Datasheet

117
2545T–AVR–05/11
ATmega48/88/168
pare Flag generates an Output Compare interrupt. The OCF1x Flag is automatically cleared
when the interrupt is executed. Alternatively the OCF1x Flag can be cleared by software by writ-
ing 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
(WGM13:0) bits and Compare Output mode (COM1x1:0) 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 119.)
A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (that
is, counter resolution). In addition to the counter resolution, the TOP value defines the period
time for waveforms generated by the Waveform Generator.
Figure 16-4 shows a block diagram of the Output Compare unit. The small “n” in the register and
bit names indicates 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 16-4. 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 Com-
pare 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 out-
put 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 dis-
abled 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
OCFnx (Int.req.)
= (16-bit comparator )
OCRnx buffer (16-bit register)
OCRnxH buffer (8-bit)
OCnx
TEMP (8-bit)
DATA BUS (8-bit)
OCRnxL buffer (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