Controlling Servos With the AT90USB162 or Teensy

I have been in the process of replacing the use of the old Atmega 8s in combination with an FTDI RS232RL for an all in one AT90USB162. I want to use the AT90USB162 for controlling servos and electronic speed controls. Instead of making my own breakout board, I recently picked up a Teensy from PJRC. I spent the weekend trying to figure out how to get computer controllable PWM signals to my old Futaba servo. I first started out trying to use the 8bit counter but eventually realized there is no way to generate the correct PWM signal on a 16Mhz AT90USB162 using the 8bit counter.

Let me elaborate. For controlling a servo, you must generate at PWM signal at 50Hz (50 times a second). You may have seen people talking about this as 20 milliseconds. You get 20ms by taking the 1000ms in one second and dividing by 50...get it? So in order to control the position of a servo you need your pulse width signal to be on for only 1 to 2ms of your full 20ms cycle.

From my understanding, if you look at the 8bit counter on the AT90USB162, you can only use 0xFF(decimal 255) or the OCR0A register for the TOP of the counter which also has a max value of 255. The formula for calculating the frequency of Fast PWM mode for the 8bit counter is as follows:
Frequency = Clock Speed / Prescaler * 256.
The Teensy has a fixed Clock speed of 16Mhz or 16,000,000. The highest Prescaler you can use is 1024. So, if we use the max prescaler...
16,000,000/ (1024 * 256) = about 61.03Hz.
You can get lower using PWM Phase Correct mode, but never hit 50Hz exact at a fixed 16Mhz clock speed; so the 8bit timer can not be used for controlling a servo!

I set up the 16bit counter to actually work for 50Hz so let me show you the settings I used below.
For the 16bit Counter/Timer (Im having it output on pin 23 or PC6) you can make TOP the value equal to the ICR1 register which is key. You can then cycle the on and off duration by changing the value of the OCR1A register. So the formula in the 16bit counter for setting PWM frequency in fast PWM mode is as follows (datasheet page 122).

Frequency = Clock Speed / Prescaler * (1 + TOP)

Lets plug in our values needed to get close to 50Hz. 16,000,000 / 8 * (1 + 40,000)

So Prescaler = 8, TOP = 40,000.

This actually works. If you vary the value of OCR1A between 2200 to 3800 youll get pretty close to the 1 to 2ms on time for the 20ms period in order to control the full range of the servo. My exact register settings for the C programming language using the AVRlibC library are below. I have the timer set to CLEAR on compare match, SET at TOP.