44.6.2.6.5 Normal Pulse-Width Modulation (NPWM) Dual-Slope Operation

For dual-slope PWM generation, the period setting (TOP) is controlled by PER, while CCy control the duty cycle of the generated waveform output. The figure below shows how the counter repeatedly counts from ZERO to PER and then from PER to ZERO. The waveform generator output is set on compare match when up-counting, and cleared on compare match when down-counting depends on DS mode (DSTOP, DSBOTTOM, DSBOTH or DSCRITICAL). Depending on Dual-Slope mode (DSTOP, DSBOTTOM, DSBOTH, or DSCRITICAL), an interrupt and/or event is generated on TOP (when counting upwards) and/or ZERO (when counting up or down).

In DSBOTH operation, the circular buffer must be enabled to enable the update condition on TOP.

The following steps must be performed to operate the TCC in Dual-slope PWM generation operation.

1. Configure the clock source for the TCC Instance in the Main Clock Controller (MCLK) and enable the APB BUS clock for the TCC Instance by writing a ‘1’ to the TCCx_ bit in the APB Mask register of the MCLK (i.e. enable CLK_TCC0_APB for TCC0 by setting TCC0_ bit (APBCMASK<3>), CLK_TCC1_APB for TCC1 by setting TCC1_ bit (APBCMASK<4>) etc.).
2. Enable Generic clock for TCC Instance (e.g. enable GCLK_TCC0 for TCC0 by setting PCHCTRL31.CHEN bit PCHCTRL31<6>, GCLK_TCC1 for TCC1 by setting PCHCTRL32.CHEN bit PCHCTRL32<6> etc.).
3. Select desired prescaler by setting CTRLA.PRESCALER bits (CTRLA<10:8>).
4. Select prescaler Synchronization PRESCSYNC bits (CTRLA <13:12>).
5. Set waveform generation operation to Dual Slope PWM by configuring WAVE.WAVEGEN bits (WAVE<2:0>)
   1. For DSCRITICAL operation, set the value of WAVE.WAVEGEN bit (WAVE<2:0> = 4)
   2. For DSBOTTOM operation, set the value of WAVE.WAVEGEN bit (WAVE<2:0> = 5)
   3. For DSBOTH operation, set the value of WAVE.WAVEGEN bit (WAVE<2:0> = 6)
   4. For DSTOP operation, set the value of WAVE.WAVEGEN bit (WAVE<2:0> = 7)
6. Load the selected Compare/Capture (CCy) register (i.e. CC0<31:0>) with the desired PWM duty cycle value.
7. Load the period register PER<31:0> with the desired time period value.
8. Set Counter to count in up direction by clearing DIR bit (CTRLBCLR<0> = 1).
9. If DSBOTH operation (WAVEGEN bit (WAVE<2:0> = 6)) is set, enable Circular buffer by setting CIPEREN bit (WAVE<7>).
10. Select matrix routing to desired port pins for generated output waveform, by configuring WEXCTRL.OTMX bits (WEXCTRL<1:0>).
11. The dead time insertion on output matrix can be enabled by setting WEXCTRL.DTIENy bit, where y=0,1,2,3.
12. Higher side and Lower side PWM Dead-time is programmed in WEXCTRL.DTHS (WEXCTRL<30:24>) and WEXCTRL.DTLS (WEXCTRL<23:16>) respectively.
13. The output polarity for each individual channel can be changed by configuring the corresponding WAVE.POLy bit, where y= 0,1,2,..7. For example, when the CC0 register is used for the duty cycle, the corresponding POL0 bit (WAVE<16>) decides the polarity.
14. The waveform output for each individual channel can be inverted by configuring the corresponding Waveform Output Invert Enable bit DRVCTRL.INVENy bits where y = 0,1,2…7. For example, when the CC0 register is used for the duty cycle, the corresponding INVEN0 bit (DRVCTRL<16>) decides inversion.
15. If overflow interrupt is used, set OVF bit (INTENSET<0>) and configure the NVIC by setting group priority, sub priority and enabling corresponding TCCx IRQ.
16. Enable TCC by setting ENABLE bit (CTRLA<1>).
17. For instructions on how to clear interrupts, see Interrupts.

Dual-slope Critical (DSCRITICAL) PWM operation is explained below.

Using dual-slope PWM results in a lower maximum operation frequency compared to single-slope PWM generation. The period (TOP) defines the PWM resolution. The minimum resolution is 1 bit (TOP=0x00000001).

The following equation calculates the exact resolution for dual-slope PWM (R_{PWM_DS}):

The PWM frequency f_{PWM_DS} depends on the period setting (TOP) and the peripheral clock frequency f_{GCLK_TCCx}, and can be calculated by the following equation:

N represents the prescaler divider used. The prescaler can be selected by CTRLA.PRESCALER bits (CTRLA<10:8>). The selected prescaler value can be 1, 2, 4, 8, 16, 64, 256, 1024.. The waveform generated will have a maximum frequency of half of the TCC clock frequency (f_{GCLK_TCCx}) when TOP is set to 0x00000001 and no prescaling is used.

The pulse width (P_{PWM_DS}) depends on the compare channel (CCy) register value and the peripheral clock frequency (f_{GCLK_TCCx}), and can be calculated by the following equation: