参数资料
型号: MB9AF312NPMC
元件分类: 微控制器/微处理器
英文描述: 32-BIT, FLASH, 40 MHz, RISC MICROCONTROLLER, PQFP100
封装: 0.50 MM PITCH, PLASTIC, LQFP-100
文件页数: 98/114页
文件大小: 1357K
代理商: MB9AF312NPMC
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
84
15.5
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 compare 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 location. The waveform generator uses the match signal to
generate an output according to operating mode set by the WGM02:0 bits and compare output mode (COM0x1:0) bits. The
max and bottom signals are used by the waveform generator for handling the special cases of the extreme values in some
Figure 15-3 shows a block diagram of the output compare unit.
Figure 15-3. 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 buffering 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.
15.5.1 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 (FOC0x) bit. Forcing compare match will not set the OCF0x flag or reload/clear the timer, but the OC0x pin
will be updated as if a real compare match had occurred (the COM0x1:0 bits settings define whether the OC0x pin is set,
cleared or toggled).
15.5.2 Compare Match Blocking by TCNT0 Write
All CPU write operations to the TCNT0 register will block any compare match that occur in the next timer clock cycle, even
when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0 without triggering an
interrupt when the Timer/Counter clock is enabled.
OCFnx (Int. Req.)
= (8-bit Comparator)
OCRnx
Waveform Generator
TCNTn
OCnx
top
bottom
FOCn
WGMn1:0
COMnX1:0
DATA BUS
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