Arduino Pro 3.3v PWM

Development Tools IDE 1.x
1

Hi guys. I had a question regarding PWM frequency. I have PWM working using a analogwrite on pin 11. I'm reading a 245hz pwm frequency with my Tektronics scope. This is half of what I've read the frequency should be (490hz).

I'm fairly certain that the scope is ok and its doing a automatic measurement of frequency and duty cycle. Duty Cycle looks right.

Am I loosing my mind or have I done something wrong? I have a Arduino Pro 3.3v 8mhz set as the board type in the IDE and that is what is marked on it.

2

I've added the following -
TCCR2B = (TCCR2B & 0xF8) | 3;

This should produce 980hz but it really produces 490hz now. Still half frequency. Odd. It is indeed selected for 8mhz in the IDE

3

I have the PID library and the Software Serial Library installed. I commented all of that out to be sure they were not messing with things and still no change. Just the basic set the pin for output and analogwrite value. Always half freq of what was stock and half frequency of what I select in the TCCR2B register.

4

No suggestions or ideas?

5

Clock Speed 8 MHz (3.3V versions) or 16 MHz (5V versions)

Clock Speed 8 MHz (3.3V model) or 16 MHz (5V model)

6

Yep....I've got the 3.3v 8Mhz Pro
Still produces half rate at baud for any particular setting. I cant figure it out.

7

The timers work by dividing the processor clock by certain amounts (called a "prescaler"). The core always sets the prescaler to 64 for the timers. For fast PWM (timer 0), the frequency is...

(CPU frequency) / 64 / 256 = 16000000 / 64 / 256 = 976.5625

If the processor is changed to run at half that speed (like on your board), the PWM frequency is also cut in half...

8000000 / 64 / 256 = 488.28125

8

OK, I understand now. I just didn't see any of that anywhere in the documents. I must have missed it.