MPM
Member
Registered: Jun 2000
Location: Sunnyvale, CA
Posts: 216 |
 Reality check...
quote:
Originally posted by JJinNJ
Overclocking any processor means it will run hotter.
When any electronic device runs hotter, it will fail earlier than it would have without over-clocking.
Ok, time for an engineering reality check...
While the statement "faster = hotter = fails sooner" is true, the Visor's CPU, the Motorola Dragonball EZ, consumes very little power. It can't consume gobs of power and still give you 1-2 months of battery life.
So let's run the numbers and see just how much hotter the CPU will run when overclocking a Visor Deluxe to 28MHz.
According to the Motorola Dragonball EZ datasheet the maximum current the CPU will draw running at 16MHz is 20mA. The CPU runs at 3.3V. So that gives us 66mW of power (heat) dissapation.
Now with CMOS type ICs (like the Dragonball), doubling the clock rate, doubles the current consumption. So increasing the CPU to 28MHz by overclocking will increase the current consumption by a factor of 1.75 (1.75=28/16). So now at 28MHz the maximum CPU current consumption will be 35mA and the power dissapation will be 115.5mW.
The temperature rise of the CPU can be computed by using the junction to ambient thermal resistance of the CPU package. Unfortuneatly, Motorola does not specify this in the datasheet. The package they use (a 100 pin TQFP) is a standard one, so I used another manufactures number. They had the thermal resistance of the 100 pin TQFP package at 31 deg C per Watt. This means the for every Watt of power dissapated by the package that the junction temperature of the chip inside the package will increase by 31 deg C.
So for our tiny increase of 49.5mW (115.5mW-66mW) the Dragonball CPU temperature will increase by just 1.5 degrees C! That's just 2.7 degrees F!
Now let's say that Motorola's package is not 31 deg C per W. Say is is double that (very unlikely). Then the worst case temperature rise is just 3 degrees C! (5.4 degrees F)
Now the above analysis does not take into account the the CPU is hooked up to other components and is not running alone. This will add a bit to the power draw because the Dragonball will have to drive the signal lines connecting it to the other components (RAM, ROM, etc...) So let's just double power dissapation (again somewhat too much). This simply doubles the temperature rise. So now we have a 6 deg C increase.
Believe me, even a 6 deg C temperature rise will NOT effect the life of the CPU. (If it did effect it that much, electronic products in the hotter regions of the world would be failing at a much faster rate then ones in the cooler regions.)
So in the worst, worst case scenario, we have a 6 deg C (11 deg F) temperature rise when overclocking to 28MHz. i.e. Don't worry about it! Overclock!
P.S. I'm going to overclock my Visor to 24MHz (default) at my next battery change, and see what effect it has on my battery life. I'll post some results when that set of batteries dies.
P.P.S. I'm going to use 24MHz instead of 26MHz or 28MHz because at 24MHz the serial port and IR beaming will work. They don't work properly at 26MHz or 28MHz.
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10-21-2000 06:55 PM
) I would definately recommend it, but you may need to back down a little if things get unstable. (The difference in 28MHz and 16MHz is VERY aparent.)
although I might try QuickBits on it.
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