PC/CP200 Electronics Lab I

Logic Gates Characteristics - Current

Objectives

1. To gain more experience with an oscilloscope.
2. To measure the current characteristics for various digital logic families.
3. To obtain and interpret information from a data sheet.
   

Preparation

Gates have current limits as well as voltage limits. Four particular quantities are of interest:

1. I_ILmin -- the minimum input current which must be drawn from a gate's input to ground to pull the input low.
2. I_IHmin -- the minimum input current which must be supplied to a gate's input to pull the input high.
3. I_OHmax -- the maximum current which the gate can source through its output and still keep the output high.
4. I_OLmax -- the maximum current which the gate can sink through its output and still keep the output low.

I_ILmin and I_IHmin may require some explanation. Most people assume that if the inputs of a gate are not attached to anything they will be treated as logic low. This is a bad assumption. An important piece of information about various logic families is what happens when inputs are left to float; i.e. remain unconnected. They may float high, low or anywhere in between. To have an input recognized as something other than its "floating" state will require that a finite amount of current be either supplied to the input (to make it high) or drawn from the input to ground (to make it low.) Never assume anything about unconnected inputs. If you want them to be in a particular state, tie them that way. Never leave inputs floating.

These current limits are referred to as the fan-in and fan-out characteristics of digital circuits. In this lab we shall attempt to measure these.

Definition of terms can sometimes become tricky. An output limit, for instance, can be seen as either how much is guaranteed to be supplied, or as how much can be safely demanded. In other words, the limit can be seen as either belonging to the device or the surrounding circuit. While they are functionally equivalent, the first view will give a minimum for a quantity while the second will give a maximum. Different manufacturers may take either view, and so it is important to understand this so that you can make sense of whichever you are given. In the previous section, these limits were discussed as limits on the device.

Equipment

• bench power, variable power supply
• breadboard
• variable resistor
• 74LS00 [1988 datasheet, 2003 datasheet]
• CD4011UB [1998 datasheet, 2003 datasheet]
• oscilloscope - one of TDS 210, TDS 1002 (old and new numbers for the same oscilloscope)
  

Procedure

Great care should be taken to avoid static discharge into CMOS (static sensitive) based chips.

Before starting the lab, review the CMOS handling procedures.

1. Always use a ground strap. If your grounding mat doesn't have two grounding straps, one for each of the partners, see the lab instructor.
2. CMOS devices should be stored pin down in conductive foam when they are not in a circuit.
3. Never leave unused inputs floating; connect to ground or +5V to prevent excessive current consumption and erratic behaviour.
4. Never connect an input signal to a CMOS device when the power is off.

Current limits; Fan-out

Do the following analysis (steps 1 to 3) first for the 74LS00 and then repeat for the 4011.

1. Wire up the following circuit; R is a variable resistor.
   
   A resistance substitution box is an alternative to using a trimmer; it doesn't have a wiper, but the resistance value can be read off the dial; you don't have to measure it.
   
   Use the oscilloscope to measure V_in and V_out. Since one end of R is tied to a known voltage, and the other end is connected to the oscilloscope, then the voltage across R is known, and thus the current through R can be calculated using Ohm's Law. Obtain the value of the output sourcing current I_OHmax from the data sheets. Determine the maximum value of current which keeps the output in the high state; i.e. the output voltage above the specified value of V_OHmin . Thus determine I_OHmax.
   Is it within the manufacturer's specifications?
   
   Demonstrate for 7LS400 to lab staff.
   
   
2. Now set up the following circuit; R is a variable resistor. Obtain the value of the output sinking current I_OLmax from the data sheets. Determine the maximum value of current which keeps the output in the low state and thus I_OLmax. Is it within the manufacturer's specifications?
   
   
   Demonstrate for CD4011 to lab staff.
   
   
3. Based on your results from voltage measurements above, can you suggest why this parameter matters; i.e. under what conditions is the output of a gate going to be pulled high, so that it has to "work" to keep the output low?

Current Limits; Fan-in

TTL

Do the following analysis for the 74LS00.

1. Wire up the following circuit. Use the oscilloscope to measure V_in and V_out. Since one end of R, a variable resistor, is tied to a known voltage, and the other end is connected to the oscilloscope, then the voltage across R is known, and thus the current through R can be calculated using Ohm's Law.
   
2. Disconnect R, so that the gate input is unconnected. This means there is no current into one of the gate inputs. Observe its output. Is it what you expect it to be? Explain your answer. Hint: What does this tell you about whether the inputs are being seen as high or low?
   
   
3. If the inputs of a device “float” so that they appear to be in a particular state with nothing connected, then by definition you don't need to supply any current to keep them in that state! [Sometimes the inputs float to a voltage which is in the indeterminate region, so that sometimes they will appear to be high and other times they will appear to be low.]  The TTL and CMOS chip will behave differently.  If the chip can hold the output state without any current, skip the next step (as changing the current will make no difference).
   
   
4. Reconnect R so that you can adjust the current into the gate. Starting with the lowest current possible, monitor the value of V_out as current is increased, so that you can determine when the output voltage just reaches the specified value of V_OLmax and thus determine I_IHmin.
   
   Demonstrate for 7LS400 to lab staff.
   
   

CMOS

The inputs to CMOS are capacitive, which is different than TTL. While the amount of input current required is very small, the speed of the gate response will depend on the current in or out of the inputs.

1. Wire up the circuit below, using the decade box for R.
   
   
   
2. Adjust the frequency generator to produce a 0-5V square wave output at 10kHz.
   
   
3. With the decade box resistance at a minimum, line up the input and output signals on the oscilloscope.
   You may want to invert one of the channels so that you can overlay them directly.
   
   
4. Increase the value of R, and see when you observe a delay between the input and output signals. Measure the time delay, and record the value of R.
   Don't confuse this with the propagation delay; this isn't the time for the signal to get through the gate, it's the time for the input capacitance to charge or discharge.
   
   
5. Given that the time constant for the RC combination is RC, use the time you have determined to estimate a value for the input capacitance of the gate.
   
   
6. Look up the input capacitance of the gate in the datasheet. Is the value you calculated within the manufacturer's specifications?
   Demonstrate for CD4011 to lab staff.