74
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
form is generated by setting (or clearing) the OC0x Register at the Compare Match between OCR0x and TCNT0,
and clearing (or setting) the OC0x 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 prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represents special cases when generating a PWM waveform output in
the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each MAX+1
timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output (depending on the
polarity of the output set by the COM0A[1:0] bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to toggle
its logical level on each Compare Match (COM0x[1:0] = 1). The waveform generated will have a maximum fre-
quency of f
OC0 = fclk_I/O/2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
11.7.4
Phase Correct PWM Mode
The phase correct PWM mode (WGM0[2:0] = 1 or 5) provides a high resolution phase correct PWM waveform gen-
eration option. The phase correct PWM mode is based on a dual-slope operation. The counter counts repeatedly
from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM0[2:0] = 1, and
OCR0A when WGM0[2:0] = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on
the Compare Match between TCNT0 and OCR0x while upcounting, and set on the Compare Match while down-
counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maxi-
mum 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.
In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the counter
reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer clock cycle. The
timing diagram for the phase correct PWM mode is shown on
Figure 11-9. The TCNT0 value is in the timing dia-
gram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare Matches
between OCR0x and TCNT0.
f
OCnxPWM
f
clk_I/O
N 256
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