Specifications

Section 43. High-Speed PWM

High-Speed PWM

43.5.4 Center-Aligned PWM

The center-aligned PWM waveforms as illustrated in Figure 43-6, align the PWM signals to a

reference point such that half of the PWM signal occurs before the reference point and the

remaining half of the signal occurs after the reference point. Center-aligned mode is enabled

when the Center-Aligned Mode Enable bit (CAM) in the PWM Control register (PWMCONx<2>)

is set.

When operating in Center-aligned mode, the effective PWM period is twice the value that is

specified in the PWM Primary Phase Shift registers (PHASEx) because the independent time

base counter in the PWM generator is counting up and then counting down during the cycle. The

up/down count sequence doubles the effective PWM cycle period. This mode is used in many

motor control and uninterrupted power supply applications. The configuration of edge-aligned or

center-aligned mode selection is shown in Example 43-4. The typical application of

center-aligned PWM mode in UPS applications is illustrated in Figure 43-7.

Figure 43-6: Center-Aligned PWM Mode

Example 43-4: Edge-Aligned or Center-Aligned Mode Selection

Figure 43-7: Center-Aligned PWM Mode in Power Inverter/UPS Applications

Note: Independent Time Base mode (ITB = 1) must be enabled to use Center-aligned

mode. If ITB = 0, the CAM bit (PWMCONx<2>) is ignored.

PHASEX

Local Time-Base Value

2 x Period

Period

PDC1

PDC2

PWM1H

PWM2H

/* Select Edge-Aligned PWM */

PWMCON1bits.CAM = 0; /* For Edge-Aligned Mode */

/* Select Center-Aligned PWM */

PWMCON1bits.CAM = 1; /* For Center-Aligned Mode */

PWMCON1bits.ITB = 1; /* Enable Independent Time Base */

VDC

–

PWM1H

PWM2H

PWM2L

PWM1L