jkeating
Posts: 11
Joined: Thu Aug 01, 2013 5:05 am

Fuses on Atmega328

Wed Sep 18, 2013 2:58 pm

I read the fuses on the default atmega328p on the gertboard and it points to an external full swing crystal. I guess my question is what is the default clock speed of the atmega on the gertboard and what should I be setting my F_CPU to when I program it?

User avatar
Gert van Loo
Posts: 2487
Joined: Tue Aug 02, 2011 7:27 am
Contact: Website

Re: Fuses on Atmega328

Thu Sep 19, 2013 11:37 am

12 MHz.
Officially according to the manufacturer even that is 2% above the maximum frequency at 3.3V.
(But a lot of user have told me they run their chip at 16MH and 3V3 so it seems there is margin)

uberscubajim
Posts: 162
Joined: Thu Jun 21, 2012 12:22 pm
Location: Ipswich, Suffolk. UK.

Re: Fuses on Atmega328

Fri Sep 27, 2013 6:46 pm

Gert,

You are aware that this seemingly innocent 2% overclock will still cause stress on the space-time continuum? Users could end up displaced into some sort of parallel universe! Mind you, the rest of the world runs them at 16MHz on 3V3 without problems....

Seriously, cool stuff! I look forward to having a serious play with my Gertboard really soon!

Jim.

User avatar
Gert van Loo
Posts: 2487
Joined: Tue Aug 02, 2011 7:27 am
Contact: Website

Re: Fuses on Atmega328

Fri Sep 27, 2013 7:00 pm

I am not your run-of-the-mill PCB hardware designer.
I am an ASIC designer and I more: I am the technical lead for the BCM2835.
I have worked a year long with all the burn-in tests, the temperature tests, the over- and under voltage tests, the FFF, FST, TSF, TTT, SSS and all other corner silicon test. And the combination of all those as well as ESD, latch-up test, package moisture test, solder, re-solder and re-work test and half a dozen more test which I have forgotten about.
I know silicon has a lot of margin but I also know that if I stick to the specs the device will work under all circumstances.
You will also find that none of the PIs running in my house are overclocked.

uberscubajim
Posts: 162
Joined: Thu Jun 21, 2012 12:22 pm
Location: Ipswich, Suffolk. UK.

Re: Fuses on Atmega328

Sat Sep 28, 2013 3:49 pm

Thanks for that Gert!

I was actually joking about the “overclocking”. I'm really sorry if I caused any offence!

In fact, you're the only person I know who has expressed any concern at running at a mere 2% overclock. Everyone else runs their 3V3 ATMegas at 16MHz (a 33% overclock!) with no concern at all! What's even more bizarre is that they all seem to work! Of course, the questions are: how reliably and for how long?

I certainly strongly respect your “rock solid” design choices! They have led to the Pi and the Gertboard being amazingly solid and reliable bits of engineering. As you say, “...if I stick to the specs the device will work under all circumstances”.

I think there's a lot of scope to educate people about “good design” and why it's important.

Also, having just written some interrupt driven code code for the ATTiny85 I know completely understand your comments made at the Cambridge Jam before last about the ATMega being very useful for teaching interrupts from the ground up (in addition to a load of other things of course).

Finally, my original comment about damage to the space-time continuum is based on http://www.bbspot.com/news/2000/5/clock_rift.html (it's well worth a read, it's a fantastic bit of satire).

Regards,

Jim.

User avatar
Gert van Loo
Posts: 2487
Joined: Tue Aug 02, 2011 7:27 am
Contact: Website

Re: Fuses on Atmega328

Sun Sep 29, 2013 8:18 am

Humm yes, now I read it again my previous post was waaaay to stern and gives the impression that I am upset.
People who know me better are used to that semi-offended tone and know to laugh about it.
Actually I am not easily offended and your post did not offend me at all.
And yes, I know I am a nitpicking, hair splitting, perfectionist.
(All good ASIC engineers I know are perfectionist. The thought of a half-million-dollar re-spin because you left an
fatal error in the design is always in the back of our minds)

uberscubajim
Posts: 162
Joined: Thu Jun 21, 2012 12:22 pm
Location: Ipswich, Suffolk. UK.

Re: Fuses on Atmega328

Sun Sep 29, 2013 3:54 pm

Hurrah for “nitpicking, hair splitting, perfectionists”! That's the way we make reliable hardware and software. And ASICs.

Return to “HATs and other add-ons”