ELE22MIC Lecture 9 MULTIPLEXOR - DATA SELECTOR DEMULTIPLEXOR - DATA DISTRIBUTOR Parallel to Serial Data Conversion External Address Bus Latching Address Strobe timing Diagram Address Decoding using a 74ls138 Serial Peripheral Interface - SPI

Multiplexor - Data Selector Multiplex (MUX) many inputs to one output Switch selects the one signal source from many input signals. Like Stereo HiFi source selection switch

Two Input Multiplexor Output = (Input0 & Select#) | (Input1 & Select)

Four Input Multiplexor

Eight Input Multiplexor

74F151 8-Input MUX

Pin Names and Loading / Fanout

Mux vs DeMux

Parallel I/O

External Address Latch The Address Bus (Bits 0..7) and Data Bus (Bits 0..7) are multiplexed onto Port C when in expanded mode - external RAM, ROM & Peripherals We want to extend the Address Bus external to the 68HC11 - MAR is presented on C0..PC7/A0..A7/D0..D7 pins The 68HC11’s AS pin strobes high to load the address into the external address latch. The address latch presents the address onto the system’s external address bus.

External Address Latch

Address Strobe Timing Diagram

Applications of a de-multiplexor The Memory Chip Select device used on the original IBM PC is a 74LS138 de-multiplexor. The 74LS138 is used to activate 1 of 8 lines based on the conditions of the three binary select inputs A, B & C, and the three enable inputs. The 74LS138 Outputs are “Active Low”.

74LS138 8-Output DEMUX De-Multiplex one input to many outputs -Reverse operation of a multiplexor 74LS138 Truth Table

DeMultiplexor The 74LS138 can be implemented by the logic shown. The 54LS138 is identical in function, but can operate over the “Mil-spec” -55°C to 125°C Temperature Range. The 74LS138 can operate over the Commercial 0°C - 70°C Temperature Range.

Memory Select

Address Decoding & Chip Select A15 -> G1#, E -> G, A14 -> A2, A13 -> A1 R/W# -> A0 Chip is enabled when A15 = 0 & E is High Y2 = (A14#) & (A13) & Write (R/W#=0) & E Y3 = (A14#) & (A13) & Read (R/W#=1) & E Y4 = (A14) & (A13#) & Write (R/W#=0) & E Y5 = (A14) & (A13#) & Read (R/W#=1) & E

Write Data Timing Diagram

68HC11 Flash Technology (1)

68HC11 Flash Technology (2) Erasure of Cells is performed by providing a tunnelling voltage to the control gate which causes the charge on the floating gate to be removed. When read, each cell returns a logical ‘1’ value.

68HC11 Flash Technology (3) Programming of Cells is performed by providing a tunnelling voltage to the control gate which causes the charge to be placed on the floating gate. The write process writes the ‘0’s into each cell.

68HC11 I/O & Control Bits (2)

Parallel to Serial Conversion (1) Many digital systems use parallel binary data communication for speed (eg CPU data bus, printer etc), but when large distances are to be covered, the number of lines required makes the parallel transmission costly and thus undesirable. Referring to the schematic on the next page: A multiplexor can be used convert parallel data to serial data. Parallel data can be latched into the 74F373 gate, then output as a serial data stream from the 74F151 as a binary count sequence is created by the 74LS160.

Parallel to Serial Conversion (2)

Logic Family - Propagation Delay (H-L)

Logic Family - Propagation Delay (L-H)

Logic Family - Propagation Delay (3)

Bus Design Rules Bus lines have very low line impedances ( Ohms). Bus lines have to be terminated to prevent line reflections (signal distortion, circuit malfunctions due to undershoots). Take care of propagation times (25 ns/m). Settling time of signals on TTL-type buses is 2 x tp (no incident wave switching). Take care of control lines (clock, read, write, etc.). Provide shielding between control lines and data / address lines.

Bus Design Rules A multiplexed data and address bus reduces design problems (50% less signal lines and 50% less line driver). Driver output current is 100 mA/line. Provide adequate and low inductance GND return path (simultaneous switching)! Rule of thumb: 25% of all backplane connector pins have to be GND lines! Use multilayer boards with separate GND and Vcc plane for backplanes.

Acknowledgements I used Altium Protel 98 and Protel DXP to create these schematic diagrams Logic Timing Diagrams are from Texas Instruments (TI) Logic Selection Guide - Digital Design Seminar National Semiconductor data sheets 74LS138. Motorola 11rm.pdf Reference Manual