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ATtiny40 [DATASHEET]
8263B–AVR–01/2013
put Compare output OC0A. For more details about advanced counting sequences and waveform generation, see
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM0[1:0]
bits. TOV0 can be used for generating a CPU interrupt.
11.5
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and OCR0B).
Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the Output Com-
pare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output
Compare Flag generates an Output Compare interrupt. The Output Compare Flag is automatically cleared when
the interrupt is executed. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit loca-
tion. The Waveform Generator uses the match signal to generate an output according to operating mode set by the
WGM0[2:0] bits and Compare Output mode (COM0x[1:0]) bits. The max 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 ofFigure 11-3. Output Compare Unit, Block Diagram
The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For the
normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buff-
ering synchronizes the update of the OCR0x Compare Registers 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 OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the OCR0x
directly.
OCFnx (Int.Req.)
8-BIT COMPARATOR
OCRnx
OCnx
DATA BUS
TCNTn
WGMn[2:0]
WAVEFORM GENERATOR
top
FOCn
COMnX[1:0]
bottom