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ATmega128 Datasheet(PDF) 95 Page - ATMEL Corporation |
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ATmega128 Datasheet(HTML) 95 Page - ATMEL Corporation |
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95 / 386 page  95 2467S–AVR–07/09 ATmega128 Signal description (internal signals): count Increment or decrement TCNT0 by 1. direction Selects between increment and decrement. clear Clear TCNT0 (set all bits to zero). clk T0 Timer/Counter clock. top Signalizes that TCNT0 has reached maximum value. bottom Signalizes that TCNT0 has reached minimum value (zero). Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clk T0). clkT0 can be generated from an external or internal clock source, selected by the clock select bits (CS02:0). When no clock source is selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless of whether clk T0 is present or not. A CPU write overrides (has priority over) all counter clear or count operations. The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/Counter Control Register (TCCR0). There are close connections between how the counter behaves (counts) and how waveforms are generated on the output compare output OC0. For more details about advanced counting sequences and waveform generation, see “Modes of Operation” on page 98. The Timer/Counter overflow (TOV0) flag is set according to the mode of operation selected by the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt. Output Compare Unit The 8-bit comparator continuously compares TCNT0 with the Output Compare Register (OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will set the output compare flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1), the output compare flag generates an output compare interrupt. The OCF0 flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0 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 WGM01:0 bits and compare output mode (COM01:0) bits. The max and bottom signals are used by the waveform generator for han- dling the special cases of the extreme values in some modes of operation (“Modes of Operation” on page 98). Figure 36 shows a block diagram of the output compare unit. |
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