CS251 - Computer Organization and Design - Spring 2008

Lecture 4 - Gates and Transistors

Practical Details

Gates

We can draw them

Make a one bit adder out of NAND, AND and OR

Transistors

How a MOS-FET transistor works

• remember the resistor
• MOS-FET transistors are voltage dividers

There are two types of MOS-FET transistor

1. nMOS
   • source and drain are doped n
   • channel is doped p
   • Enough positive charge on the gate draws electrons into a conducting "inversion layer". The transistor switches from insulating to conducting.
2. pMOS
   • source and drain are doped p
   • channel is doped n
   • Enough negative charge on the gate draws holes (inverse electrons) into a conducting "inversion layer". The transistor switches from insulating to conducting.

Problems:

1. When nMOS or pMOS conduct they draw current: not good because it drains the battery and creates heat

Solution: cMOS

• Use a complementary MOS-FET as the resistor

More complex patterns of transistors create more complex patters of gates

From now on, whew, we can leave the transistors to the electrical engineers.

• But there is one thing we have to remember: never drive a single input with two outputs

Back to Gates

Multiple gates on a chip is the level of technology we call medium scale integration, MSI.

Here is the sort of thing you could buy in that era

Decoder/Encoder

Example: 2 to 4 decoder.

Where are decoders used

1. Buses: read/write line
2. Bus arbitration
3. DRAM decoding

Multiplexor/Demultiplexor

Example: 2 to 1 multiplexor

Where are multiplexors used

1. Driving a bus, together with 3-state outputs
2. Inserting sprites in video games
3. USARTs

Drawing a bus with a slash

General Logic

ROM

PLA

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