Why are PWM ports not all created equal?

When I posted about my H-Bridge motor control, I indicated that it didn't work on ports 10 and 11, but when I switched to ports 5 and 6 everything started to work correctly. I also mentioned that some code examples have you connect to specific ports, not just any ports. What bothered me was that it wasn't obvious why you "must" connect to a specific port. "Are all PWM ports created equal?" kind of question on my part. Well, while I was trolling the Arduino forum I found out about equality when someone asked about "the best" PWM ports to use.

It so happens that the PWM ports are modulated by timers. Ok I knew that. What I didn't know is that ports are assigned to specific timers, not any available timer. Now that makes a difference. From this link we have the following port/timer assignments:

3    PD 3        PWM T2B, Ext Int 1

5    PD 5        PWM T0B
6    PD 6        PWM T0A

9    PB 1        PWM T1A
10   PB 2        PWM T1B

11   PB 3        PWM T2A, MOSI


So timer zero is assigned PWM ports 5 and 6; timer 1 is assigned to pins 9 and 10; and timer 2 is assigned to 3 and 11.

In my application, timer 2 is used for the Quantum Leap Framework, so I could try to use pin 11 for PWM but it will be taken over by my QF/AD framework. So this is a good reason that I couldn't drive the motor from pin 11 via. PWM.

What this chart tells me is that for my specific application, I can't use pins 3 and 11 for PWM, but they are fine as standard digital I/O.

I'm thinking of adding a PWM driven speaker to my project and now I know what ports I can use.