L instead (geez i've stocked so many Mega8)?

Thanks.

KISS - Keep It Simple Stupid!

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
since it is violate the power supply recommendation from datasheet

If this is a hobbiest project then try it and if it works then fine. If it's a commecial product it would be suicide to operate the device outside the datasheet recommendations.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Keep the mega8 at 5 volts. It is not difficult to connect devices with different voltages to the same i2c bus.
Here's an example:
http://www.nxp.com/acrobat_download/applicationnotes/AN10441_1.pdf

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:

Keep the mega8 at 5 volts.

Why? AVRs will run fine at spec'ed voltages. I'd suggest the problem is elsewhere, such as needing to adjust I2C pullup values for the lower voltage.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

but a mega8 is not spec'd to run at 3V3.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:

but a mega8 is not spec'd to run at 3V3.

...but a Mega8L is.

Is this a one-off? Then whatever works. If production, then you'd really need a lot of stock to avoid using the "right" part. If using the "right" part then maybe Mega48 or Mega88.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I think you'll also find that your LCD display no longer works down at 3.3V. I have a bunch of LCDs with the HD44780 controllers on them and although the controller will work down to those voltages, the LCD itself does not.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

theusch wrote:

Quote:

but a mega8 is not spec'd to run at 3V3.

...but a Mega8L is.

I know, but the OP has a big stock of non-L versions ;)

Quote:
I think you'll also find that your LCD display no longer works down at 3.3V. I have a bunch of LCDs with the HD44780 controllers on them and although the controller will work down to those voltages, the LCD itself does not

The display is still fed with 5V.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You have problems with the ATMega8 I2C interface and are running the part at below VCC spec. Hmm. Shouldn't you expect that? Get your circuit in spec - crank up the ATMega8's VCC to 5.0V. Pull up your I2C pins to 3.3V with 3K each. Then test for proper operation and let us know the result. I2C connections are open drain and will work fine with one end powered by 3.3V and the other by 5.0V. Should not be a problem.

"It's easier to ask forgiveness than it is to get permission" - Admiral "Amazing" Grace Hopper.