1. Lecture No. 2 Computer Logic Design

2. Binary Digits The two state number system is called binary. Its two digits are 0 and 1 A binary digit is called a bit. In digital circuits, two different voltage levels are used to represent the two bits

3. Logic There are two types of logic Positive Logic –High = 1, Low = 0. Negative Logic –High = 0, Low = 1. Codes –In digital system such as computers, combination of the two states, called codes –used to represent numbers, symbols, alphabetic characters and other type of information

4. Logic Levels The voltages used to represent a 1 and 0 are called logic levels. One voltage level represent a HIGH Another voltage level represent a LOW In a practical digital circuit, a HIGH can be any voltage between a specified minimum value and specified maximum value. A LOW can be any voltage between a specified minimum value and specified maximum value. There is no overlap between accepted HIGH levels and the accepted LOW levels.

5. Voltage Range for Logic Levels 2.0 V – 5V (TTL), 3.5 V (CMOS) TTL (Transistor –Transistor Logic) CMOS (Complementary Metal Oxide Semiconductor) 0.5 V – 0.8V (TTL), 1.5 V (CMOS)

6. Digital System Representation Two binary digits also called –Two Voltage Levels (5V, 0V) –Two States (On/High, Off/Low) –Two Values (1, 0) –Two Logic Levels (True, False) How to represent other quantities?

7. Digital Waveforms Digital waveforms consist of voltage levels that are changing back and forth between the HIGH and LOW levels or states

8. Waveform Characteristics

9. Periodic –Repeat itself at a fixed interval Non-periodic –Does not repeat itself at a fixed interval

10. Waveform Characteristics Frequency –Rate at which it repeats itself (hertz (Hz)) Time Period –Time to complete one cycle (sec) Duty Cycle –Ratio of the pulse width to the period

11. Computer Pictorial Diagram

12. Information Processing Numbers Text Formula and Equations Drawings and Pictures Sounds

13. Logic Gates Processors of Binary Information Basic Gates: –AND, OR, NOT Other frequently used Gates: –NAND, NOR, XOR, XNOR Integrated Circuits (ICs)

14. Logic Gate Symbol and ICs 123 456 G N D V c c 1 3 1 2 1 1 1 0 98 7400