Datasheet

dsPIC30F3010/3011
DS70141F-page 100 © 2010 Microchip Technology Inc.
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
functions 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 the PTCON
register 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
instances:
• 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 be determined using
Equation 15-1:
EQUATION 15-1: PWM PERIOD
(FREE-RUNNING MODE)
If the PWM time base is configured for one of the
Continuous Up/Down Count modes, the PWM period is
given by Equation 15-2.
EQUATION 15-2: PWM PERIOD (UP/DOWN
COUNTING 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 =
T
CY• 2 • (PTPER + 1) • PTMR Prescale Value
Resolution =
log (2
• TPWM / TCY)
log (2)