Unlock Microcontroller ATMEGA128PA Heximal

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When an external clock input is used for Timer0, it must meet certain requirements. The external clock requirement is due to internal phase clock (TOSC) synchronization. Also, there is a delay in the actual incrementing of Timer0 after synchronization when Unlock Microcontroller.

When no prescaler is used, the external clock input is the same as the prescaler output. The synchronization of an external clock with the internal phase clocks is accomplished by sampling the prescaler output on the Q2 and Q4 cycles of the internal phase clocks (Figure 7-4) if Unlock Microcontroller.

Therefore, it is necessary for T0CKI or the comparator output to be high for at least 2 TOSC (and a small RC delay of 2 Tt0H) and low for at least 2 TOSC (and a small RC delay of 2 Tt0H). Refer to the electrical specification of the desired device before Unlock Microcontroller.

When a prescaler is used, the external clock input is divided by the asynchronous ripple counter type prescaler, so that the prescaler output is symmetrical after Unlock Microcontroller.

For the external clock to meet the sampling requirement, the ripple counter must be taken into account. Therefore, it is necessary for T0CKI or the comparator output to have a period of at least 4 TOSC (and a small RC delay of 4 Tt0H) divided by the prescaler value when Unlock Microcontroller.

The only requirement on T0CKI or the comparator output high and low time is that they do not violate the minimum pulse width requirement of Tt0H. Refer to parameters 40, 41 and 42 in the electrical specification of the desired device before Unlock Microcontroller.

Since the prescaler output is synchronized with the internal clocks, there is a small delay from the time the external clock edge occurs to the time the Timer0 module is actually incremented after Unlock Microcontroller.

Figure 7-4 shows the delay from the external clock edge to the timer incrementing. An 8-bit counter is available as a prescaler for the Timer0 module or as a postscaler for the Watchdog Timer (WDT), respectively (see Figure 9-6) if Unlock Microcontroller.

For simplicity, this counter is being referred to as “prescaler” throughout this data sheet when Unlock Microcontroller.


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