Motorola M68CPU32BUG User Manual download pdf (Page 12)

MOTOROLA MC68332TUT/D

or, on the other hand, a working oscillator could be moved into a region of no oscillation at all. Therefore, it

is important to measure oscillator performance indirectly. This can be done through the CLKOUT pin, which

is a buffered form of the internal system clock. Monitoring the CLKOUT pin with an oscilloscope does not

affect the oscillator and provides an accurate representation of oscillator problems. If the MCU is running

off the internal PLL and a 32.768 kHz crystal, the CLKOUT frequency should be 8.389 MHz.

The CLKOUT signal is likely to do one of three things when power is turned on. It will either remain at a

constant DC level, jump quickly to the proper frequency, or, first jump to the desired frequency, then enter

a very high frequency metastable state and then jump back to the fundamental frequency. With a small

amount of practice, these metastable states, which last for approximately 100 to 500 ms, can be easily de-

tected on an oscilloscope. In the third case, the MCU generally takes almost a second to reach steady state,

which provides plenty of time for it to attempt operation while the clock is in a metastable state.

2.5.2.6 Using a Canned Oscillator

A second option when using the internal frequency synthesizer circuit is to hold MODCLK high during reset

and connect an external clock reference or canned oscillator (a single package that includes the oscillator

and required external components) to the EXTAL pin. Leave the XTAL pin floating, but connect the filter

circuit shown in Figure 7 to V

DDSYN

and XFC. The allowable frequency range is 20-50 kHz.

One manufacturer of canned oscillators is:

Oak Frequency Control Group — (717) 486-3411

2.5.3 Using an External Clock

To use an external clock, connect a clock signal to the EXTAL pin and hold MODCLK low during reset.

Leave the XTAL and XFC pins floating, but connect V

DDSYN

to power. The frequency control bits in the

SYNCR register have no effect; the signal applied to the EXTAL pin should appear unchanged on the CLK-

OUT line. The external clock must comply with the following expression.

Minimum external clock high/low time is a specification given in the device electrical characteristics.

2.6 Getting Out of Reset

Asserting and releasing the RESET

line was once a relatively simple task. However, as microcontrollers

have become more complex, bidirectional reset pins have become standard. Bidirectional reset lines allow

an external device to reset the MCU, and also allow the MCU to assert reset for associated peripherals.

Bidirectional pins must be driven with open collector devices. A typical circuit for driving the MCU RESET

pin is shown in Figure 8. The RESET pin is driven by an open collector device, and it is pulled to a logic 1

by an 820 Ω resistor.

Minimum external clock period

Minimum external clock high/low time

50% percentage variation of external clock input duty cycle–

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Motorola M68CPU32BUG User Manual Page 12

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