CMOS Dynamic Behavior

Introduction

Objectives

• Measure gate propagation delay using direct and indirect measurement methods
• Measure gate output rise and fall times
• Investigate relationship between gate switching and supply current spikes
• Study the effect of decoupling capacitors as a method of reducing supply noise

 Parts List

• SN74HC04N hex inverter
• 100 pF capacitor
• 1000 pF capacitor
• 0.1 uF capacitor
• 10-ohm resistor

 Equipment

• Agilent 54622D MSO
• Agilent 33120A Function/Arb Generator
• Fixed 5V power supply
• Breadboard

Prelab

1. Obtain a data sheet for the SN74HC04N hex inverter. Study the “Parameter Measurement Information” section to determine how to measure rise and fall times and propagation delay.
    
2. Consider the ring oscillator circuit below:
   
   A ring oscillator is a cascade of an odd number of inverters with the final invert output fed back to the input of the first inverter. The circuit is self-oscillating, and has no input from another circuit.
   
   Draw the timing diagram for the ring oscillator circuit using N = 5 inverters; the diagram will show all five inverter output waveforms. Assume a constant finite propagation delay t_P for the inverters. [Hint: Assume that the first inverter has just produced an output transition from low to high, and hence an input transition to the next inverter of low to high, then follow the resulting behavior around the loop].
    
3. Derive an equation that describes the oscillation frequency of the ring oscillator in terms of t_P and the number of inverters N. Explain how this equation could be used to measure propagation delay.
    
4. The supply current I_DD is defined as positive when it enters the V_DD pin of a device. Develop a method using an oscilloscope and a 10-ohm resistor that would allow you to display the dynamic supply current waveform i_DD. Explain your technique and draw a circuit diagram. [Hint: Remember that the oscilloscopes in our lab must have their scope probe ground clips attached to ground! Also remember that oscilloscopes measure voltage, not current.]
    
5. Make a photocopy of your prelab pages, and bring to class the day before lab.

Rise and Fall Time Measurement

1. Set up the function generator to produce a 0 to 5V squarewave at 1 MHz. Apply this signal to a single inverter (the remaining inverter inputs should be tied low). Monitor both the inverter input and output with the oscilloscope.
    
2. Adjust the oscilloscope output waveform display to maximize use of the screen in the vicinity of a rising edge on the inverter output.
    
3. The MSO can measure rise and fall time directly using “Measure -> Quick Meas” followed by “Softkey -> more” and “Softkey -> Rise Time” (or “Fall Time”). Note the positions of marker lines, and verify that you understand how this relates to your Prelab Step 1. Record the rise time t_r and fall time t_f of the inverter output and compare to the published specifications – the data sheet may use the symbol “t_t” to denote transition time when t_r and t_f are the same. [Hint: Recall that “compare” is lab handout code for “calculate percentage error and discuss your findings.”]
    
4. Repeat Step 3 with a 100 pF capacitive load on the inverter output (connect between output terminal and ground), and then with a 1000 pF capacitive load. Discuss the impact of capacitive loading on rise and fall time.
    
5. Allow the inputs on the right side of the chip to float, rather than being grounded.  What effect do floating inputs have on the output waveform? Why?

Dynamic Supply Current

1. Set up your equipment to display the dynamic supply current i_DD. Seek help from your instructor if you are in doubt about your method!
    
2. Drive one inverter with your 1 MHz squarewave signal (the other inverters should be tied low). Show the output of the inverter on the other probe of your scope. Record the waveform in your lab book and measure the numerical value of the peak current from your waveform.
    
3. Next, drive two inverters with the same squarewave input signal (the two inverter inputs would be tied together at this point). Record the waveform and measure the peak current.
    
4. Repeat the process by driving three inverters, then four, and so forth, each time recording the waveform and measuring the peak current.
    
5. Plot peak current as a function of the number of simultaneously switched inverters. Discuss any trends in your data, and propose an explanation for the trend.
    
6. Now that all six inverters are switching simultaneously, remove the 10-ohm resistor, then look at the dynamic supply voltage v_DD at pin 14. Record the waveform and measure the peak deviation from the nominal value V_DD.
    
7. Connect a 0.1 uF capacitor (called a decoupling capacitor or bypass capacitor in this application) directly from the V_DD to ground. Make sure that the capacitor is physically close to the package; you may even want to cut the leads a bit in order to minimize lead length.
    
8. Repeat Step 6. How much did the decoupling capacitor “clean up” the supply voltage? (Do a percentage change calculation).

Indirect Measurement of Propagation Delay

1. Construct a ring oscillator using N = 5 inverters. Use inverters 1 to 5 of the 74HC04 hex inverter for the ring oscillator. Use the remaining inverter as a buffer between the ring oscillator and the instrumentation (frequency counter on the oscilloscope). That is, select one of the inverter outputs from the ring oscillator, and apply this to the input of the sixth inverter. Measure the output of the sixth inverter.
    
2. Measure the oscillation frequency of the sixth inverter’s output using the “quick measurement” feature of the oscilloscope: press “Measure -> Quick Meas” followed by “Softkey -> Frequency”. You may find it necessary to power cycle the 74HC04 device a few times to “kick start” the oscillations.
    
3. Once you have a stable reading of frequency, try probing inside the ring with your other oscilloscope probe. Note any differences in waveform quality, and note the degree to which the oscilloscope probe alters the measured frequency. What is the effective capacitance of the probe? (Hint: look at the probe connection to the oscilloscope).
    
4. Use the equation you derived in the prelab to estimate the propagation delay t_P for a single inverter, and compare to the published specification and as well as your direct measurement results.
    
5. Use the digital probe pod and digital waveform display to create a measured timing diagram of all five inverter outputs. Compare to your prelab prediction.

All Done!

• Clean up your work area
• Remember to submit your lab notebook for grading at the beginning of next week's lab