1 BUSES DE DATOS CON MULTIPLEXORES Y DECODIFICADORES

2 Tri-State and Open-Collector Non-inverting buffer'sThe Third State Logic States: "0", "1" Don't Care/Don't Know State: "X" (must be some value in real circuit!) Third State: "Z" — high impedance — infinite resistance, no connection Tri-state gates: output values are "0", "1", and "Z" additional input: output enable (OE) When OE is high, this gate is a non-inverting "buffer" When OE is low, it is as though the gate was disconnected from the output! This allows more than one gate to be connected to the same output wire, as long as only one has its output enabled at the same time A X 1 OE 1 F Z 1 Non-inverting buffer's timing waveform "Z" "Z"

3 Tri-state and Open CollectorUsing tri-state gates to implement an economical multiplexer: When SelectInput is asserted high Input1 is connected to F When SelectInput is driven low Input0 is connected to F This is essentially a 2:1 Mux

4 Tri-state and Open CollectorAlternative Tri-state Fragment Active low tri-state enables plus inverting tri-state buffers

5 Tri-State and Open Collector4:1 Multiplexer, Revisited Decoder + 4 tri-state Gates

6 Tri-State and Open Collectoranother way to connect multiple gates to the same output wire gate only has the ability to pull its output low; it cannot actively drive the wire high this is done by pulling the wire up to a logic 1 voltage through a resistor OC NAND gates Wired AND: If A and B are "1", output is actively pulled low if C and D are "1", output is actively pulled low if one gate is low, the other high, then low wins if both gates are "1", the output floats, pulled high by resistor Hence, the two NAND functions are AND'd together!

7 Tri-State and Open Collector4:1 Multiplexer Decoder + 4 Open Collector Gates

8 Línea bidireccional de datosBUS BIDIRECCIONAL Registro de salida Línea bidireccional de datos D C Registro de entrada D C Sout Control del bus Salida de datos Control de entrada Sin Bus inhabilitado (alta impedancia)

9 Read-Only Memories ROM: Two dimensional array of 1's and 0'sRow is called a "word"; index is called an "address" Width of row is called bit-width or wordsize Address is input, selected word is output +5V +5V +5V +5V n 2 -1 i Word Line 0011 Dec j Word Line 1010 Bit Lines n-1 Address Internal Organization

10 Read-Only Memories Example: Combination Logic ImplementationF0 = A' B' C + A B' C' + A B' C F1 = A' B' C + A' B C' + A B C F2 = A' B' C' + A' B' C + A B' C' F3 = A' B C + A B' C' + A B C' ROM 8 word by 4 bits A B C F F 1 F 2 F 3 address outputs

11 Read-Only Memories Memory array 2 n words by m lines Decoder word bitsoutput lines address Decoder word

12 Read-Only Memories 2764 EPROM 8K x 8 16K x 16 Subsystem

13 DISEÑO Diseñar una unidad de memoria que posea las siguientes características: Un microprocesador de 20 bits de direcciones y 8 bits de datos (tipo 8088) manejará 4 bancos de memoria ROM del tipo 27256, para direccionar los 128 kbytes más altos de direcciones.

14 Mapa de memoria 1M FFFFF H 32K F8000 H F7FFF H F0000 H EFFFF H E0000 HE7FFF H E8000 H EFFFF H F0000 H F7FFF H F8000 H FFFFF H E0000 H 128K 1M 1FFFF H 00000 H

15 Mapa de decodificación de memoria Mapa de decodificación de memoria  Habilitación Selección

16 Decodificación de memoria27256 27256 27256 27256 A0 A0 A0 A0 M E M E M E A0 M E Direc A14 A14 A14 A14 A19 32Kx8 32Kx8 32Kx8 32Kx8 D0 D0 D0 D0 D0 Datos D7 D7 D7 D7 D7 OE OE OE OE RD CS CS CS CS MICRO PROCE SADOR A17 A18 A19 DEC Y0 Y1 Y2 Y3 A15 A16