Do I Have to Change Internal Interrupt Setups when I Change Clock Speed?

Question

Do I Have to Change Internal Interrupt Setups when I Change Clock Speed? The reason I ask is because I am trying to modify this code:

https://github.com/DccPlusPlus/BaseStation/blob/master/DCCpp_Uno/DCCpp_Uno.ino (Beginning at Line 289)

so that if will work on an ATmega328P-derivative board running at 3.3V and 8 MHz. I need the waveforms to be generated at the same speed, so is there anything I have to change? The original code is written for a 16 MHz uno or mega.

• Majenko: Yes, you will have to recalculate all the timer compare values (halve them for half the clock speed?)
• Edgar Bonet: Halve the values and round down: 3199 → 1599, etc.

Answer 1

The equation for timer period is given by the following formula, where N is the value of the prescaler, TOP is the value given by either MAX (i.e. 0xFF, 0xFFFF) or the value in register OCRnx, and f is the frequency of the I/O clock.

T = (N ×ばつ (TOP+1)) ÷ f

If you're using phase-correct or phase- and frequency-correct mode, the factor (TOP+1) in the formula above should be replaced by 2 ×ばつ TOP.

• Edgar Bonet: It's not TOP. It's either 2 × TOP (in the phase-correct and phase-and-frequency-correct PWM modes) or TOP + 1 (in any other mode).