Thread: Overclock the N900?
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Flandry is offline Flandry
2010-04-05 , 12:29
Posts: 1,559 | Thanked: 1,786 times | Joined on Oct 2009 @ Boston
#904
From Igor Stoppa, the engineer who gave the warning.

Originally Posted by Igor Stoppa
Hi,
the damage is not directly related to the temperature, but rather to the overvoltage used @600MHz.

There are few temperature sensors, but not all of them are accessible to the normal SW and most are just thermal shutdown safeguards.

But the battery, for example, has a temperature sensor nearby.

However please notice that usually their readings are meaningless apart from indicating the _local_ temperature, since there are so many heat sources on the board.

Finally, being the device basically plastic, not perceiving high temperature at surface level is not so relevant, since plastic is not such a good thermal conductor and allows for higher and longer power peaks. Phones with metallic casing have lower max temperature allowed at surface level because of the higher transfer efficiency (the delta being 15C, on top of my head).

Cheers, Igor
Takeaway messages:
  • Don't use temperature sensor readings as a guide to what is or is not going to damage the system
  • The N900 can not be expected to handle sustained overclocked heat production as well as a phone using metallic casing. More generally: device heat sinking is an important parameter.
  • The operational voltage (as was stated by Matan) is the main reason for the decreased lifetime when running at top frequencies.

What's not directly included in that short message, but is known for modern CPUs, is that operating at elevated temperatures accelerates the process known as electromigration, which eventually leads to a short or other fault in the processor. It's a cumulative effect, so e.g. if the CPU is at 40C and top voltage for 1 hour, the decrease in lifetime could be equivalent to 100 hours at 30C and bottom voltage. It's a consequence of the exponential dependence on temperature of the relevant physical processes.

Realistically, if you are going to throw away your N900 in a year or even two, overclocking the top speed step to 800 MHz and using it normally is probably not going to be a problem.

Edit: oops Titan beat me to the link.
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Last edited by Flandry; 2010-04-05 at 12:42. Reason: Linked to wikipedia discussion of electromigration.
 

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