mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 48

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

Because of the complexity of the MCU, there are a considerable number of potential ‘fatal flaws’ that can
cause a prototype application to either not operate from power up, or to fail soon after. This section covers
common problems, causes, and fixes.This is not an exhaustive discussion, but is intended to be used as a
check list of the main problem areas that can cause an application to fail.
5.1 Critical Signals to Check
5.2 Common Problems and Solutions
5.2.1 Problem: Device Stays in Reset
5.2.2 Problem: Device Resets Every 16 ms
5.2.2.1 Problem: CLKOUT Frequency Drifts at Higher Temperatures
If using the PLL, and the CLKOUT signal is fine at room temperature but begins to become unstable or drift
at about 60 – 70 degrees Celsius, make sure that the VCO is not operating over the maximum frequency
specification. See 4.2.2 Clock Synthesizer Control Register (SYNCR) for further explanation.
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• RESET should stay low for at least 512 clocks during a power-on reset. If using the internal PLL, RE-
• CLKOUT should be at the system clock frequency. If MODLCK is held high at the release of reset,
• Immediately after reset, CSBOOT should pulse low five times for a 16-bit port and nine times for an
• FREEZE should be low and HALT should be high. Otherwise, the MCU is halted, or is in BDM.
• BR and BGACK should be high. Otherwise, the external bus is granted away.
• Make sure that the data bus pins are configured correctly during reset.
• Make sure that IRQ7 is high during reset.
1. There is no pull-up resistor on RESET. RESET needs an 820
2. A capacitor on RESET can also prevent the device from coming out of reset. Do not put any capac-
3. MODCLK is pulled high at the release of RESET, and the VCO is not locking. Check the components
4. MODCLK is pulled low at the release of RESET, and there is no external clock signal. Make sure that
1. The software watchdog is enabled but is not being serviced by the program. When the watchdog is
2. If the code does disable the watchdog, but the device is still resetting every 16 ms, then the code is
SET will remain low for a little longer because the VCO must lock first. RESET should then go high
and remain high.
CLKOUT should be 512 times the frequency going into EXTAL (8.389 MHz for a 32.768 kHz crystal).
Make sure that the frequency is exact, as a measurable error may indicate limp mode and oscillator
faults. If MODCLK is held low at the release of reset, the frequency on CLKOUT should be the fre-
quency going into EXTAL.
8-bit port.
Data Bus Pins to Configure the MCU and 2.3 Pins that Need Pull-Up Resistors.
itors on RESET. See 2.1 Using Data Bus Pins to Configure the MCU and 2.3 Pins that Need
in the crystal circuit to ensure that they are correct. Check the layout to ensure that the board is clean
and that there are no noisy signals nearby to affect operation of the oscillator, and make sure that
power is applied to V
else fails, change crystals. See 2.5 Clock Circuitry.
there is a signal going into EXTAL. See 2.5 Clock Circuitry.
enabled, the program must write a sequence to the software service register to prevent the watchdog
from timing out and resetting the MCU. The software watchdog is enabled out of reset, and the pro-
gram must disable it by clearing the SWE bit in the SYPCR register. Note that this is a write-once-
only register.
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.
Pull-Up Resistors.
DDSYN
Freescale Semiconductor, Inc.
For More Information On This Product,
. Also, make sure that the crystal frequency is within specifications. If all
5 TROUBLESHOOTING
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resistor to 5 volts. See 2.1 Using
M68331/332TUT/D
MC68331/332