ATA8742-PXQW【PDF 126/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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Sheet目录52

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126

9151AINDCO07/09

ATA8742

Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is actively changed by changing the TOP value, using the OCR1A as

TOP is clearly a better choice due to its double buffer feature.

In phase and frequency correct PWM mode, the compare units allow generation of PWM wave-

forms on the OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and

an inverted PWM output can be generated by setting the COM1x1:0 to three (See Table 21-3 on

page 130). The actual OC1x value will only be visible on the port pin if the data direction for the

port pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing)

the OC1x Register at the compare match between OCR1x and TCNT1 when the counter incre-

ments, and clearing (or setting) the OC1x Register at compare match between OCR1x and

TCNT1 when the counter decrements. The PWM frequency for the output when using phase

and frequency correct PWM can be calculated by the following equation:

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the

output will be continuously low and if set equal to TOP the output will be set to high for

non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.

21.10 Timer/Counter Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clk

) is therefore shown as a

clock enable signal in the following figures. The figures include information on when interrupt

flags are set, and when the OCR1x Register is updated with the OCR1x buffer value (only for

modes utilizing double buffering). Figure 21-10 on page 126 shows a timing diagram for the set-

ting of OCF1x.

Figure 21-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling

Figure 21-11 on page 127 shows the same timing data, but with the prescaler enabled.

OCnxPFCPWM

clk_I/O

2 N TOP

?nbsp ?/DIV>

---------------------------------

clk

I/O

/1)

CFnx

clk

I/O

CRnx

CNTn

OCRnx Value

OCRnx - 1

OCRnx

OCRnx + 1

OCRnx + 2

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