# EET 252 Digital Systems II Professor Nick Reeder.

## Presentation on theme: "EET 252 Digital Systems II Professor Nick Reeder."— Presentation transcript:

EET 252 Digital Systems II Professor Nick Reeder

Reminders  Please turn off cell phones.  No food or soft drinks in the classroom.  Stow water bottles at floor level.

EET 252 Unit 1 Review; Arithmetic Logic Units  Review Floyd, Chapter 2 and Chapters 6 to 9.  Study Unit 1 e-Lesson.  Do Lab #1.  Homework #1 and Lab #1 due next week.  Quiz next week.

Overview of This Week’s Lecture  Unused Inputs  Pull-up Resistors  Review of Logical Operations  Review of Arithmetic Operations  Arithmetic Logic Units

Unused Gate Inputs (Floyd, p. 143)

Tying Other Inputs HIGH or LOW  The same rules apply to other input pins (such as enable inputs).  To force them HIGH, tie them to V CC through a 1-kΩ resistor for TTL.  To force them LOW, tie them to ground.

But what about the Trainer’s Data Switches?  When you connect a TTL input pin to one of the trainer’s data switches and set the switch to HIGH, do you need a 1−kΩ resistor between the switch and the input pin?

From the Trainer’s Schematic Diagram

Don’t Tie Outputs HIGH or LOW  Remember that, in general, unused output pins should be left unconnected. Do not tie them HIGH or LOW.

Overview of This Week’s Lecture  Unused Inputs  Pull-up Resistors  Review of Logical Operations  Review of Arithmetic Operations  Arithmetic Logic Units

Pull-Up Resistor  A pull-up resistor is a resistor with one end connected to a HIGH voltage level and the other end connected to a point in a digital circuit.  The resistor’s purpose is to pull up the voltage at that point to a HIGH level when it would otherwise be in a float condition (not HIGH or LOW).

Pull-Up Resistor (Continued)  Three common uses of pull-up resistors: 1. On a switch connected to an input pin. 2. When interfacing two logic families that use different voltage levels. 3. On open-collector outputs (TTL) or open-drain outputs (CMOS).

First Use for Pull-Up Resistor: Switch on an Input Pin  Good explanation at http://www.seattlerobotics.org/encoder /mar97/basics.html http://www.seattlerobotics.org/encoder /mar97/basics.html  See also example on next slide from your textbook.

Second Use for Pull-Up Resistor: Interfacing Logic Families  Suppose we want to connect the output of a TTL gate to the input of a CMOS gate.  A TTL HIGH output may be as low as 2.4 V.  But CMOS expects at least 3.3 V for a HIGH.  Problem: The CMOS gate may interpret the TTL gate’s HIGH output as a LOW.  Solution? See next slide.

Digital Electronics: A Practical Approach, Eighth Edition William Kleitz Copyright ©2008 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Figure 9–27 Using a pull-up resistor to interface TTL to CMOS.

Third Use for Pull-Up Resistor: Open Collector Outputs  As we’ll see next week, some TTL gates are intentionally designed with a missing transistor on the output. To work properly, such an output needs an external pull-up resistor.  One of the output pins on the chip in tonight’s lab is an open-collector output, and therefore needs a pull-up resistor.

Overview of This Week’s Lecture  Unused Inputs  Pull-up Resistors  Review of Logical Operations  Review of Arithmetic Operations  Arithmetic Logic Units

How Many Logical Operations?  You already know how to perform some logical operations on two input bits, A and B. Examples: X = AB X = A+B  Question: How many possible logical operations are there on two input bits?

How Many Logical Ops? (Continued)  Let’s list them all: AB 00 01 10 11

Overview of This Week’s Lecture  Unused Inputs  Pull-up Resistors  Review of Logical Operations  Review of Arithmetic Operations  Arithmetic Logic Units

Overview of This Week’s Lecture  Unused Inputs  Pull-up Resistors  Review of Logical Operations  Review of Arithmetic Operations  Arithmetic Logic Units

Arithmetic Logic Unit (ALU)  Central to any computer system is its ALU, which performs mathematical and logical operations on data.  In modern systems, the ALU is contained on the computer’s microprocessor chip. (See next slide, or Figure 13-3 on p. 724 of Floyd.)  In older systems, the ALU was a separate chip, such as the 74181.

74181 ALU chip  Can perform 16 logical operations (bit- by-bit) and 16 arithmetic operations on two four-bit input numbers.  Data Sheet shows two truth tables: Table 1 if you’re considering data inputs & outputs to be active-low, and Table 2 if you’re considering data inputs & outputs to be active-high.  Data Sheet: 74LS18174LS181

Positive Logic versus Negative Logic  Any gate or logic circuit can be looked at from either an active-HIGH perspective (“positive logic”) or an active-LOW perspective (“negative logic”).  Example: The gates on a 7408 chip can be considered either positive-AND gates or negative-OR gates.  Data Sheet: 74087408

74181 ALU (Continued)  Caution: In the “Arithmetic Operations” columns of the 74181 truth tables, the + symbol always means logical OR, not addition. The word “PLUS” is used for addition.

74181 ALU (Continued)  Fourteen Input Pins: M is the mode pin (arithmetic or logic). S0 to S3 select the operation performed. C n is the carry-in bit, used only during arithmetic ops (ignored during logic ops). A0 to A3 form one of the 4-bit inputs. B0 to B3 form the other 4-bit input.

74181 ALU (Continued)  Eight Output Pins: F0 to F3 form the 4-bit output. C n=4 is carry-out bit, meaningful only for arithmetic ops. (Ignore it for logic ops.) A=B is comparison bit, meaningful only when performing “A MINUS B” operation. (Ignore it for all other ops.) P and G are carry-look-ahead bits for high-speed arithmetic, when 74181 is used in conjunction with 74182 chip. (We’ll ignore these.)