In fast PWM mode, the counter is incremented until the counter value matches the MAX value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 13-6. The TCNT0 value is in the timing diagram shown as a his-
togram for illustrating the single-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare
matches between OCR0A and TCNT0.
Figure 13-6. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCn
OCn
1
2
3
4
5
6
7
Period
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches MAX. 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 PWM waveforms on the OC0A pin.
Setting the COM0A1:0 bits to two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM0A1:0 to three (See Table 13-4 on page 103). The actual
OC0A value will only be visible on the port pin if the data direction for the port pin is set as out-
put. The PWM waveform is generated by setting (or clearing) the OC0A Register at the compare
match between OCR0A and TCNT0, and clearing (or setting) the OC0A Register at the timer
clock cycle the counter is cleared (changes from MAX 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, 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 COM0A1:0
bits.)
A frequency (with 50ꢀ duty cycle) waveform output in fast PWM mode can be achieved by set-
ting OC0A to toggle its logical level on each compare match (COM0A1:0 = 1). The waveform
generated will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero. This
98
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
