
115
ATmega64A [DATASHEET]
8160D–AVR–02/2013
Figure 16-4. Output Compare Unit, Block Diagram
The OCRnx Register is double buffered when using any of the twelve Pulse Width Modulation (PWM) modes. For
the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double
buffering synchronizes the update of the OCRnx Compare Register to either TOP or BOTTOM of the counting
sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby
making the output glitch-free.
The OCRnx Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCRnx Buffer Register, and if double buffering is disabled the CPU 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 bits 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 eight bits of either the OCRnx
Buffer or OCRnx Compare Register in the same system clock cycle.
16.7.1
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).
16.7.2
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 trigger-
ing an interrupt when the Timer/Counter clock is enabled.
OCFnx (Int.Req.)
= (16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf. (8-bit)
OCnx
TEMP (8-bit)
DATA BUS (8-bit)
OCRnxL Buf. (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit)
TCNTnL (8-bit)
COMnx1:0
WGMn3:0
OCRnx (16-bit Register)
OCRnxH (8-bit)
OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM