Nasty Realities EECS 150 – Lab6 Spring 2001 By Steve Fang

Topics Capacitive loading vs. Propagation delay Reflection vs. Termination Capacitive coupling

Ideal vs. Reality Ideal Reality

Components (1) Figure 1a Breadboard connections Breadboard Connected underneath with metal contacts Know how to make parallel resistors

Components (2) Figure 1b Ribbon cable DIP plug Ribbon Cable with 16-pin Dip connector

Components (3) Resistors and Capacitors Lab handout has some examples of how to identify a resistor and a capacitor Example: red-black-brown = 200 ohm (red=2, black=0, brown=1)

Components (4) 74F04PC hex inverter chip It has 6 inverters in it Don’t forget to ground pin 7 and connect pin 14 to Vdd Figure 2b 74F04PC hex inverter pinouts

Propagation delay (1) What is it??? It is defined to be the time between the 50% transition points of the input and output

Propagation delay (2) T pLH and T pHL together define the total propagation delay T p = (T pLH + T pHL ) / 2 What causes the delay???

Capacitive Loading Treat capacitor as a water bucket The bigger the capacitor, the bigger the bucket Treat inverter as a faucet Filling the bucket takes time

Capacitive Loading vs. Propagation Delay Bigger fanout means lumping more capacitors at the output and thus longer time to fully charge up the capacitor

Ring Oscillator An odd number of inverters in a circular chain No stable operation point exists; thus oscillates Oscillates with a period T = 2 * T p * N In the lab, you will see that the frequency is dependent on the capacitive load at the output of the inverter Figure 2a 5-stage ring oscillator

Transmission Line (1) Figure 3 More realistic model of a wire A more realistic and accurate model of a wire is with inductance, resistance, and capacitance As the wire gets longer, the effect can dominate the speed of the signal

Transmission Line (2) First major problem is that signals with high frequencies might get smeared out Second major problem is that signals might reflect back and forth on the wire, taking a long time to settle down to a definite value

Termination Wire has a characteristic impedance Your job is to match that impedance at the end of the wire Example: If the ribbon cable has a characteristic impedance of 100 ohms, then you should have 100 ohms at the output end of the cable

Capacitive Coupling Figure 4 Capacitive Coupling In between two wires, there is always capacitive coupling which introduces an interference called cross-talk We need to eliminate the cross-talk

Shielding Shield the wires with either Gnd or Vdd

Note In this lab, you will not need to use Xilinx The lab is really easy and so you should spend more time studying for the midterm