Pulse Generator circuit

Posted on Feb 5, 2014

A simple pulse generator with a variable duty cycle and frequency is often useful in the lab to trigger or test other devices, e. g a servo with a frequency of 50 Hz and pulses between 1 and 2 ms. This is where the project started but for reasons we see later this idea was good but just not in this case. How is it done From the Atmel datasheet we

Pulse Generator circuit
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see that the duty cycle of Timer1 is basically D = (OCR1A+1) / (OCR1C+1). The PWM frequency is fpwm = 1 / (OCR1C+1). So the duty cycle D = fpwm * (OCR1A+1). This is calculated in the Basic program. We can set the frequency and keep the duty cycle constant at e. g 50% or any other percentage when the frequency is changed. The duty cycle D scale is linear but the frequency scale is not linear, bad luck it`s hyperbolic as with most analog frequency generators, so the upper convenient frequency setting is at about 1 MHz. The frequency and duty cycle are set independently by reading 2 potentiometers with 2 ADC in a loop. The controller uses an internal 8Mhz system clock (unprogram fuse CKDIV8) and also a fast PLL clock for Timer1 at 8 times the system clock. The PLL must be enabled and takes some time to lock, then Timer1 can run on a 64 MHz clock frequency. It takes now 1 pushbutton to toggle between PLL clock and system clock, one more pushbutton walks through the Timer1 prescaler values between 1 and 15 and back to 1 and so on. The main program is in Basic and the part dealing with the PLL is in assembler. It`s digital and it`s 8 bit pwm so it has some granularity when you turn the potentiometers which may be a limitation or not, depends on the application. As an example, with the system clock and clock/2048 for the prescaler the Timer1 will then get one clock pulse every 512 usec, that is the limit for the resolution, in...

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