610 likes | 786 Views
Documentation Standards. Circuit specification. Description of what the system is supposed to do, including a description of all inputs and outputs and the functions that are to be performed (page 312). Block diagram. Informal pictorial description of the system’s major functional modules.
E N D
Documentation Standards • Circuit specification. • Description of what the system is supposed to do, including a description of all inputs and outputs and the functions that are to be performed (page 312). • Block diagram. • Informal pictorial description of the system’s major functional modules. • Schematic or logic diagram. • Formal specification of the electrical components of the system, their interconnections, and all details needed to construct it (IC type, pin numbers, etc.).
Documentation Standards • Timing diagram. • Shows the values of various logic signals as a function of time, including cause and effect delays between critical signals. • Structured logic device description. • Describes the internal function of a programmable logic device (PLD), a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC).
Active Levels For Pins • Each signal should have an active level associated with it. • A signal is active high if it performs the named action or denotes the named condition when it is HIGH or 1. • A signal is active low if it performs the named action or denotes the named condition when it is LOW or 0. • A signal is said to be asserted when it is at its active level.
Buses • Collection of two or more related signal lines.
Timing Diagrams • Illustrate the logical behavior of the signals in a digital circuit as a function of time. • Causality: which input transition causes which output transition. • Propagation delay: the time it takes for a change at the input to produce a change at the output. • Note that propagation delays when outputs change from LOW to HIGH may differ from when they change from HIGH to LOW.
Decoders • A decoder is a multiple-input multiple-output logic circuit that converts coded inputs into coded outputs, where the inputs and outputs codes are different.
Decoders • Generally the input code has fewer bits than the output codes. • In a one-to-one mapping each input code word produces a different output code word. • Most commonly used codes: • For input: • The n-bit binary code. • For output: • The 1-out-of-m code.
Binary Decoder • Binary decoder: • The most common decoder. • n-to-2n. • The input code is the n-bit binary code. • The output code is the 1-out-of-m code.
Encoders • Reverses what a decoder does. • If a device’s output code word has fewer bits than the input code word then the device is usually called an encoder. • The simplest encoder is the binary encoder or the 2n-to-n encoder.
Priority Encoders • Priority encoders assigns priority to the input lines, so that when multiple lines are asserted at the same time, the encoder will produce the highest priority requestor.
Tri-State Devices • Devices whose outputs may be in one of three states, 0, 1, or Hi-Z (high impedance). • These devices have an extra input which is used to control if the output is floating (Hi-Z) or if it is behaving normally (page 385).
Multiplexers • Digital switch (page 398). • A multiplexer with n data sources requires s = log2n select lines. • Commercially available MUX have n= 1, 2, 4, 8 or 16. • EN enables the output.
Multiplexers • Y = EN S’ D0 + EN S D1
Multiplexers • General logic equation for a multiplexer is show above.
Demultiplexers • Digital switch (page 405). • A demultiplexer with n data outputs requires s = log2n select lines.
Exclusive-Or Gates • An exclusive-or (XOR) gate is a 2 input device whose output is 1 if exactly one of its inputs is 1 (page 410). • An exclusive-nor (XNOR) or equivalence is just the opposite.
Parity Circuits • Parity detectors are circuits that detects if the numbers of ones at its input is odd or even. • Used to detect errors during the transmission of binary information, by using a parity bit. • Parity bit is an extra bit included with the binary message to make the number of ones in the message either even or odd.
Comparators • Comparators are circuits that compare two binary words and indicates whether they are equal or not equal.