參數(shù)資料
型號(hào): GPTRM
英文描述: General-Purpose Timer Reference Manual
中文描述: 通用定時(shí)器參考手冊(cè)
文件頁數(shù): 41/90頁
文件大?。?/td> 647K
代理商: GPTRM
GPT
REFERENCE MANUAL
PULSE-WIDTH MODULATION UNIT
MOTOROLA
6-1
SECTION 6 PULSE-WIDTH MODULATION UNIT
The pulse width modulation (PWM) unit has two PWM outputs, PWMA and PWMB
which are controlled by the same clock output of the prescaler multiplexer (mux). This
clock is the input to a 16-bit counter which is used by both of the PWM channels. The
output of the counter is multiplexed to provide two operational modes: fast mode or
slow mode. This gives a clocking rate 1/256 (fast mode) or 1/32768 (slow mode) of the
output of the prescaler mux. The duty cycle ratios of the two PWM channels are indi-
vidually controlled by software. The PWM pins can be used as output pins if not used
for PWM functions. The PWMA pin may also be used to output the clock used to drive
the PWM counter.
Figure 6-1
is a block diagram of the pulse-width modulation unit.
6.1 Counter
The 16-bit counter in the PWM unit is similar to the timer counter in the compare/cap-
ture unit. After reset, the GPT is configured to use the system clock divided by two as
the input to the free-running counter. Initialization software may reconfigure the
counter to use one of seven prescaler outputs or an external clock from the PLCK input
pin. Refer to
5.1 Prescaler
for additional information on the prescaler.
Software can read the counter at any time without affecting its value. A read of the
PWM count register (PWMCNT) must be a word access to ensure coherency, but byte
accesses can be made if coherency is not needed. The counter is cleared to $0000
during reset and is a read-only register except in freeze or test mode. Refer to
SEC-
TION 9 SPECIAL MODES
. Any value may be written to the counter when in either of
these modes. Note, however, that any writes to PWNCNT which result in changing the
most significant bit of the eight bits used for the PWM frequency from a logic level one
to a logic level zero, will be seen as a zero detect by the PWM logic.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.
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