Datasheet

dsPIC30F2010
DS70118J-page 84 © 2011 Microchip Technology Inc.
14.1.4 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 cen-
ter-aligned PWM waveforms can be generated, which
are useful for minimizing output waveform distortion in
certain motor control applications.
14.1.5 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.
14.1.6 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.
14.2 PWM Period
PTPER is a 15-bit register and is used to set the count-
ing 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
instances:
• Free Running and Single Shot modes
: When the
PTMR register is reset to zero after a match with
the PTPER register.
• Up/Down Counting 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 be determined using
Equation 14-1:
EQUATION 14-1: PWM PERIOD
If the PWM time base is configured for one of the Up/
Down Count modes, the PWM period is found using
Equation 14-2.
EQUATION 14-2: PWM PERIOD (UP/DOWN
COUNT MODE)
The maximum resolution (in bits) for a given device
oscillator and PWM frequency can be determined using
Equation 14-3:
EQUATION 14-3: 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 =
TCY • 2 • (PTPER + 1) • PTMR Prescale Value
Resolution =
log (2 • TPWM/TCY)
log (2)