ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 151

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Fast PWM Mode
2549K–AVR–01/07
OCRnA for defining TOP (WGMn3:0 = 15) since the OCRnA then will be double
buffered.
For generating a waveform output in CTC mode, the OCnA output can be set to toggle
its logical level on each compare match by setting the Compare Output mode bits to tog-
gle mode (COMnA1:0 = 1). The OCnA value will not be visible on the port pin unless the
data direction for the pin is set to output (DDR_OCnA = 1). The waveform generated will
have a maximum frequency of f
waveform frequency is defined by the following equation:
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOVn Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) pro-
vides a high frequency PWM waveform generation option. The fast PWM differs from
the other PWM options by its single-slope operation. The counter counts from BOTTOM
to TOP then restarts from BOTTOM. In non-inverting Compare Output mode, the Output
Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx, and
set at BOTTOM. In inverting Compare Output mode output is set on compare match and
cleared at BOTTOM. Due to the single-slope operation, the operating frequency of the
fast PWM mode can be twice as high as the phase correct and phase and frequency
correct PWM modes that use dual-slope operation. This high frequency makes the fast
PWM mode well suited for power regulation, rectification, and DAC applications. High
frequency allows physically small sized external components (coils, capacitors), hence
reduces total system cost.
The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either
ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to
0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM
resolution in bits can be calculated by using the following equation:
In fast PWM mode the counter is incremented until the counter value matches either
one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in
ICRn (WGMn3:0 = 14), or the value in OCRnA (WGMn3:0 = 15). The counter is then
cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
shown in Figure 54. The figure shows fast PWM mode when OCRnA or ICRn is used to
define TOP. The TCNTn value is in the timing diagram shown as a histogram for illus-
trating the single-slope operation. The diagram includes non-inverted and inverted PWM
outputs. The small horizontal line marks on the TCNTn slopes represent compare
matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a com-
pare match occurs.
ATmega640/1280/1281/2560/2561
f
OCnA
OC
R
FPWM
n
A
= f
=
clk_I/O
-------------------------------------------------- -
2 N
=
⋅
log
---------------------------------- -
/2 when OCRnA is set to zero (0x0000). The
⋅
(
f
(
1
clk_I/O
log
TOP
+
2 ( )
OCRnA
+
1
)
)
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