125
2467S–AVR–07/09
ATmega128
An interrupt can be generated at each time the counter value reaches the TOP value by either
using the OCFnA or ICFn flag according to the register used to define the TOP value. If the inter-
rupt is enabled, the interrupt handler routine can be used for updating the TOP value. However,
changing the TOP to a value close to BOTTOM when the counter is running with none or a low
prescaler value must be done with care since the CTC mode does not have the double buffering
feature. If the new value written to OCRnA or ICRn is lower than the current value of TCNTn, the
counter will miss the compare match. The counter will then have to count to its maximum value
(0xFFFF) and wrap around starting at 0x0000 before the compare match can occur. In many
cases this feature is not desirable. An alternative will then be to use the fast PWM mode using
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 toggle 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 fre-
quency of f
OC
n
A
= f
clk_I/O
/2 when OCRnA is set to zero (0x0000). The 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.
Fast PWM Mode
The
fast Pulse Width Modulation
or fast PWM mode (WGMn3:0 = 5,6,7,14, or 15) provides 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 setat 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 cor-
rect 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, capaci-
tors), 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 max-
imum 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
. 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 illustrating 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 compare match occurs.
f
OCnA
f
clk_I/O
2
N
1
OCRnA
+
(
)
⋅ ⋅
---------------------------------------------------
=
R
FPWM
TOP
1
+
(
)
log
2
( )
log
-----------------------------------
=
