Jameco-electronics Rabbit 3000 Manual de usuario descargar pdf (Pagina 237)

100

101

228 Rabbit 3000 Microprocessor

Example

The spreader and doubler are enabled, with 8 ns nominal delay in the doubler. The high

and low clock are equal to within 1 ns. This violates the duty cycle requirement by 3 ns

since (clock low - clock high) can be as small as -1 ns, but the requirement is that it not be

less than 2 ns. Thus, 3 ns must be added to the minimum period of 21 ns, giving a mini-

mum period of 24 ns, and a maximum frequency of 41.6 MHz (commercial).

Since the built-in high-speed oscillator buffer generates a clock that is very close to having

a 50% duty cycle, to obtain the highest clock speeds using the clock doubler you must use

an external oscillator buffer that will allow for duty-cycle adjustment by changing the

resistance of the power and ground connections as shown below.

Figure 16-8. External Oscillator Buffer

+3.3 V

Adjust the values of

these resistors to

vary the duty cycle

XTALA1

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Jameco-electronics Rabbit 3000 Manual de usuario Pagina 237

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