Intel 386 manual Debugging Considerations, Hardware Debugging Features

PHYSICAL DESIGN AND DEBUGGING

Case temperature calculations offer several advantages over ambient temperature calculations:

•Case temperature is easier to measure accurately than ambient temperature because the measurement is localized to a single point (top .center of the package).

•The worst-case junction temperature (Tj ) is lower when calculated with case temper- ature for the following reasons:

The junction-to-case thermal coefficient (Sjc) is lower than the junction-to- ambient thermal coefficient (Sja); therefore, calculated junction temperature var- ies less with power dissipation (PD).

Sjc is not affected by air flow in the system; Sja varies with air flow.

With the case-temperature specification, the designer can either set the ambient temper- ature or use fans to control case temperature. Finned heat sinks or conductive cooling may also be used in environments where the use of fans is precluded. To approximate the case temperature for various environments, the two equations above should be com-

, bined by setting the junction temperature equal for both, resulting in this equation:

The current data sheet should be consulted to determine the values of Sja (for the system's air flow) and ambient temperature that will yield the desired case temperature.

Whatever the conditions are, the case temperature is easy to verify. '

11.8 DEBUGGING CONSIDERATIONS

This section outlines an approach to building and debugging Inte1386 DX microproces- sor hardware incrementally. In a short time, a complete Intel386 DX microprocessor- based ,system can be built and working. This approach does not have to be followed to the letter, but it provides several valuable debugging concepts and useful hints. Use these guidelines in conjunction witli the Intel386 DX microprocessor data sheet, which contains detailed information about the Intel386 DX microprocessor. .

11.8.1 Hardware Debugging Features

Even before a system is built, debugging can be made easier by planning a suitable environment for the Inte1386 DX microprocessor. The Intel386 DX microprocessor board (whether it is a printed circuit board or a wire-wrap board) must have power and ground planes. The user should provide a decoupling capacitor between Vec and GND next to each Ie on the board. All Inte1386 DX microprocessor Vcc and GND pins should be connected individually to the appropriate power or ground plane; multiple power or ground pins should not be daisy-chained.

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