Mitsubishi DS907x SIP, DS5000TK manual Troubleshooting, Unexplained Device Resets

USER'S GUIDE

SECTION 18: TROUBLESHOOTING

Dallas Semiconductor's Secure Microcontroller family has proven itself to be a reliable and easy±to±use prod- uct. As with any highly±integrated device, however, questions and or problems can arise during its use and development. Many of these stem from inadvertent at- tempts to design with the Secure Microcontroller as though it were exactly an 8051. To reduce these difficul- ties, Dallas Semiconductor has gathered the common problems in this section. These are the result of thou- sands of application questions and represent the most likely sources of trouble. The following section is orga- nized by symptom, with suggested remedies. If these fail, Dallas Semiconductor applications engineers are available to assist you. The next section lists specific do's and don'ts for designing with Secure Microcontrol- lers. These are largely based on the default practices of 8051 and other microcontroller users.

UNEXPLAINED DEVICE RESETS

Several features in the device can cause a reset. Be- cause many of these are unique to the Secure Micro- controller Family, a traditional 8051 user may be unaware of them.

1.Watchdog Timer. If the Watchdog Timer is enabled, it will cause a reset every 122,800 machine cycles. At 12 MHz, this is 122.8 ms. The Watchdog may be operating even though it was never deliberately enabled. If the Watchdog is not used, deliberately disable it in software as part of the reset vector. If it is used, the code may be missing an opportunity to strobe the Watchdog leading to an accidental reset.

2.Power Supply Glitches. The Soft Microprocessor

monitors VCC for a power failure. When power drops below its VCCmin threshold, the microprocessor will reset. Good decoupling can eliminate resets due to noise. A 10 μF and a 0.1 μF capacitor are reason-

able values, but actual selections depend on the system. Note, be especially wary of synchronous resets. That is, if every time an event occurs, the microprocessor resets. The event (i.e., turning on a motor ) could be causing a dip in VCC.

3.Electrostatic Discharge. Most microprocessors will loose control during a large static burst. The watch-

dog timer will catch an out of control processor. This will appear as a watchdog timer reset.

During the debugging process, it may be necessary to isolate the cause of an unexpected device reset. Because resets are initiated by a limited number of sources, it is relatively easy to determine their source by interrogating a few bits. These bits should be interrogated early in the code following a reset to determine its source. As a debug tool, software could set the state of one or more port pins to indi- cate the type of reset to the designer. Note that power supply problems or glitches will appear as unplanned power±on resets.

SOURCE	POR BIT	WTR BIT
	PCON. 6	PCON.4
Power±on reset	0	0
Watchdog reset	0	1
External reset	1	0

TIME MICROCONTROLLER READS THE WRONG TIME

1.Shift register corruption of a DS1215 type clock. When using a DS5000T or DS2250T and ECE2=1, any MOVX will increment the clock pointer. If the micro receives an interrupt while reading the clock, a MOVX done as part of the ISR will alter the clock pointer. Either disable interrupts while in the clock or clear ECE2 as soon as an interrupt occurs.

2.Time is not changing. The timekeeper oscillator must be enabled if the RTC is to be used. If the oscil- lator is off, the time will remain as it was written.

RAM LOSES DATA WHEN POWERED DOWN

The lithium cell is drained. Under loading, the lithium cell has insufficient capacity to create a voltage that sus- tains data in the absence of power. This could occur if a negative voltage (below ±0.3V) has been applied to the part on any pin. Look for undershoots on power or sig- nals. Also, the power could have been applied in reverse polarity or a DS5000(T) could have been plugged in backwards. If this happens to a module, the part may still work, but will not retain memory. Note that

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