ATtiny167-ESSZ datasheet(126/282 Pages) ATMEL

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126

7728D–AVR–07/09

ATtiny87/ATtiny167

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. How-

ever, 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 OC1A/B

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 COM1A/B1:0 to three (see Table 12-2 on page

133). The actual OC1A/B value will only be visible on the port pin if the data direction for the

port pin is set as output (DDR_OC1A/B) and OC1A/Bi is set. The PWM waveform is generated

by setting (or clearing) the OC1A/B Register at the compare match between OCR1A/B and

TCNT1, and clearing (or setting) the OC1A/B 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 OCR1A/B Register represents special cases when generating a

PWM waveform output in the fast PWM mode. If the OCR1A/B is set equal to BOTTOM

(0x0000) the output will be a narrow spike for each TOP+1 timer clock cycle. Setting the

OCR1A/B equal to TOP will result in a constant high or low output (depending on the polarity

of the output set by the COM1A/B1:0 bits.)

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

setting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). The wave-

form generated will have a maximum frequency of fOC1A = 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 buf-

fer feature of the Output Compare 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 (WGM13:0 = 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

(OC1A/B) is cleared on the compare match between TCNT1 and OCR1A/B while upcounting,

and set on the compare match while downcounting. In inverting Output Compare mode, the

operation is inverted. The dual-slope operation has lower maximum operation frequency than

single slope operation. However, due to the symmetric feature of the dual-slope PWM modes,

these modes are preferred for motor control applications.

The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or

defined by either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A

set to 0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM

resolution in bits can be calculated by using the following equation:

fOCnxPWM

clk_I/O

TOP

()

⋅

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

RPCPWM

TOP 1

()

log

()

log

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

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