ATtiny167-ESSZ datasheet(124/282 Pages) ATMEL

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124

7728D–AVR–07/09

ATtiny87/ATtiny167

An interrupt can be generated at each time the counter value reaches the TOP value by either

using the OCF1A or ICF1 flag according to the register used to define the TOP value. If the

interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.

However, changing the TOP to a value close to BOTTOM when the counter is running with

none or a low prescaler value must be done with care since the CTC mode does not have the

double buffering feature. If the new value written to OCR1A or ICR1 is lower than the current

value of TCNT1, the counter will miss the compare match. The counter will then have to count

to its maximum value (0xFFFF) and wrap around starting at 0x0000 before the compare match

can occur. In many cases this feature is not desirable. An alternative will then be to use the

fast PWM mode using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will

be double buffered.

For generating a waveform output in CTC mode, the OC1A output can be set to toggle its log-

ical level on each compare match by setting the Compare Output mode bits to toggle mode

(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction

for the pin is set to output (DDR_OC1A = 1) and OC1Ai is set. The waveform generated will

have a maximum frequency of f

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

waveform frequency is defined by the following equation:

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

As for the Normal mode of operation, the TOV1 flag is set in the same timer clock cycle that

the counter counts from MAX to 0x0000.

12.9.3

Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 5, 6, 7, 14, or 15) provides a

high frequency PWM waveform generation option. The fast PWM differs from the other PWM

options by its single-slope operation. The counter counts from BOTTOM to TOP then restarts

from BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1A/B) is set

on the compare match between TCNT1 and OCR1A/B, and cleared at TOP. In inverting Com-

pare Output mode output is cleared on compare match and set at TOP. Due to the

single-slope operation, the operating frequency of the fast PWM mode can be twice as high as

the phase correct and phase and frequency correct PWM modes that use dual-slope opera-

tion. This high frequency makes the fast PWM mode well suited for power regulation,

rectification, and DAC applications. High frequency allows physically small sized external

components (coils, capacitors), hence reduces total system cost.

The PWM resolution for fast PWM 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:

fOCnA

clk_I/O

OCRnA

()

⋅⋅

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

RFPWM

TOP 1

()

log

()

log

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

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