Datasheet

will access the OCRnx directly. The content of the OCR1x (Buffer or Compare) Register is only changed
by a write operation (the Timer/Counter does not update this register automatically as the TCNTn and
ICRn Register). Therefore OCRnx is not read via the High byte temporary register (TEMP). However, it is
a good practice to read the Low byte first as when accessing other 16-bit registers. Writing the OCRnx
Registers must be done via the TEMP Register since the compare of all 16-bit is done continuously. The
High byte (OCRnxH) has to be written first. When the High byte I/O location is written by the CPU, the
TEMP Register will be updated by the value written. Then when the Low byte (OCRnxL) is written to the
lower eight bits, the High byte will be copied into the upper 8-bits of either the OCRnx buffer or OCRnx
Compare Register in the same system clock cycle.
For more information of how to access the 16-bit registers refer to Accessing 16-bit Registers on page
140.
20.7.1. Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (FOCnx) bit. Forcing Compare Match will not set the OCFnx Flag or
reload/clear the timer, but the OCnx pin will be updated as if a real Compare Match had occurred (the
COMn1:0 bits settings define whether the OCnx pin is set, cleared or toggled).
20.7.2. Compare Match Blocking by TCNTn Write
All CPU writes to the TCNTn Register will block any Compare Match that occurs in the next timer clock
cycle, even when the timer is stopped. This feature allows OCRnx to be initialized to the same value as
TCNTn without triggering an interrupt when the Timer/Counter clock is enabled.
20.7.3. Using the Output Compare Unit
Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNTn when using any of the Output Compare channels,
independent of whether the Timer/Counter is running or not. If the value written to TCNTn equals the
OCRnx value, the Compare Match will be missed, resulting in incorrect waveform generation. Do not
write the TCNTn equal to TOP in PWM modes with variable TOP values. The Compare Match for the
TOP will be ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNTn value
equal to BOTTOM when the counter is downcounting.
The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe
bits in Normal mode. The OCnx Register keeps its value even when changing between Waveform
Generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the
COMnx1:0 bits will take effect immediately.
20.8. Compare Match Output Unit
The Compare Output mode (COMnx1:0) bits have two functions. The waveform generator uses the
COMnx1:0 bits for defining the Output Compare (OCnx) state at the next Compare Match. Secondly the
COMnx1:0 bits control the OCnx pin output source. The figure below shows a simplified schematic of the
logic affected by the COMnx1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are
shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected
by the COMnx1:0 bits are shown. When referring to the OCnx state, the reference is for the internal OCnx
Register, not the OCnx pin. If a System Reset occur, the OCnx Register is reset to “0”.
Atmel ATmega64A [DATASHEET]
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