106
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
(COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform Generator for handling the special
A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (that is, counter reso-
lution). In addition to the counter resolution, the TOP value defines the period time for waveforms generated by the
Waveform Generator.
Figure 15-4 shows a block diagram of the Output Compare unit. The small “n” in the register and bit names indi-
cates the device number (n = n for Timer/Counter n), and the “x” indicates Output Compare unit (x). The elements
of the block diagram that are not directly a part of the Output Compare unit are gray shaded.
Figure 15-4. 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 TCNT1 and ICR1 Register). Therefore OCR1x 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 8-bits of either the OCRnx
buffer or OCRnx Compare Register in the same system clock cycle.
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