PIC16F87T-I/ML【PDF 6/200 页】IC MCU FLASH 4KX14 EEPROM 28QFN

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103

8008H–AVR–04/11

ATtiny48/88

The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP

value. The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low

value when the counter is running with none or a low prescaler value, there is a risk that the new

ICR1 value written is lower than the current value of TCNT1. The result will then be that the

counter will miss the compare match at the TOP value. The counter will then have to count to the

MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.

The OCR1A Register however, is double buffered. This feature allows the OCR1A I/O location

to be written anytime. When the OCR1A I/O location is written the value written will be put into

the OCR1A Buffer Register. The OCR1A Compare Register will then be updated with the value

in the Buffer Register at the next timer clock cycle the TCNT1 matches TOP. The update is done

at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is set.

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 fast PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins.

Setting the COM1x[1:0] bits to two will produce a non-inverted PWM and an inverted PWM out-

put can be generated by setting the COM1x[1:0] to three (see Table on page 110). 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, and clearing (or setting) the OC1x Register at

the timer clock cycle the counter is cleared (changes from TOP to BOTTOM).

The PWM frequency for the output 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 fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the out-

put will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP

will result in a constant high or low output (depending on the polarity of the output set by the

COM1x[1:0] bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC1A to toggle its logical level on each compare match (COM1A[1:0] = 1). This applies only

if OCR1A is used to define the TOP value (WGM1[3:0] = 15). The waveform generated will have

a maximum frequency of f

OC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is

similar to the OC1A toggle in CTC mode, except the double buffer feature of the Output Com-

pare unit is enabled in the fast PWM mode.

12.9.4

Phase Correct PWM Mode

The phase correct Pulse Width Modulation or phase correct PWM mode (WGM1[3:0] = 1, 2, 3,

10, or 11) provides a high resolution phase correct PWM waveform generation option. The

phase correct PWM mode is, like the phase and frequency correct PWM mode, based on a dual-

slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from

TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is

cleared on the compare match between TCNT1 and OCR1x while upcounting, and set on the

compare match while downcounting. In inverting Output Compare mode, the operation is

fOCnxPWM

clk_I/O

TOP

()

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