Figure 13-7. Fast PWM Mode, Timing Diagram
OCRnx/ TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
TCNTn
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
1
2
3
4
5
6
7
8
Period
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
two will produce a non-inverted PWM and an inverted PWM output can be generated by setting the COMnx1:0 to three (see
Table on page 110). 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:
f
clk_I/O
----------------------------------
f
=
OCnxPWM
N 1 + TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform output in the fast
PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow spike for each TOP+1 timer clock
cycle. Setting the OCRnx equal to TOP will result in a constant high or low output (depending on the polarity of the output set
by the COMnx1:0 bits.)
104
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
