AVR-Chat

> -----Original Message-----
> From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On
> Behalf Of Clark Martin
> Sent: Thursday, October 13, 2011 12:41 PM
> To: AVR-Chat@yahoogroups.com
> Subject: Re: [AVR-Chat] OCDEN Fuse (minor rant)
> 
> 
> On Oct 13, 2011, at 8:56 AM, Phillip Vogel wrote:
> 
> >
> > I've been scratching my head for a while, trying to figure out why a
> Mega644PA was drawing something over 1.6mA while in
> SLEEP_MODE_PWR_DOWN. I had all the fuses set, all the peripherals
> disabled, all the ports set to draw the least current. The chip looked
> like it was going to sleep - it obediently sat and waited for a pin-
> change interrupt, and then continued execution when I pressed the
> appropriate button. But this was something like 1000 times the expected
> current.
> >
> > I tried shorting the crystal to ground, just to see what would
> happen, and the current went down a bit. This indicated to me that,
> even though it was supposed to be stopped, the clock was actually
> running. WTF?
> >
> > Then, I reconnected the JTAGICE MKII (for the hundredth time today),
> and this time I exited debug mode before disconnecting it. That fixed
> the problem and the chip now sleeps at sub-2uA consumption. So, it
> appears that activation the JTAG debugger also activates the OCDEN
> fuse, and unless you exit debug mode methodically, OCDEN remains
> programmed (and the clock continues to run in all sleep modes).
> >
> > Now that I'm aware of this, I can live with it. But, WHERE IN THE
> DOCUMENTATION AM I WARNED ABOUT THIS? I really don't recall seeing it
> anywhere.
> 
> I ran across this the other day while checking fuse settings...
> 
> In doc 8272 (AT Mega 644 data sheet), page 299 it says:
> 
> "Never ship a product with the OCDEN Fuse programmed regardless of the
> setting of Lock bits and JTAGEN Fuse. A programmed OCDEN Fuse enables
> some parts of the clock system to be running in all sleep modes. This
> may increase the power consumption."
> 
> Clark Martin
> Redwood City, CA, USA
> Macintosh / Internet Consulting
> 
> "I'm a designated driver on the Information Super Highway"

Right. I had read that as well, but the point is that I DID disable OCDEN, and the JTAG debugger re-enabled it and left it that way. Turning off OCDEN has to be the last thing you do before disconnecting the JTAGICE.