Datasheet

Figure 19-4. Output Compare Unit, Block Diagram
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)
COMnx1:0WGMn3:0
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM
The OCR1x 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-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.
19.7.1. 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
COMnx1:0 bits settings define whether the OCnx pin is set, cleared or toggled).
Atmel ATmega32A [DATASHEET]
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