Intel 386 manual Latch-Up, Clock Considerations, Requirements

PHYSICAL DESIGN AND DEBUGGING

One way to minimize this is by decreasing the lengths of the interconnections. Overall route lengths are shorter in multilayer printed-circuit boards than in double layer boards because ground and power traces are not present. In addition to adding ground planes, a routing program can help to shorten the paths.

The guidelines discussed so far are prominent at higher operating frequencies. Debug- ging an Inte1386 DX processor-based system at higher frequencies requires careful plan- ning of the layout and the physical design. The following sections cover latch-up and thermal characteristics which are system design considerations that stem from the device itself.

11.5 LATCH-UP

Latch-up is a condition in CMOS devices which occurs when Vee becomes shorted to Vss. Much attention has been directed at eliminating this phenomenon under normal conditions. It is necessary for board designers to be aware of latch-up, of its causes, and of how to prevent it.

Latch-up is triggered when the voltage limits on the I/O pins are exceeded, causing the internal PN junction to become forward-biased. The following steps ensure the preven- tion of latch-up.

•Observe the maximum input voltage rating of I/O pins.

•Never apply power to Inte1386 DX processor pin or to any device connected to it before applying power to the Inte1386 DX processor.

•Use good termination techniques to prevent overshoot and undershoot.

•Ensure a proper layout to minimize reflections and to reduce noise on the signals.

11.6CLOCK CONSIDERATIONS

11.6.1 Requirements

For performance at high frequencies, the clock signal (CLK2) for the Intel386 DX microprocessor must be free of noise and within the specifications listed in the Intel386 DX microprocessor data sheet. These requirements can be met by following these guidelines:

•Construct the Clock Generating circuit as shown in Figure 11-24.

•Terminate the CLK2 output to obtain a clean signal.

•Avoid placing too many loads on a single driver and carefully plan the traces to minimize reflection.

•Use an oscilloscope to verify the waveform of CLK2 against the specification in the Inte1386 DX microprocessor data sheet.

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