Common (and uncommon) Problems

Mysterious Oscilloscope Traces

A few important tips on using the lab oscilloscopes:

  1. Use the Auto Set function to reset the scope to a reasonable state, but don't rely on it to always do the right thing with displaying your signal. It's always best to configure it manually with the following steps:
  2. Connect the scope probe's ground clip to the circuit's/kit's earth ground.
  3. Verify that the scaling on the probe (1X/10X) matches the setting on the scope.
  4. Ground couple the scope channel you are using. Adjust the vertical position to where you would like the "zero" voltage level to be for that trace.
  5. Adjust the vertical (volts) scale on the channel to something reasonable (say 2.00V/DIV for TTL logic levels), and DC couple the scope channel in question. Do not AC couple your scope's channels unless you really know what you're doing. AC coupling will provide you with some pretty strange behavior.
  6. Make sure the scope is in Run/Roll mode, not stopped (mainly applicable to the digital scopes).
  7. Change the horizontal scaling (time) to see different levels of detail.
  8. If you're lost when it comes to oscilloscopes, take a look at the Tektronix guide on the Miscellaneous page.

Connecting Devices that are on

It is always a bad idea to connect a cable between two devices if one or both is on. The only exception is that you can plug the the serial cable into the R31JP if the R31JP is off but the computer is on. But when you're connecting a bench power supply to your kit, running wires to a new chip on your board, hooking up the lamp, or anything at all, make sure both devices are powered off. Often chips will fail in mysterious ways when they are connected while powered, and powering up pins out of sequence can damage chips.

Using the correct book

Yeralan is a guide to programming the 8051; it is not a reference. Always keep the 8051 manual with you (it is the spiral-bound book labeled "MCS 51 Microcontroller Family User's Manual") to look up information. Yeralan is known to contain a few errors that will mess you up if you do not also cross-check with the Intel manual.

Vertical rails

The vertical rails on the breadboard are not connected across the break in the middle. You have to make jumpers for them to be connected. Jumper them all right now, as it will save you many hours of debugging later. If you don't understand what we're talking about, ask a TA to show you.

Keypad debouncing capacitors

Debouncing capacitors are required on the 74HC922 for correct keypress behavior. The datasheet does not tell you exactly what values to use, but it does give their relative sizes, and you can figure out reasonable values from looking at the graphs.