Intel 386 manual Other Simple Diagnostic Software

PHYSICAL DESIGN AND DEBUGGING

Mter installing the EPROMs, the READY# line should be tied high (negated) so that the Inte1386 DX microprocessor begins its first bus cycle after reset and then continues to add wait states. While the system is in this state, the circuit should be probed to verify signal states, using a voltmeter or oscilloscope probe.

The programmer should check whether the address latches have latched the first address and whether the address decoder is applying a chip-select signal to the EPROMs. The EPROMs should be emitting the first four opcode bytes of the first code to be executed (90H, 90H, EBH, FCH for the 4-byte program of Figure 11-28), and the opcode should be propagating through the data transceivers to the. Intel386 DX microprocessor data pins.

Then the READY# input should be connected to the READY# generation logic, the Intel386 DX microprocessor, and the results should be tested when the simple program runs. Because the program loops back on itself, it runs continuously. At this point, the system has progressed to running multiple bus cycles, so a logic analyzer is needed to observe the dynamic behavior of the system.

When the EPROMs programmed with the simple 4-byte diagnostic program are

.. installed and the Inte1386 DX microprocessor is executing the code, the LED indicator for ADS# (if included in the system) glows, because ADS# is generated for each bus cycle by the Inte1386 DX microprocessor. It is necessary to check that the EPROMs are selected for each code fetch cycle. After system operation is verified with the simple program, more complex programs can be run.

11.8.5 Other Simple Diagnostic Software

Other simple programs can be used to check the other operations the system must perform. The program described here is longer than the 4-byte program illustrated pre- viously; it tests the abilities to write data into RAM and read the data back to the Inte1386 DX microprocessor.

This second diagnostic program, shown in Figure 11-29, is also suitable for placing into EPROMS. Because this diagnostic loops back to itself, the ADS# LED should glow continuously, just as it does when running the 4-byte program.

The program in Figure 11-29 is based on the assumption that hardware exists to report whether the data being read back from RAM is correct. This hardware consists of a writable output latch that can display a byte value written to it. The byte value written is a function of the RAM data comparison test. If the data is correct, the byte value written is AAH (10101010); if the data is incorrect, 55H (01010101) is written.

This diagnostic program is not comprehensive, but it does exercise EPROM, RAM, and an output latch to verify that the basic hardware works.

The program is short (45 bytes) to be easily understood. Because it is short and because it loops continuously, a logic analyzer or even an oscilloscope can be used to observe system activity.

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