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ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 17-4 shows a block diagram of the Output Compare unit. The small “n” in the register and bit names indi-
cates the device number (n = 1 for Timer/Counter 1), and the “x” indicates Output Compare unit (A/B). The
elements of the block diagram that are not directly a part of the Output Compare unit are gray shaded.
Figure 17-4. Output Compare Unit, Block Diagram
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 OCR1x 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 OCR1x Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCR1x Buffer Register, and if double buffering is disabled the CPU will access the OCR1x
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 OCR1x Registers must be done via the TEMP Register since the
compare of all 16-bit is done continuously. The High byte (OCR1xH) 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
(OCR1xL) is written to the lower eight bits, the High byte will be copied into the upper 8-bits of either the OCR1x
buffer or OCR1x Compare Register in the same system clock cycle.
17.7.1
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 (FOC1x) bit. Forcing Compare Match will not set the OCF1x Flag or reload/clear the
timer, but the OC1x pin will be updated as if a real Compare Match had occurred (the COM1x1:0 bits settings
define whether the OC1x pin is set, cleared or toggled).
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