Motorola M68CPU32BUG Manual de usuario descargar pdf (Pagina 47)

MC68332TUT/D MOTOROLA

2. If the code does disable the watchdog, but the device is still resetting every 16 ms, then the code is

probably not being executed. Either the memory is not programmed properly, or something is pre-

venting the MCU from executing code. Check all pull-up resistors for good connections and correct

values. Also check the frequency of CLKOUT.

5.2.3 Problem: CLKOUT Frequency is Incorrect

1. MODCLK is not driven correctly during reset. To use a crystal and the internal PLL, MODCLK must

be driven high during reset. To use an external clock and bypass the internal PLL, MODCLK must be

driven low during reset.

2. The crystal is settling into overtones due to a poor quality crystal or incorrect components in the crys-

tal circuit.

3. If the frequency is unstable, it is possible that the crystal is being overdriven. Increase Rs to reduce

crystal drive.

4. There is residue on the PCB. Since low frequency crystal circuits tend to be very high impedance,

the PCB must be clean, dry, and free of conductive material such as solder rosin and excessive mois-

ture from high humidity.

5. In the absence of other circuit problems, the series resistor is the most probable culprit when an os-

cillator will not start. The resistor limits the power that starts the crystal oscillating. If the resistance is

too low, the crystal will start oscillating in unpredictable modes and could even become damaged. If

the resistance is too high, the oscillator will start very slowly or not at all.

6. If the value of the series resistor is correct, check for the presence of metastable states during power-

up. If there is extremely high frequency oscillation on the CLKOUT pin during the first few hundred

milliseconds of operation, and increasing the size of Rs does not fix the problem, the only real solution

is to find a different brand and/or style of crystal. There is no practical way to compensate for a crystal

that exhibits poor self-suppression of the first overtone and first harmonic. Once again, if a particular

crystal type and brand is prone to starting at overtones or harmonics, just don’t use it. No amount of

circuit design will ever compensate for a bad or poor quality crystal.

Usually, it is impossible to observe oscillator operation with an oscilloscope connected to one of the oscil-

lator pins. The oscilloscope adds 3-30 pF and 1-10 MΩ of loading to V

, which will usually affect oscillator

operation. When the oscilloscope is connected to the EXTAL input, the 10 MΩ to V

(oscilloscope input)

forms a resistive divider with Rf and often disables the oscillator by biasing the circuit out of the linear region

of the EXTAL input. This problem can sometimes be overcome by capacitively coupling the oscilloscope

with a very small capacitor (1-5 pF) between the oscilloscope probe and the oscillator pin.

It is better to observe the CLKOUT signal, since this does not alter the operation of the oscillator. It may be

possible to observe XTAL since it is isolated from rest of oscillator by Rs. Observe I

without oscilloscope

connected and again with it connected. If I

is unchanged, it is usually safe to assume the oscillator was

unaffected. For additional information, see 2.5 Clock Circuitry.

5.2.4 Problem: System Crashes after Fetching Reset Vector

1. Incorrect reset configuration of boot memory width is causing the address bus to increment by the

wrong amount during fetches of the reset vectors. Check DATA0 to make sure that it is being driven

to the correct state during reset. If CSBOOT is a 16-bit port, drive DATA0 high during reset; if it is an

8-bit port, drive DATA0 low. See 2.1 Using Data Bus Pins to Configure the MCU.

2. An IRQ7 interrupt is received during or immediately after reset. The MCU will recognize the interrupt

after fetching the reset information and first instruction. In a typical system that is booting out of ROM,

stack RAM will not be enabled at this point, and the first bus cycle to write the stack frame will hang

the MCU. Make sure that the IRQ[7:1] lines are either pulled up through resistors to 5 volts or config-

ure the pins as PORTF I/O lines by pulling DATA9 low during reset. Also, start-up software should

enable the stack RAM (by configuring the appropriate chip-select circuits) before enabling the inter-

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Motorola M68CPU32BUG Manual de usuario Pagina 47

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