Investigation of PWM

This week I investigated the Velleman K8055 board for suitablility of driving an H-Bridge motor controller. I am using a good fast opto-isolator. I originally intended to use the TIP523 opto isolator with darlington output but it is slow and has a max bandwidth of 12.5kHz so I have switched to a non-darlington type which can switch in 3 microseconds. This has added the need for an additional transistor between the opto-isolator and each transistor of the H-Bridge.
Investigaton of the Vellemans outputs allowed me to calculate its frequency. It does not seem to be documented anywhere other than the PIC microcontroller datasheet which gives a number of possibilities. It was time to connect it up to an oscilloscope.

 

As you can see from these pictures there is 20us per div giving approximately 40us for the time period. 1/40us gives a frequency of 25kHz. The closest frequency I can calculate is to take the oscilator frequency of 6MHz and divide by 16 twice which gives 23437.5Hz. This frequency will need to be divided down a little to be suitable for the motors.