Single-Slope PWM Generation

For single-slope Pulse-Width Modulation (PWM) generation the period (T) is controlled by TCAn.PER, while the values of the TCAn.CMPm registers control the duty cycles of the generated waveforms. The figure below shows how the counter counts from BOTTOM to TOP and then restarts from BOTTOM. The waveform generator output is set at BOTTOM and cleared on the compare match between the TCAn.CNT and TCAn.CMPm registers.

CMPn = BOTTOM will produce a static low signal on WOn while CMPn > TOP will produce a static high signal on WOn.

Figure 1. Single-Slope Pulse-Width Modulation

The TCAn.PER register defines the PWM resolution. The minimum resolution is 2 bits (TCA.PER = 0x0002), and the maximum resolution is 16 bits (TCA.PER = MAX-1).

The following equation calculates the exact resolution in bits for single-slope PWM (R_{PWM_SS}):

RPWM_SS=logPER+2log2

The single-slope PWM frequency (f_{PWM_SS}) depends on the period setting (TCA_PER), the system’s peripheral clock frequency f_{CLK_PER} and the TCA prescaler (CLKSEL in TCAn.CTRLA). It is calculated by the following equation where N represents the prescaler divider used:

fPWM_SS=fCLK_PERN(PER+1)