Datasheet

Figure 20-7 Fast PWM Mode, Timing Diagram
TCNTn
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 7
Period
2 3 4 5 6 8
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In addition the OCnA
or ICFn Flag is set at the same timer clock cycle as TOVn is set when either OCRnA or ICRn is used for
defining the TOP value. If one of the interrupts are enabled, the interrupt handler routine can be used for
updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
Compare Match will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP
values the unused bits are masked to zero when any of the OCRnx Registers are written.
The procedure for updating ICRn differs from updating OCRnA when used for defining the TOP value.
The ICRn Register is not double buffered. This means that if ICRn is changed to a low value when the
counter is running with none or a low prescaler value, there is a risk that the new ICRn value written is
lower than the current value of TCNTn. The result will then be that the counter will miss the Compare
Match at the TOP value. The counter will then have to count to the MAX value (0xFFFF) and wrap around
starting at 0x0000 before the Compare Match can occur. The OCRnA Register, however, is double
buffered. This feature allows the OCRnA I/O location to be written anytime. When the OCRnA I/O location
is written the value written will be put into the OCRnA Buffer Register. The OCRnA Compare Register will
then be updated with the value in the Buffer Register at the next timer clock cycle the TCNTn matches
TOP. The update is done at the same timer clock cycle as the TCNTn is cleared and the TOVn Flag is
set.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the
OCRnA Register is free to be used for generating a PWM output on OCnA. However, if the base PWM
frequency is actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better
choice due to its double buffer feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the
COMnx1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COMnx1:0 to 3. Refer to Table 20-3 Compare Output Mode, Fast PWM on page 160. The
actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the
Compare Match between OCRnx and TCNTn, and clearing (or setting) the OCnx 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:

OCnxPWM
=

clk_I/O
 
1 + TOP
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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