ELE22MIC Lecture 10• MULTIPLEXOR - DATA SELECTOR
• DEMULTIPLEXOR - DATA DISTRIBUTOR
• External Address Bus Latching
• Address Strobe timing Diagram
• Address Decoding using a 74LS138
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)
Two Input MuxTruth Table
OutputSelectInput 00Input 11
Four Input Multiplexor4 Input MultiplexorTruth TableSelect Line:
Output01Input 000Input 110Input 201Input 311
Eight Input Multiplexor
Eight Input MultiplexorTruth Table
OutputSelect Line:012
Input 0000Input 1100Input 2010Input 3110Input 4001Input 5101Input 6011Input 7111
74F151 8-Input MUX
74F151 8-Input MUX
Pin Names and Loading / Fanout
Mux vs DeMux
Parallel I/O
External Address Latch• The Address Bus and Data Bus are multiplexed,
and output to the 68HC11’s Port C pins. So that we can utilise external RAM, ROM & Peripherals (in expanded mode MODA/B)
• The M.A.R. is presented on A0..A7/D0..D7 pins• The AS pin goes high to load the low 8 bits of the
address into the external address latch. • An external address latch extends the internal
memory address to create a 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 DEMUXDe-Multiplex one input to many outputs -Reverse operation of a multiplexor74LS138 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)
Logic Family - Propagation Delay (H-L)
Logic Family - Propagation Delay (L-H)
Logic Family - Propagation Delay (3)
Bus Design RulesBus lines have very low line impedances (20 .. 40 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 drivers).
• 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
Top Related