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ATMEGA8515-16AU Datasheet(PDF) 94 Page - ATMEL Corporation |
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ATMEGA8515-16AU Datasheet(HTML) 94 Page - ATMEL Corporation |
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94 / 254 page  94 ATmega8515(L) 2512G–AVR–03/05 Timer/Counter0 and Timer/Counter1 Prescalers Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters can have different prescaler settings. The description below applies to both Timer/Counter1 and Timer/Counter0. Internal Clock Source The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system clock frequency (f CLK_I/O). Alternatively, one of four taps from the prescaler can be used as a clock source. The prescaled clock has a frequency of either f CLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024. Prescaler Reset The prescaler is free running, i.e., operates independently of the clock select logic of the Timer/Counter, and it is shared by Timer/Counter1 and Timer/Counter0. Since the pres- caler is not affected by the Timer/Counter’s clock select, the state of the prescaler will have implications for situations where a prescaled clock is used. One example of pres- caling artifacts occurs when the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024). It is possible to use the Prescaler Reset for synchronizing the Timer/Counter to program execution. However, care must be taken if the other Timer/Counter that shares the same prescaler also uses prescaling. A Prescaler Reset will affect the prescaler period for all Timer/Counters it is connected to. External Clock Source An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock (clk T1/clkT0). The T1/T0 pin is sampled once every system clock cycle by the pin syn- chronization logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 45 shows a functional equivalent block diagram of the T1/T0 synchroni- zation and edge detector logic. The registers are clocked at the positive edge of the internal system clock (clk I/O). The latch is transparent in the high period of the internal system clock. The edge detector generates one clk T1/clkT0 pulse for each positive (CSn2:0 = 7) or neg- ative (CSn2:0 = 6) edge it detects. Figure 45. T1/T0 Pin Sampling The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an edge has been applied to the T1/T0 pin to the counter is updated. Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated. Each half period of the external clock applied must be longer than one system clock cycle to ensure correct sampling. The external clock must be guaranteed to have less than half the system clock frequency (f ExtClk < fclk_I/O/2) given a 50/50% duty cycle. Since Tn_sync (To Clock Select Logic) Edge Detector Synchronization DQ DQ LE DQ Tn clk I/O |
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