CS150 Fall 2000 Lab6 - Nasty Realities

EECS150 Fall 2000 Lab6 - Nasty Realities by Sammy Sy Dont let the title scare you. Even though the final project might be nasty, this lab is not. Sections

Some information on the electrical components in this lab Capacitance vs Propagation delay Reflection and Termination Capacitive Coupling Extra Xilinx tips Components (1) Breadboard Which holes are connected with which?

Figure 1a Breadboard connections Components (2) Ribbon cable Is interference a problem? Well find out Figure 1b Ribbon cable DIP plug Components (3)

Resistors and capacitors Time to figure out how those colors on resistors and numbers on capacitors mean Detailed description on the lab handout Example: red-black-brown = 200 ohm (red = 2, black = 0, brown 1) Components (4)

74F04PC hex inverter chip Essentially just a bunch of inverter gates Remember an inverter requires input, and VCC and GND. Figure 2b 74F04PC hex inverter pinouts

Components (5) We also need a bunch of normal wires to hook them up Remind yourself how to play with the pulse generator More practice with the mighty oscilloscope What to do in this lab

You will be building several simple circuits to measure gate delays, and observe how reflection and capacitance affect electrical signals. The lab handout gives you the step-bystep procedures. Some motivations. Capacitance vs Propagation delay (1)

Capacitance is evil Why? It limits how fast a gate transition can take place. In fact, all conductors are subject to this evilness - for example, two wires together. Capacitance vs Propagation delay (2)

Ring oscillator No stable state Therefore, those inverters will be switching as fast as their capacitance allows. T = 2 * delay * N Figure 2a 5-stage ring oscillator

Reflection and Termination (1) Whats reflection? If you view electrical signal as propagation of E&M waves, then a wire is no different than optical fiber. When the signal hits a dead end, it bounces back and interferes with itself. Can be solved by proper termination.

Reflection and Termination (2) Terminate by eliminating those dead ends. Add some load (i.e.: resistance). I once heard that TV cable companies detect undesired fanouts by sending a strong pulse to the wire and count the

number of reflections received. Now you can outsmart them. Capacitive Coupling (1) Recall the ribbon cable Signal on wire interferes with the neighboring wires. Its especially noticeable in long and crowded wires. An undriven wire is more easily affected.

Capacitive Coupling (2) For example, say we have 3 wires A, B and C arranged linearly (i.e. B is between A and B). Let A = some signal Let B = undriven Let C = some other signal

Because B is undriven, its easily influenced by A and C Capacitive Coupling (3) Simultaneously, B affects A and C. We can introduce a shield to protect the signals in A and C. If we ground B, it becomes much less

likely to be manipulated by A and C. Thus, A and C wont step onto each others toes. Xilinx Tips (1) This lab doesnt need Xilinx (relieved!) Difference between FDC and FDR (similarly applies to CC8CE vs CC8RE,

etc) FDC = D-type flipflop with asynchronous clear FDR = D-type flipflop with synchronous reset Xilinx Tips (2) Keep the clock signal clean! Read the implementation log

Once youve made something into a macro, the original schematic file is not used by the project anymore. Ground yourself