117
ATmega64A [DATASHEET]
8160D–AVR–02/2013
tion Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx) must be set as
output before the OCnx value is visible on the pin. The port override function is generally independent of the Wave-
The design of the Output Compare pin logic allows initialization of the OCnx state before the output is enabled.
The COMnx1:0 bits have no effect on the Input Capture unit.
16.8.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COMnx1:0 bits differently in Normal, CTC, and PWM modes. For all modes,
setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the OCnx Register is to be performed on
A change of the COMnx1:0 bits state will have effect at the first Compare Match after the bits are written. For non-
PWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits.
16.9
Modes of Operation
The mode of operation, that is, the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generation mode (WGMn3:0) and Compare Output mode (COMnx1:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COMnx1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted
PWM). For non-PWM modes the COMnx1:0 bits control whether the output should be set, cleared or toggle at a
16.9.1
Normal Mode
The simplest mode of operation is the Normal mode (WGMn3:0 = 0). In this mode the counting direction is always
up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum
16-bit value (MAX = 0xFFFF) and then restarts from the BOTTOM (0x0000). In normal operation the Timer/Coun-
ter Overflow Flag (TOVn) will be set in the same timer clock cycle as the TCNTn becomes zero. The TOVn flag in
this case behaves like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow
interrupt that automatically clears the TOVn flag, the timer resolution can be increased by software. There are no
special cases to consider in the Normal mode, a new counter value can be written anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the maximum interval between the
external events must not exceed the resolution of the counter. If the interval between events are too long, the timer
overflow interrupt or the prescaler must be used to extend the resolution for the capture unit.
The Output Compare units can be used to generate interrupts at some given time. Using the Output Compare to
generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.
16.9.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register are used to manipu-
late the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNTn) matches
either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn define the top value for the
counter, hence also its resolution. This mode allows greater control of the Compare Match output frequency. It also
simplifies the operation of counting external events.