121
2467S–AVR–07/09
ATmega128
Output Compare
Units
The 16-bit comparator continuously compares TCNTn with the
Output Compare Register
(OCRnx). If TCNT equals OCRnx the comparator signals a match. A match will set the
Output
Compare Flag
(OCFnx) at the next timer clock cycle. If enabled (OCIEnx = 1), the output com-
pare flag generates an output compare interrupt. The OCFnx flag is automatically cleared when
the interrupt is executed. Alternatively the OCFnx flag can be cleared by software by writing a
logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an
output according to operating mode set by the
Waveform Generation mode
(WGMn3:0) bits and
Compare Output mode
(COMnx1:0) bits. The TOP and BOTTOM signals are used by the wave-
form generator for handling the special cases of the extreme values in some modes of operation
(
See “Modes of Operation” on page 124.
A special feature of output compare unit A allows it to define the Timer/Counter TOP value (i.e.,
counter resolution). In addition to the counter resolution, the TOP value defines the period time
for waveforms generated by the waveform generator.
shows a block diagram of the output compare unit. The small “n” in the register and bit
names indicates the device number (n = n
for Timer/Counter n), and the “x” indicates output
compare unit (A/B/C). The elements of the block diagram that are not directly a part of the output
compare unit are gray shaded.
Figure 49.
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 dou-
ble 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.
OCFnx
(Int.Req.)
=
(16-bit Comparator )
OCRnx
Buffer (16-bit Register)
OCRnxH
Buf. (8-bit)
OCnx
TEMP
(8-bit)
DATABUS
(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
