19.9.4 Phase Correct PWM Mode

The Phase Correct Pulse Width Modulation or Phase Correct PWM modes (WGMn[3:0]= 0x1, 0x2, 0x3, 0xA, and 0xB) provide a high resolution, phase correct PWM waveform generation option. The Phase Correct PWM mode is, like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx while up-counting, 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.

The PWM resolution for the Phase Correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following equation:

R PCPWM = log TOP + 1 log 2

In Phase Correct PWM mode the counter is incremented until the counter value matches either one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn[3:0]= 0x1, 0x2, or 0x3), the value in ICRn (WGMn[3:0]=0xA), or the value in OCRnA (WGMn[3:0]=0xB). The counter has then reached the TOP and changes the count direction. The TCNTn value will be equal to TOP for one timer clock cycle. The timing diagram for the Phase Correct PWM mode is shown below, using OCRnA or ICRn to define TOP. The TCNTn 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 lines on the TCNTn slopes mark compare matches between OCRnx and TCNTn. The OCnx interrupt flag will be set when a compare match occurs.

Figure 19-7. Phase Correct PWM Mode, Timing Diagram
Note: The “n” in the register and bit names indicates the device number (n = 1, 3, 4 for Timer/Counter 1, 3, 4), and the “x” indicates output compare unit (A/B).

The Timer/Counter Overflow flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same timer clock cycle as the OCRnx registers are updated with the double buffer value (at TOP). The interrupt flags can be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value of all of the compare registers. If the TOP value is lower than any of the compare registers, a compare match will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values, the unused bits are masked to zero when any of the OCRnx registers is written. As illustrated by the third period in the timing diagram, changing the TOP actively while the Timer/Counter is running in the phase correct mode can result in an unsymmetrical output. The reason for this can be found in the time of update of the OCRnx. Since the OCRnx update occurs at TOP, the PWM period starts and ends at TOP. This implies that the length of the falling slope is determined by the previous TOP value, while the length of the rising slope is determined by the new TOP value. When these two values differ the two slopes of the period will differ in length. The difference in length gives the unsymmetrical result on the output.

It is recommended to use the Phase and Frequency Correct mode instead of the Phase Correct mode when changing the TOP value while the Timer/Counter is running. When using a static TOP value, there are practically no differences between the two modes of operation.

In Phase Correct PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Writing COMnx[1:0] bits to 0x2 will produce a non-inverted PWM. An inverted PWM output can be generated by writing the COMnx[1:0] to 0x3. The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx register at the compare match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx register at compare match between OCRnx and TCNTn 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 2 ⋅ N ⋅ TOP

N represents the prescale divider (1, 8, 64, 256, or 1024).

The extreme values for the OCRnx represent special cases when generating a PWM waveform output in the Phase Correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously low and if set equal to TOP the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. If OCRnA is used to define the TOP value (WGMn[3:0]=0xB) and COMnA[1:0]=0x1, the OCnA output will toggle with a 50% duty cycle.