Sine Wave Generation

As seen in the previous section, amplitude of the analog waveform is directly proportional to duty cycle of PWM signal. So, if a sine wave is to be generated, the duty cycle of the PWM signal has to be varied accordingly. In ATtiny26, the OCR value determines the duty cycle of the PWM signal. In order to control the variation in amplitude, the duty cycle has to be varied, thereby generating a sine wave. The application uses a sine table that has the sample values (duty cycle of PWM), which will be loaded at every Timer overflow. To generate a sine table, the following parameters must be considered:
Table 1. Configuration Parameters
Parameter Value used in the application
CPUFreq 8MHZ
TimerFreq 16MHZ
TimerTop 255
PWMFreq 62.5kHz
SineFreq ~244Hz
  1. 1.CPUFreq: This is the frequency at which the system is operated. In the application demonstrated, the CPU runs at 8MHz using Internal RC Oscillator.
  2. 2.TimerFreq: This is the frequency at which the Timer/Counter1 is operated. In the application demonstrated, 1MHz output of Internal RC Oscillator acts as Timer1 PLL reference to generate a 64MHz clock. This clock is then prescaled down to 16MHz to demonstrate the prescaling capability of Timer/Counter1.
  3. 3.TimerTop: This is the Timer/Counter1 top value that is one of the deciding parameters of PWM frequency. In the application demonstrated 255 is loaded to 8-bit register OCR1C.
  4. 4.PWMFreq: This is the frequency of the PWM output. It can be calculated by PWMFreq = TimerFreq / TimerTop.
  5. 5.SineFreq: This is the frequency of Sine Wave generated after passing the PWM signal to a Low-Pass filter. This can be calculated by the formula, PWMFreq / TimerTop.
  6. 6.SampleValue: This is the instantaneous sample of Sine Wave that will be loaded to OCR1A on every Timer overflow. The sample value decides the duty cycle of PWM which in turn controls the amplitude of the analog waveform (sine wave) generated. The formula to generate SampleValue is shown as follows:
Note:
  1. 1.Here, n is the number of samples in one cycle of sine wave. It ranges from 0 to TimerTop.
  2. 2.It has to be noted that the CPU should be running fast enough to load the duty cycle values to the OCR1A register after every Timer1 overflow.

Configurations to generate different frequencies of sine wave are listed in the following table:

Table 2. Various Combinations to Generate Desired Frequency of Sine Wave
CPUFreq [MH] TimerFreq [MH] PWMFreq [kH] SineFreq [Hz] TimerTop Number of CPU cycles before overflow
8 16 62.5 250 249 125
8 16 62.5 500 124 62.5
8 16 62.5 800 77 39
16 16 62.5 800 77 78
16 16 62.5 1000 62 63
16 16 62.5 1500 41 42
Note: The values shown in the table has some restrictions on CPU speed and PWM speed. For more details, refer Interrupt Service Routine.
Parent topic: Description