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参数资料
型号： ST72F324K2B6
厂商： STMICROELECTRONICS
元件分类： 微控制器/微处理器
英文描述： 8-BIT, FLASH, 8 MHz, MICROCONTROLLER, PDIP32
封装： 0.400 INCH, PLASTIC, SDIP-32
文件页数： 123/156页
文件大小： 1027K
代理商： ST72F324K2B6
ST72324J/K
69/156
16-BIT TIMER (Cont’d)
10.3.3.6 Pulse Width Modulation Mode
Pulse Width Modulation (PWM) mode enables the
generation of a signal with a frequency and pulse
length determined by the value of the OC1R and
OC2R registers.
Pulse Width Modulation mode uses the complete
Output Compare 1 function plus the OC2R regis-
ter, and so this functionality can not be used when
PWM mode is activated.
In PWM mode, double buffering is implemented on
the output compare registers. Any new values writ-
ten in the OC1R and OC2R registers are taken
into account only at the end of the PWM period
(OC2) to avoid spikes on the PWM output pin
(OCMP1).
Procedure
To use pulse width modulation mode:
1. Load the OC2R register with the value corre-
sponding to the period of the signal using the
formula in the opposite column.
2. Load the OC1R register with the value corre-
sponding to the period of the pulse if (OLVL1=0
and OLVL2=1) using the formula in the oppo-
site column.
3. Select the following in the CR1 register:
– Using the OLVL1 bit, select the level to be ap-
plied to the OCMP1 pin after a successful
comparison with the OC1R register.
– Using the OLVL2 bit, select the level to be ap-
plied to the OCMP1 pin after a successful
comparison with the OC2R register.
4. Select the following in the CR2 register:
– Set OC1E bit: the OCMP1 pin is then dedicat-
ed to the output compare 1 function.
– Set the PWM bit.
– Select the timer clock (CC[1:0]) (see Table 16
Clock Control Bits).
If OLVL1=1 and OLVL2=0 the length of the posi-
tive pulse is the difference between the OC2R and
OC1R registers.
If OLVL1=OLVL2 a continuous signal will be seen
on the OCMP1 pin.
The OCiR register value required for a specific tim-
ing application can be calculated using the follow-
ing formula:
Where:
t
= Signal or pulse period (in seconds)
fCPU
= CPU clock frequency (in hertz)
PRESC = Timer prescaler factor (2, 4 or 8 depend-
ing on CC[1:0] bits, see Table 16 Clock
Control Bits)
If the timer clock is an external clock the formula is:
Where:
t
= Signal or pulse period (in seconds)
fEXT
= External timer clock frequency (in hertz)
The Output Compare 2 event causes the counter
to be initialized to FFFCh (See Figure 46)
Notes:
1. After a write instruction to the OC
iHR register,
the output compare function is inhibited until the
OC
iLR register is also written.
2. The OCF1 and OCF2 bits cannot be set by
hardware in PWM mode therefore the Output
Compare interrupt is inhibited.
3. The ICF1 bit is set by hardware when the coun-
ter reaches the OC2R value and can produce a
timer interrupt if the ICIE bit is set and the I bit is
cleared.
4. In PWM mode the ICAP1 pin can not be used
to perform input capture because it is discon-
nected to the timer. The ICAP2 pin can be used
to perform input capture (ICF2 can be set and
IC2R can be loaded) but the user must take
care that the counter is reset each period and
ICF1 can also generates interrupt if ICIE is set.
5. When the Pulse Width Modulation (PWM) and
One Pulse Mode (OPM) bits are both set, the
PWM mode is the only active one.
Counter
OCMP1 = OLVL2
Counter
= OC2R
OCMP1 = OLVL1
When
= OC1R
Pulse Width Modulation cycle
Counter is reset
to FFFCh
ICF1 bit is set
OC
iR Value =
t * fCPU
PRESC
-5
OC
iR=
t * fEXT -5
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