Rainbow Electronics ATmega128L - Manual (Page 119)

119
ATmega128(L)
2467B–09/01
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 com-
pare 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 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
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) 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 set on the compare match between TCNTn and OCRnx, and cleared
at TOP. In inverting compare output mode output is cleared on compare match and set
at TOP. 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 51. 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.
f
OC nA
f
clk_I/O
2 N 1 OCRnA + ( ) ⋅ ⋅
--------------------------------------------------- =
R
FPWM
TOP 1 +
( ) log
2 ( ) log
----------------------------------- =