The control routines have a fixed "off" period and a variable "on" period. The on cycle repeats at approximately 60hz, but varies slightly depending on the "on" period. That has no noticeable effect on the servo's behavior.
By calculation and experiment, a center neutral position can be acheived by using $0BE0 as the "on" period. This is a period of 1520 micro seconds when using a MC68HC11 with 8Mhz XTAL and with the timer prescaler set to run at it's fastest (reset default). I got the 1520 micro second figure from the back of the package of a Tower Hobbies clone Airtronics servo. I find the Airtronics coonector convienient to plug directly into my ciruit boards through 0.1 inch headers.
I have found that the servos are linear and repeatable. By experiment I found approxamately 90 degrees left and right in terms of pulse length number. The numbers were equidistant from center and provided good 45 degree estimates.
Theoretically, a single HC11 could handle 30-40 servos with a little demux-latch hardware.
Brian Olson, 8/26/95
bolson+@andrew.cmu.edu
http://www.andrew.cmu.edu/user/bolson/robot.html