Micro overclocking

Jun 2 7:59pm

For a circuit I’m working on it would be preferable to use a 3.3v supply for a microcontroller, as it interfaces with a 3.3v sensor, and the uC in question needs to run at 12Mhz. The problem: the uC, the Atmel ATtiny88, is not specified to run at 12Mhz off a 3.3v supply. Solution: overclocking, with the hope that it won’t cause any critical issues. We’ll see.

The probability of it not working is dependent on how much you overclock the uC. Below is a run through of the simple calculations to figure out roughly how much the uC would be operating out of it’s guaranteed operating range.

You’re given the following speed grades with their voltages in the datasheet:

• 8Mhz at 2.7v
• 12Mhz at 4.5v

My voltage, 3.3v, is in between so we calculate the corresponding frequency that 3.3v could operate at according to the manufacturer, based on a linear relationship between the two data points given (note: note linear outside these points).

Basic line math, you know, slope and stuff:
12Mhz – 8Mhz = 4Mhz
4.5v – 2.7v = 1.8v
4Mhz/1.8v = 2.2 Mhz/v
3.3v – 2.7v = 0.6v
0.6v * 2.2Mhz/v + 8Mhz = 9.32 Mhz

Quite a bit short of the 12Mhz I was looking for. I’d be overclocking it 29% which is significant. But knowing that this project is not a demanding application and that manufacturers (usually) underspec parts it just might work. Or it might seeming work fine in the lab only to fail sporadically in random ways in the field rendering the unit effectively garbage.