Download CSE22MAL - Test Page for Apache Installation

ELE2MIC Lecture 19
•
•
•
•
•
MULTIPLEXOR - DATA SELECTOR
DEMULTIPLEXOR - DATA DISTRIBUTOR
External Address Bus
Timing Diagrams
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
Two Input Mux
Truth Table
Select Output
0 Input 0
1 Input 1
Output = (Input0 & Select#)
| (Input1 & Select)
Four Input Multiplexor
4 Input Multiplexor
Truth Table
Select Line:
1
0 Output
0
0 Input 0
0
1 Input 1
1
0 Input 2
1
1 Input 3
Eight Input Multiplexor
Eight Input Multiplexor
Truth Table
Select Line: Output
2
1
0
0
0
0 Input 0
0
0
1 Input 1
0
1
0 Input 2
0
1
1 Input 3
1
0
0 Input 4
1
0
1 Input 5
1
1
0 Input 6
1
1
1 Input 7
74F151 8-Input MUX
74F151 8-Input MUX
Pin Names and Loading / Fanout
Mux vs DeMux
AVR
On-Chip SRAM Timing
AVR Data Ram
• When 4KB is enough RAM for an
application, the On-Chip SRAM is
sufficient.
• When 4KB is insufficient, an external RAM
chip can be used to expand the address
range to 64K bytes.
• There are four memory configuration
options for external RAM.
AVR External Data Ram
• By setting the XMEM bit to 1, the eXternal
MEMory interface is enabled, and the
dedicated external memory control lines
become active.
• The dedicated controls are ALE#, RE#,
WE# and the multiplexed address & data
bus bits 0..7 and the address bits 8..15 take
control, overriding the port A, port C and
port G (pins 0..2) functions.
AVR External Data Ram
• The dedicated control signals are:
• RE# - Read Enable - Active Low
– Data is read from the external memory (or
device) into the AVR microcontroller.
• WE# - Write Enable - Active Low
– Data is written from the AVR to the external
memory (or device).
AVR External Data Ram
• ALE - Address Latch Enable - Active High.
• When ALE transitions high, the Memory
Address Register is asserted onto the
Multiplexed Address & Data bus lines
• the bus enters a write-address phase
• the address is latched into an external
address latch which is used to form an
system’s external address bus.
AVR External Data Ram
• ALE - Address Latch Enable - Active High.
• When ALE is low, a data phase commences
and data can be read or written to the
external memory or device.
• (external in this context refers to off-chip
memory)
AVR External Address Latch
AVR External Mem Timing
AVR External Mem Timing
68HC11 External Address Latch
HC11 Strobe Timing Diagram
Applications of a de-multiplexor
• The Memory Chip Select device used on the
original IBM PC is a 74xx138 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
EEPROM Technology (1)
EEPROM 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.
EEPROM 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.
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 (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
• Altium Protel 98, DXP or Altium 6 to create
these schematic diagrams
• Logic Timing Diagrams are from Texas
Instruments (TI) Logic Selection Guide Digital Design Seminar
• National Semiconductor data sheets 74LS138.
• http://www.sea.vg/mic/2007/Atmel/Atmega128
ManualDoc2467.pdf
• Paul Main - sea.net.au, October 2007