Download Module 12: Elementary Input/Output

68000 Microprocessor
Module 12: Elementary Input/Output
I/O Concepts
Typical Organization of an I/O Device
Memory-mapped vs Isolated I/O
Elementary output with latch and LED
Elementary input with buffer and switch
7-segment interfacing
Keypad interfacing (extra)
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11-1
68000 Microprocessor
Input and Output (I/O)
One of the basic and essential features designed in a computer system is
its ability to exchange data with other external devices, and to allow the
user to interact with the system:
Input Devices include:
Output devices include:
Switches, Keyboards, Mice, Scanners, Cameras, etc.
Lamp/LED/LCD displays, Video monitors, Speakers, Printers, etc.
One or more interface circuits usually are used between I/O devices and
the CPU to:
Handle transfer of data between CPU and I/O interface.
Handle transfer of data between I/O device and interface.
Enable the CPU to request the status of data sent/received by the interface.
Common I/O interfaces:
Elementary I/O: Simple two-state devices such as LED and switches
Peripheral I/O: Non-trivial interaction with complex I/O devices such as
printers and keyboards
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11-2
68000 Microprocessor
Elementary I/O
Two-state peripheral devices
May involve more than one bit - e.g., BCD
Requirements
Bit signals can be written to output devices under program control
Bit signals can be read from input devices under program control
Devices required
Latches such as 74LS374 for output
Tri-state buffers such as 74LS244 for input
Example Output devices
LED, bulbs
Relay coils
7-segment display
Example Input devices
Push button
Proximity switch
Rotary BCD coder
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11-3
68000 Microprocessor
I/O Schemes
Address
Memory-mapped I/O
The I/O devices and memory
components are resident in the
same memory space
The microprocessor uses the
same instruction set for I/O
operations
Data
CPU
Read
Write
Memory
I/O Device
Isolated I/O
The microprocessor has a
separate address space for I/O
devices
The microprocessor has
dedicated instructions for I/O
operations
Special bus lines
Traditionally, Intel microprocessors use the isolated
I/O method, whereas Motorola microprocessors use
the memory-mapped I/O method.
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Address
CPU
Data
Memory I/O
Read
Write
I/O Port
Memory
I/O Device
11-4
68000 Microprocessor
Memory Mapped I/O
Part of the CPU's normal memory space is
dedicated to I/O operations and all I/O
ports look exactly like normal memory
locations.
Pros
Rich set of addressing modes for I/O
operation (e.g. bit manipulation)
No overhead on special instructions
00
000000
0000
004000
Program
Cons
Processor’s address space is allocated to
I/O devices
Risk of errors due to spurious accesses
(elaborated address decoding)
Lack of special purpose I/O signals to
control the I/O operation
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200000
80
800000
0000
80
800003
0003
Data
I/O
11-5
68000 Microprocessor
Peripheral I/O Concepts
I/O devices (peripheral devices)
Interface (peripheral) chip
Equipment that exchange data with a computer.
May itself contain a microprocessor!
Examples include printers, modems, keyboards, and disk drives.
A chip whose function is to synchronize data transfer between the CPU
and I/O devices.
An interface may consist of control registers, status registers, data
registers, and control circuitry.
An interface chip has data pins that are connected to the CPU and I/O
port pins that are connected to the I/O devices.
Data transfer between an I/O device and the CPU can be
proceeded bit-by-bit (serial) or in multiple bits (parallel).
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68000 Microprocessor
I/O Transfer Synchronization
The role of an interface chip
Synchronizing data transfer between the CPU and the interface chip.
Synchronizing data transfer between the interface chip and the I/O
device.
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68000 Microprocessor
Synchronizing µP and the Interface Chip
The polling method
Repeatedly checks the status of an I/O device (through a status
register of the associated I/O controller) until the desired condition is
met
This process is called polling
Polling involves overhead
Repeated testing of condition
Can be used to handle only anticipated event
The interrupt-driven method
Processor gets interrupted when a specified event occurs
Very efficient
Can be used to handle unanticipated events
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68000 Microprocessor
Polled I/O
Usually in the interface chip we have
Control register
Data register
a register where the processor is placing commands
input and output data registers where the data is made available
to/from the processor
Status register
a register that shows the status of the peripheral device
For input -- the microprocessor checks a status bit of the interface chip to
find out if new data has been received from the input device.
For output -- the microprocessor checks a status bit of the interface chip to
find out if it can send new data to the interface chip.
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68000 Microprocessor
A Generic Interface Chip
Bidirectional
data bus
CPU
Chip select
Register select
Register select
R/W
I/O data
Port B
register
I/O data
Bus
buffers
CS
RS1
RS0
Control
register
Timing
and
Control
Status
register
R/W
Inputs
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Port A
register
Register Selected
E
RS1
RS0
1
X
X
None
0
0
0
Port A Register
0
0
1
Port B Register
0
1
0
Control Register
0
1
1
Status Register
Control
Status
I/O
Device
68000 Microprocessor
Communicating with External Devices : Parallel Interface
Several ports each 1-byte wide
A full byte of data or more is transferred
on data cycle.
Example devices:
MC6821 PIA for 6800 CPU
MC68230 PIT for 68000 CPU
They are general purpose devices
Direction of data flow is programmable,
unlike in elementary I/O
Strobe and handshaking lines for more
efficient data transfers.
May interrupt the CPU if peripheral is
ready for data
These devices are often quite complex
and can operate in one of several modes
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68000 Microprocessor
Communicating with External Devices : Serial Interface
Serial data transmission
One bit is transferred for each clock cycle.
cheap, simple, easy to use
Connects the µP to another µP or
peripheral such as modem
Example devices:
MC6850 ACIA for 6800 CPU
MC68681 DUART for 68000 CPU
They are programmable devices
To use, the designer must program
number of bits per character (not
necessarily 8 bit/char), parity etc
Can interrupt µP when peripheral is ready
Possible to connect many device to
single serial line (multidrop).
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68000 Microprocessor
LEDs – Simples Elementary Output
An LED is a semiconductor
device that converts electrical
energy directly into a discrete
color of light
Can be visible or IR
White LEDs are blue LEDs plus
phosphor
1.5 - 2 V voltage drop
Typically draws 10-20 mA
Semiconductor
Diode
Wire
Bond
Anode
Lead
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Epoxy Lens
Epoxy Case
Reflective Cup
Cathode Lead
11-13
68000 Microprocessor
Turning on an LED
Note: A zero turns on the LED
No current
no light
+5V
+5V
R
LED
1
output pin
light
Current
+3.2V
+5V
R
LED
R=
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voltage
current
=
+0.4V
output pin
0
2.8
13 x 10 -3
~ 220 ohms
11-14
68000 Microprocessor
Interfacing LED with Latch
Unlike diodes, Light-emitting diodes have a
forward voltage drop from 1.7 to 2.5 volts and
most of them flow a forward current 10-20mA.
Poor circuit
Fair circuit
TTL outputs can't source above 1mA so the LED
will be very dim.
The LED will conduct heavily at about 2V and the
extra 3V has to be dropped in the TTL circuitry. This
causes high power dissipation in the TTL or the
LED fails.
Good circuit
The resistor limits the current. The resistance can
be calculated by assuming its voltage is about 2V
and the TTL output is 0.4V. See previous page.
The resistor should not less than 100ohm or the
LED would fail.
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11-15
68000 Microprocessor
Simple Interfacing CPU to LED
LED
EQU
$E00010
START
ORG
$1000
+5V
74LS374
MOVEQ
LOOP
DELAY
MOVE.B
MOVE.L
SUBQ.L
BNE
ROR.B
#1,D0
D0,LED
#200000,D1
#1,D1
DELAY
#1,D0
BRA
LOOP
END
START
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D7
D6
D5
D4
D3
D2
D1
D0
D7
D6
D5
D4
D3
D2
D1
D0
Q7
Dari penyahkod alamat
LE
Q1
OE
Q0
Dari
bas
data
68000
Q6
Q5
Q4
Q3
Q2
220ž
220ž
220ž
220ž
220ž
220ž
220ž
220ž
11-16
68000 Microprocessor
7-Segment Display
There are applications where you need to display a decimal
digit using a seven segment LED display.
The display could represent e.g. the number of times a
switch was pressed.
Digits 0-9 and hex A-F can be displayed by giving the
proper 7-segment codes
q
g
f
e
d
c
b
a
q
g
f
e
d
c
b
a
0
0
1
1
1
1
1
1
8
1
1
1
1
1
1
1
1
0
0
0
0
1
1
0
9
1
1
0
0
1
1
1
2
1
0
1
1
0
1
1
A
1
1
1
0
1
1
1
3
1
0
0
1
1
1
1
b
0
0
1
1
1
1
1
4
1
1
0
0
1
1
0
C
0
1
1
1
0
0
1
5
1
1
0
1
1
0
1
d
1
0
1
1
1
1
0
6
1
1
1
1
1
0
1
E
1
1
1
1
0
0
1
7
0
0
0
0
1
1
1
F
1
1
1
0
0
0
1
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68000 Microprocessor
Types of 7 Segment Displays
Common-anode (CA):
LED ON when Output is LOW.
8 cathode legs, 1 anode
Common-cathode (CC):
LED ON when Output is HIGH.
8 anode legs, 1 cathode
TTL and CMOS devices are
normally not used to drive the
common-cathode display directly
because of current (mA)
requirement. A buffer circuit is
used between the decoder chips
and common-cathode display
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11-18
68000 Microprocessor
Using Lookup Tables
Using lookup table to drive the 7-segment displays will result
in efficient programs.
; Driver subroutine to display digits
; Digit to be displayed in D0
; Address of interface is "DISP"
; Table of segments is "DIGTAB"
DSPLAY: MOVEM.L A0/DO,-(A7)
MOVE.L #DIGTAB,A0
EXT.W
D0
EXT.L
D0
ADD.L
D0,A0
MOVE.B (A0),DISP ;OUTPUT
MOVEM.L (A7)+,A0/D0
RTS
; Format of 7-segment: 0gfedcba Common-anode
DIGTAB: DC.B
%00111111, %00000110
DC.B
%01011011, %01001111
DC.B
%01100110, %01101101
DC.B
%01111101, %00000111
DC.B
%01111111, %01100111
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11-19
68000 Microprocessor
Elementary Input with 68000
Simplest input device is the three-state buffer .
Buffer is very fast (around 20 ns), so DTACK* does not need to be
delayed.
Needs only 1 memory address.
68000
Penimbal
D0-D7
atau
D8-D15
D0-D7
A1-A23
Penyahkod
alamat
OE
AS
Vcc
DTACK
Punca DTACK
daripada peranti lain
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11-20
68000 Microprocessor
Interfacing Switch with 68000
Poor circuit
Logic 1 is stable when the switch is
closed. But when the switch is open, the
input floats to a noise-sensitive high rather
than a low. An open TTL pin is usually
read as logic 1 but the pin may picks up
noise like an antenna. Impossible to get a
stable low input.
Fair circuit
Requires a very small the pull-down
resistor. Otherwise, the pin will rise above
0.9V when the resistor passes the 1.6mA
sinking current. When the switch is
closed, the circuit waste a large current
since virtually no current flows into the
pin. The only advantage is that a switch
closure gives logic 1.
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11-21
68000 Microprocessor
Interfacing Switch with 68000
Good Circuit
When the switch is open, the 10k resistor
supplies very small current needed for logic
1. When it is closed, the port pin is short to
ground. The voltage is 0V and all the sinking
current requirement is met, so it is logic 0.
The 10k resistor will pass 0.5 mA (5 Volt/10k
ohm). Thus the circuits waste very little
current in either state. The drawback is that
the closure of switch gives logic 0 and
people like to think of a switch closure gives
logic 1. But this is not a matter because it is
easy to handle in software.
To conclude, driving a TTL input should
always consider current sinking (pulling
input to 0V).
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11-22
68000 Microprocessor
Interfacing Switch with 68000
;
; ATURCARA UNTUK MENGUJI SUIS
;
SUIS
EQU
$800001
;ALAMAT SEBENAR BERGANTUNG SISTEM
LED
EQU
$A00001
+5V
74LS244
ORG
$1000
1kž
D7
Q7
D7
START
MOVE.B SUIS,LED
1kž
Q6
D6
D6
Ke
Q5
D5
BRA
START
1kž
bas
Q4
D4
D5
data
END
1kž
Q3
D3
68000
D2
D1
D0
Q2
Q1
Q0
D4
D3
D2
Dari penyahkod alamat
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OE1
D1
OE2
D0
1kž
1kž
1kž
1kž
11-23
68000 Microprocessor
Switch Debouncing
When mechanical switches are
opened or closed there are brief
oscillations due to mechanical
bouncing (switch bounce).
VDD
Set
Q
R
Q
Reset
key bounce, tBOUNCE
Set
Reset
R
(a) Set-reset latch
VDD
4050
3 basic hardware debouncing
methods
SR latches
Non-inverting CMOS gates
Integrating debouncer
R
The most popular and simple one has
been the wait and see method.
The program simply waits for about
10 ms and reexamines the same key
again to see if it is still pressed.
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Vout
(b) CMOS gate debouncer
VDD
R
Vout
C
H
Threshold level
L
Switch closed
(c) Integrating RC circuit debouncer
Figure 7.42 Hardware debouncing techniques
11-24
68000 Microprocessor
One Hardware Debounce Method
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68000 Microprocessor
BCD-to-7-Segment Decoder/Driver
(a)
BCD-to-7segment
decoder/driv
er driving a
commonanode 7segment
LED display;
(b) segment
patterns for
all possible
input codes.
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11-26
68000 Microprocessor
Keypad Decoding: 4x4 or 4x3 keypad
CPU
checks
these lines
CPU sends
low signal
to these
lines
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11-27
68000 Microprocessor
Decoding the Keyboard Matrix
Example: press 5 and E
0
1
2
3
Row 0
4
5
6
7
Row 1
8
9
A
B
Row 2
C
D
E
F
Row 3
Pressing 5
Row 1 Col 1
01
01
Pressing E (=14)
Col 0 Col 2
Col 1 Col 3
Row 3 Col 2
11
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10
11-28
68000 Microprocessor
74C922 Keypad Encoder
Performs the decoding and debouncing in hardware
Connect to column pins
Connect to row pins
Connect to oscillator
Low to enable output
1
2
3
4
11
10
8
7
5
6
13
Y1
Y2
Y3
Y4
X1
X2
X3
X4
DOA
DOB
DOC
DOD
OSC
KBM
OE
DA
74C922
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17
16
15
14
12
Row & col position
DA means Data
Available
High when a key
is pressed
11-29
68000 Microprocessor
74C922 Keypad Encoder
74LS244
74C922
1
2
3
4
11
10
8
7
L
C1
0.068µF
Y1
Y2
Y3
Y4
X1
X2
X3
X4
5
6
DOA
DOB
DOC
DOD
17
16
15
14
OSC
KBM
13
DA
OE
C1
0.68µF
12
D0
D1
D2
D3
D4
D5
D6
D7
To 68000
Data Bus
H
U9C
6
5
74LS14
Wait until DA pin high before accepting the 4-bit data
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11-30
68000 Microprocessor
Interfacing Other Devices with Latches
Buzzers:
Relays:
A relay is an excellent way of isolating
the latch & CPU from high voltage
circuits.
Use a relay to control motors,
solenoids, lamps etc.
Most latches do not have the current
capacity to drive a buzzer directly so a
transistor is added.
Diode protects coil transient voltages.
+5V
+5V
Relay
2N3904
+
D1
N.S.
Piezo
Buzzer
D1
-
To Indicating
Light
N.O.
12 VDC
From latch
2N3904
NPN
2.2KΩ
C
Q1
B
C
From latch
Q1
B
E
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2N3904
NPN
2.2KΩ
DC Motor
E
11-31
68000 Microprocessor
Example: System Having Only Elementary I/O
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11-32
68000 Microprocessor
Liquid Crystal Displays: HD44780
The HD44780 is an industry standard LCD controller.
An I/O port can easily be used to interface with the HD44780.
By controlling the value of RS and R/W the MCU can easily
either sends instructions or data to the controller.
Most operations require 40 micro seconds to complete.
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11-33
68000 Microprocessor
Multiplexing
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