ST10F280 datasheet(63/186 Pages) STMICROELECTRONICS

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ST10F280

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11.2.1.2 - Mode 1: Symmetrical PWM

Generation (Center Aligned PWM)

Mode 1 is selected by setting the respective bit

XPMx in register XPWMCON1 to ‘1’. In this mode

the timer XPTx of the respective XPWM channel

is counting up until it reaches the value in the

associated period shadow register.

Upon the next count pulse the count direction is

reversed and the timer starts counting down now

with subsequent count pulses until it reaches the

value 0000H. Upon the next count pulse the count

direction is reversed again and the count cycle is

repeated with the following count pulses.

The XPWM output signal is switched to a high

level when the timer contents are equal to or

greater than the contents of the pulse width

shadow register while the timer is counting up.

The signal is switched back to a low level when

the respective timer has counted down to a value

below the contents of the pulse width shadow reg-

ister. So in mode 1 this PWM value controls both

edges of the output signal.

Note that in mode 1 the period of the PWM signal

is twice the period of the timer:

PWM_PeriodMode1 = 2 * ([XPPx] + 1)

The figure below illustrates the operation and out-

put waveforms of a XPWM channel in mode 1 for

different values in the pulse width register.

This mode is referred to as Center Aligned PWM,

because the value in the pulse width (shadow)

symmetrically.

Figure 21 : Operation and Output Waveform in Mode 1

XPPx

Period=7

XPTx Count

Value

XPWx Pulse

Width=0

XPWx=1

XPWx=2

XPWx=4

XPWx=6

XPWx=7

XPWx=8

Latch Shadow Registers

Interrupt Reques

Change Count

LSR

Duty Cycle

100%

87.5%

75%

50%

25%

12.5%

Direction

LSR

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