21.7.4 Phase Correct PWM Mode

The phase correct PWM mode (WGM2[2:0] = 0x1 or 0x5) provides a high resolution phase correct PWM waveform generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM2[2:0] = 0x3, and OCR2A when MGM2[2:0] = 7. In non-inverting Compare Output mode, the Output Compare (OC2x) is cleared on the compare match between TCNT2 and OCR2x while counting up, and set on the compare match while down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation frequency than single-slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control applications.

In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the counter reaches TOP, it changes the count direction. The TCNT2 value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown in Figure 21-7. The TCNT2 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare matches between OCR2x and TCNT2.

Figure 21-7. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow flag (TOV2) is set each time the counter reaches BOTTOM. The interrupt flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.

In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC2x pin. Setting the COM2x[1:0] bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by setting the COM2x[1:0] to three. TOP is defined as 0xFF when WGM2[2:0] = 0x3, and OCR2A when WGM2[2:0] = 7. The actual OC2x value will only be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the OC2x Register at the compare match between OCR2x and TCNT2 when the counter increments, and setting (or clearing) the OC2x register at compare match between OCR2x and TCNT2 when the counter decrements. The PWM frequency for the output when using phase correct PWM can be calculated by the following equation:

f OCnxPCPWM = f clk_I/O N ⋅ 510

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2A represent special cases when generating a PWM waveform output in the phase correct PWM mode. If the OCR2A is set equal to BOTTOM, the output will be continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.

At the very start of period 2 in the above figure OC2x has a transition from high to low even though there is no compare match. The point of this transition is to guarantee symmetry around BOTTOM. There are two cases that give a transition without compare match.
  • OCR2A changes its value from MAX, as shown in the preceding figure. When the OCR2A value is MAX the OC2 pin value is the same as the result of a down-counting compare match. To ensure symmetry around BOTTOM the OC2 value at MAX must correspond to the result of an up-counting Compare Match.

  • The timer starts counting from a value higher than the one in OCR2A, and for that reason misses the compare match and hence the OC2 change that would have happened on the way up.