Auto Page CPX-3600 Especificaciones Pagina 88

  • Descarga
  • Añadir a mis manuales
  • Imprimir
  • Pagina
    / 243
  • Tabla de contenidos
  • MARCADORES
  • Valorado. / 5. Basado en revisión del cliente
Vista de pagina 87
PWM auto-reload timer (ART) ST72321xx-Auto
88/243 Doc ID 13829 Rev 1
12.2 Functional description
12.2.1 Counter
The free running 8-bit counter is fed by the output of the prescaler, and is incremented on
every rising edge of the clock signal.
It is possible to read or write the contents of the counter on the fly by reading or writing the
Counter Access register (ARTCAR).
When a counter overflow occurs, the counter is automatically reloaded with the contents of
the ARTARR register (the prescaler is not affected).
12.2.2 Counter clock and prescaler
The counter clock frequency is given by:
f
COUNTER
= f
INPUT
/ 2
CC[2:0]
The timer counter’s input clock (f
INPUT
) feeds the 7-bit programmable prescaler, which
selects one of the 8 available taps of the prescaler, as defined by CC[2:0] bits in the
Control/Status Register (ARTCSR). Thus the division factor of the prescaler can be set to 2
n
(where n = 0, 1,..7).
This f
INPUT
frequency source is selected through the EXCL bit of the ARTCSR register and
can be either the f
CPU
or an external input frequency f
EXT
.
The clock input to the counter is enabled by the TCE (Timer Counter Enable) bit in the
ARTCSR register. When TCE is reset, the counter is stopped and the prescaler and counter
contents are frozen. When TCE is set, the counter runs at the rate of the selected clock
source.
12.2.3 Counter and prescaler initialization
After RESET, the counter and the prescaler are cleared and f
INPUT
= f
CPU
.
The counter can be initialized by:
writing to the ARTARR register and then setting the FCRL (Force Counter Re-Load)
and the TCE (Timer Counter Enable) bits in the ARTCSR register
writing to the ARTCAR counter access register
In both cases the 7-bit prescaler is also cleared, whereupon counting will start from a known
value.
Direct access to the prescaler is not possible.
12.2.4 Output compare control
The timer compare function is based on four different comparisons with the counter (one for
each PWMx output). Each comparison is made between the counter value and an output
compare register (OCRx) value. This OCRx register can not be accessed directly, it is
loaded from the duty cycle register (PWMDCRx) at each overflow of the counter.
This double buffering method avoids glitch generation when changing the duty cycle on the
fly.
Vista de pagina 87
1 2 ... 83 84 85 86 87 88 89 90 91 92 93 ... 242 243

Comentarios a estos manuales

Sin comentarios