PIC16F87T-E/ML【PDF 8/200 页】IC MCU FLASH 4KX14 EEPROM 28QFN

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105

8008H–AVR–04/11

ATtiny48/88

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

Note that when using fixed TOP values, the unused bits are masked to zero when any of the

OCR1x Registers are written. As the third period shown in Figure 12-8 illustrates, changing the

TOP actively while the Timer/Counter is running in the phase correct mode can result in an

unsymmetrical output. The reason for this can be found in the time of update of the OCR1x Reg-

ister. Since the OCR1x update occurs at TOP, the PWM period starts and ends at TOP. This

implies that the length of the falling slope is determined by the previous TOP value, while the

length of the rising slope is determined by the new TOP value. When these two values differ the

two slopes of the period will differ in length. The difference in length gives the unsymmetrical

result on the output.

It is recommended to use the phase and frequency correct mode instead of the phase correct

mode when changing the TOP value while the Timer/Counter is running. When using a static

TOP value there are practically no differences between the two modes of operation.

In phase correct 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 output can be generated by setting the COM1x[1:0] to three (See Table on page 111).

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

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 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 represent 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 continuously high for

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

OCR1A is used to define the TOP value (WGM1[3:0] = 11) and COM1A[1:0] = 1, the OC1A out-

put will toggle with a 50% duty cycle.

12.9.5

Phase and Frequency Correct PWM Mode

The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM

mode (WGM1[3:0] = 8 or 9) provides a high resolution phase and frequency correct PWM wave-

form generation option. The phase and frequency correct PWM mode is, like the phase 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 Compare Output

mode, the operation is inverted. The dual-slope operation gives a lower maximum operation fre-

quency compared to the single-slope operation. However, due to the symmetric feature of the

dual-slope PWM modes, these modes are preferred for motor control applications.

fOCnxPCPWM

clk_I/O

NTOP

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