
68
ATtiny40 [DATASHEET]
8263B–AVR–01/2013
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 OCR0x Register represents special cases when generating a PWM waveform output in
the fast PWM mode. If OCR0x is set equal to BOTTOM, the output will be a narrow spike for each TOP+1 timer
clock cycle. Setting the OCR0x equal to TOP will result in a constantly high or low output (depending on the polarity
of the output set by the COM0x[1:0] bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0A to toggle
its logical level on each Compare Match (COM0A[1:0] = 1). The waveform generated will have a maximum fre-
quency of f
clk_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.
Figure 11-7. Phase Correct PWM Mode, Timing Diagram
fOCnxPWM
f
clk_I/O
NTOP 1
+
-----------------------------------
=
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1
2
3
TCNTn
Period
OCnx
OCxn
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update