Datasheet
178
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
the pin is set to output. The waveform generated will have a maximum frequency of f
OC2A
=
f
clk_I/O
/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by the following
equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the
TOV2 Flag is set in the same timer clock cycle that the
counter counts from MAX to 0x00.
20.4.3 Fast PWM Mode
Figure 20-4. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches TOP. If the inter-
rupt is enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PW M waveforms on the OC2x pin.
Setting the COM2x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM2x1:0 to three. TOP is defined as 0xFF when WGM2:0 = 3,
and OCR2A when WGM2:0 = 7 (see Table 20-3 on page 187). The actual OC2x value will only
be visible on the port pin if the data direction for the port pin is set as output. The PWM wave-
form is generated by setting (or clearing) the OC2x Register at the compare match between
OCR2x and TCNT2, and clearing (or setting) the OC2x 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, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will
be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result
f
OCnx
f
clk_I/O
2 N 1 OCRnx+
--------------------------------------------------=
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period
2 3
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Interrupt Flag Set
4 5 6 7
f
OCnxPWM
f
clk_I/O
N 256
------------------=