DSPIC30F5015 datasheet(96/236 Pages) MICROCHIP

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dsPIC30F4011/4012

DS70135E-page 94

15.1.4

DOUBLE UPDATE MODE

In the Double Update mode (PTMOD<1:0> = 11), an

interrupt event is generated each time the PTMR regis-

ter is equal to zero, as well as each time a period match

occurs. The postscaler selection bits have no effect in

this mode of the timer.

The Double Update mode provides two additional func-

tions to the user. First, the control loop bandwidth is

doubled because the PWM duty cycles can be

updated, twice per period. Second, asymmetrical

center-aligned PWM waveforms can be generated

which are useful for minimizing output waveform

distortion in certain motor control applications.

15.1.5

PWM TIME BASE PRESCALER

The input clock to PTMR (FOSC/4) has prescaler

options of 1:1, 1:4, 1:16 or 1:64, selected by control

bits, PTCKPS<1:0>, in the PTCON SFR. The prescaler

counter is cleared when any of the following occurs:

• a write to the PTMR register

• a write to the PTCON register

• any device Reset

The PTMR register is not cleared when PTCON is

written.

15.1.6

PWM TIME BASE POSTSCALER

The match output of PTMR can optionally be post-

scaled through a 4-bit postscaler (which gives a 1:1 to

1:16 scaling).

The postscaler counter is cleared when any of the

following occurs:

• a write to the PTMR register

• a write to the PTCON register

• any device Reset

The PTMR register is not cleared when PTCON is written.

15.2

PWM Period

PTPER is a 15-bit register and is used to set the

counting period for the PWM time base. PTPER is a

double-buffered register. The PTPER buffer contents

are loaded into the PTPER register at the following

instants:

•Free-Running and Single-Shot modes: When the

PTMR register is reset to zero after a match with

the PTPER register.

• Continuous Up/Down Count modes: When the

PTMR register is zero.

The value held in the PTPER buffer is automatically

loaded into the PTPER register when the PWM time

base is disabled (PTEN = 0).

The

PWM

period

can

determined

using

Equation 15-1:

EQUATION 15-1:

PWM PERIOD

If the PWM time base is configured for one of the

Continuous Up/Down Count modes, the PWM period is

provided by Equation 15-2.

EQUATION 15-2:

PWM PERIOD

(CENTER-ALIGNED

MODE)

The maximum resolution (in bits) for a given device

oscillator and PWM frequency can be determined using

Equation 15-3:

EQUATION 15-3:

PWM RESOLUTION

Note:

Programming a value of 0x0001 in the

Period register could generate a continu-

ous interrupt pulse, and hence, must be

avoided.

TPWM =

TCY

• (PTPER + 1)

(PTMR Prescale Value)

TPWM =

2 TCY

• PTPER + 0.75

(PTMR Prescale Value)

Resolution =

log (2

• TPWM/TCY)

log (2)

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