ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 119

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Output Compare Units
2490G–AVR–03/04
The 16-bit comparator continuously compares TCNTn with the Output Compare Regis-
ter (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 Compare 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 Gen-
erator 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 Waveform Generator
for handling the special cases of the extreme values in some modes of operation (See
“Modes of Operation” on page 122.)
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.
Figure 49 shows a block diagram of the Output Compare unit. The small “n” in the regis-
ter 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 Mod-
ulation (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
OCRnxH Buf. (8-bit)
OCRnxH (8-bit)
BOTTOM
OCRnx Buffer (16-bit Register)
TEMP (8-bit)
TOP
OCRnx (16-bit Register)
OCRnxL Buf. (8-bit)
OCRnxL (8-bit)
DATA BUS
Waveform Generator
WGMn3:0
=
(16-bit Comparator )
(8-bit)
COMnx1:0
TCNTnH (8-bit)
OCFnx (Int.Req.)
TCNTn (16-bit Counter)
ATmega64(L)
TCNTnL (8-bit)
OCnx
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