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# chapter3notes - (CHAPTER THREE DIGITAL LOGIC STRUCTURES...

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(CHAPTER THREE) DIGITAL LOGIC STRUCTURES SIMPLE SWITCH CIRCUIT - switch open: no current through circuit; light is off - switch closed: short circuit across switch; current flows; light is on - this simple idea is what makes the computer work VOLTAGE - potential difference” à kind of like water pressure - to produce an electric current along a wire it is necessary that there b e a difference in potential (pressure) between the two ends of the wire - definition: the pressure required for a current ---blahblah blah CURRENT - flow of electrons forced into motion by voltage - current is measured in amperes (A) but we use milli-amps or micro-amps in electronics - ampere = 1 coulomb (6.28 x 10 18 ) per second RESISTANCE - resist” the flow of electrons - measured in ohms THE TRANSISTOR - most computers today are constructed out of metal-oxide semiconductor (MOS) transistors - if they somehow start misbehaving, we are completely screwed over - there are two types of MOS transistors: n-type and p-type - microprocessors contain millions of transistors (it keeps going according to Moore’s law)

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- logically, each transistor acts like a switch and these are combine to implement logic functions such as AND, OR, and NOT a) n-type: - it works with a gate - there are 3 terminals: the gate, the source, and the drain - if the transistor is supplied with 2.9 volts, the connection is fixed (closed circuit) - if the transistor is supplied with 0 volts, the connection is broken (open circuit) - if you only have an n-type: fixed circuit: output of 0 b) p-type: - when the gate is supplied with 0 volts, the transistor fixes the connection (closed) - if the transistor is supplied with 2.9 volts, it acts as an open circuit c) circuit that contains both types - p-type and n-type are complementary - if a circuit contains both, they are called complementary metal-oxide semiconductor (CMOS) circuits - duality: o always a path to exist from output to either the power source or ground o all paths to the volage source must be the complement of the set of all paths to ground o this can be easily accomplished by defining one in terms of the NOT of the other o p-type in parallel and n-type in series LOGIC GATES - logic gates are one step up from transistors - they are called AND, OR, and NOT gates - 0 volts = ‘0’ - 2.9 volts = ‘1’ - if you push it down, it connects. If you pop it back up, it disconnects - logic functions can be represented as a truth table, a logic expression, or a logic circuit a) the NOT gate (inverter): - we call this circuit a NOT gate, or an inverter
- has one p-type and one n-type transistor - n-type connected to ground, p-type connected to the power source - if the p-type transistor is connected (input of 0 volts), then the output is 2.9 volts. - If the n-type transistor is connected (input of 2.9 volts), then the output is 0 volts b) the OR and NOR gates: - NOR: has two p-type and two n-type transistors - NOR: both p-types have to be connected (input of 0 volts) for the output to be 2.9 volts - OR: has three p-type and two n-type transistors - OR: if either p-types are connected (input of 0 volts), then the output is 2.9 volts

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chapter3notes - (CHAPTER THREE DIGITAL LOGIC STRUCTURES...

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