Some basic lab rules have been posted on the course web site, so please refer to them. Remember to close the lab door when you are the last person to leave the room and remember to turn off the soldering iron.


Q) When I measure signals using the oscilloscope, the same signal looks different depending on where I've connected the ground lead for the scope probe. Why?

A) You want to measure signals referenced to a local ground so always try to hook the ground lead of the scope to a ground pin which is close to the signal that you're measuring. For instance, if you're measuring a signal coming from the 8051, hook up the ground lead to the ground pin on the 8051. If you hook the ground lead far away, you will see additional noise on the signal due to ground bounce from the inductance of the wires which are carrying the current from other chips which are switching. Be careful when you attach the scope probes to the circuit that you do not short two adjacent pins or wires on your board. Since we are using single ended probes and our oscope is not floating, only connect the ground lead of your scope to a ground in your circuit and not to a different voltage level. For more information about scopes and probes, refer to the documents available on the course web site.


A few students have seen what looks to be a very noisy signal when they look at their oscillator signals and ALE. A good part of this is due to using a low quality 1x oscilloscope probe. I'd recommend using 10x probes to look at your signals. You'll probably see a big difference. Please be very careful with the 10x probes, as they are not inexpensive. Also, note that the probe capacitance impacts how your circuit operates, especially in circuits like the oscillator circuit, which requires small load capacitances.


Be careful when attaching a probe to pins in your circuit. I've noticed many cases where in the process of connecting the oscope probe to their chip, students short together two adjacent pins on that chip. Take care to avoid these shorts, as you may end up damaging your chips if you short two outputs together. Also, be sure that the calibration dials on each of the voltage and time base knobs on any older oscilloscope is turned fully clockwise; otherwise, your measurements may not be accurate. You can use the calibration output on the oscilloscopes to check the accuracy of the oscilloscopes - that output should generate a 1000Hz square wave which is 500mV peak to peak. The newer digital storage oscilloscopes also provide this calibration feature. You can also use that calibration output to adjust the compensation in your probe, so that your scope accurately represents the signals you are measuring.


Some documentation related to logic analyzers is available on the course web site. The Agilent application note 1337 'Feeling Comfortable with Logic Analyzers' gives a good overview of when you want to use a logic analyzer versus a digital storage oscilloscope, and when to use the state versus timing modes of the logic analyzer. Note that for examining sequences of addresses/data on the bus, the 'state' mode of the logic analyzers is very useful - just hook up the appropriate clock source to tell the logic analyzer when to sample the state of the system - depending on the particular information you are trying to capture, ALE, /PSEN, or /READ may be appropriate triggers to use. We'll typically use the logic analyzer in transitional mode rather than conventional mode, since transitional mode allows much more information to be collected with the same limited amount of logic analyzer memory.


If you just want to check whether a specific line is toggling or is at a specific logic level, a digital logic probe is much easier to use than the logic analyzer. Some students take the time to hook up the logic analyzer when they want to check whether a chip select line or other control signal is getting activated. While that is a valid approach, a simple digital logic probe can give you the answer very easily. If you haven't used a digital logic probe yet, please try one out very soon. There should be several available in the lab. If you want to measure very precise timing or transient voltage levels, the correct piece of equipment to use is a digital storage oscilloscope.


Turn off the soldering iron when you are done using it. Excessive heat can damage the tip and leaving the soldering iron on is a fire hazard. Keep the sponge wet with regular water and clean the iron tip before using it. Tin the iron tip with a small amount of solder before touching the tip to your circuit. Read the article on soldering which is available in the lab and visit for more information about soldering techniques.