ATmega48/88/168
Figure 59. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCnx
OCnx
1
2
3
4
5
6
7
Period
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches TOP. If
the interrupt is enabled, the interrupt handler routine can be used for updating the com-
pare value.
In fast PWM mode, the compare unit allows generation of PWM 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 MGM2:0 = 7. (See Table 61 on
page 143). 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 waveform 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:
f
clk_I/O
f
= -----------------
OCnxPWM
N ⋅ 256
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 in a constantly high or low output (depending on the polarity of
the output set by the COM2A1:0 bits.)
A frequency (with 50ꢀ duty cycle) waveform output in fast PWM mode can be achieved
by setting OC2x to toggle its logical level on each compare match (COM2x1:0 = 1). The
waveform generated will have a maximum frequency of foc2 = fclk_I/O/2 when OCR2A is
set to zero. This feature is similar to the OC2A toggle in CTC mode, except the double
buffer feature of the Output Compare unit is enabled in the fast PWM mode.
139
2545D–AVR–07/04
ATMEL [ ATMEL ]
