Download ECE 447 Fall 2009

ECE 447 Fall 2009
Lecture 6: TI MSP430
IO Interfacing
Agenda
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Internal/External Resistors
LEDs
Switch Debouncing
Buffers
Port Multiplexing
ECE 447: MSP430 Current Limits
VDD= 3 V
Maximum sink current
VOHmin= 2.4 V
IOLmax = 40 mA
Maximum source current
IOHmax= 40 mA
VOLmax= 0.6 V
GND=0 V
Internal Pull Up Resistor
ECE 447: Light Emitting Diode (LED)
+
Forward current
Forward voltage
IF
VF
–
Typical values for common LEDs
IF = 10-20 mA
VF = 1.7-2.4 V
ECE 447: LED
ECE 447: LED Interfacing
VDD=3 V
R=?
VDD – VF – VOL
Forward current
I ≈ IF
VF Forward voltage
VOL Low Level Output Voltage
VOL
buffer
IOL Low Level Output Current
(sink current)
I ≈ IF  IOLmax
R ≈ (VDD – VF – VOL) / IF
ECE 447: LED Interfacing
VDD=3 V
R1=?
VDD – VF – VCE
Forward current
I ≈ IF
R2=?
VBE
Base-Emitter
Voltage
VF Forward voltage
VCE Collector-Emitter Voltage
ICE Collector-Emitter Current
I ≈ IF  ICEmax
IOH  IOHmax
R1 ≈ (VDD – VF – VCE) / IF
R2  (VOH – VBE) / IOHmax
ECE 447: MAN-72: 7-Segment LED
ECE 447: Single Switch/Push Button
ECE 447: Switch Key Bouncing
key bounce, tBOUNCE
key bounce, tBOUNCE
typically, tBOUNCE < 10 ms
ECE 447: Key Debounce in Software
key bounce, tBOUNCE
key bounce, tBOUNCE
typically, tBOUNCE < 10 ms
debouncing delay
update display
debouncing delay
ECE 447: Key Debounce in Hardware
Schmidt Trigger Debounce
Schmidt Trigger Performance
MSP430 and Compatible Logic Devices
• HCT Family
– Uses CMOS technology but TTL (Transistortransistor logic) input logic levels
– Slower performance than HC, replaces TTL
• HC Family
– High performance CMOS Logic chips
– Combines strengths of TTL with CMOS
– Can operate at lower voltages (i.e. 3V)
Data In
OE
DIR
74HC245
Data Out
ECE 447: Octal Buffer 74HC245
Note: All 8 outputs controlled by one enable, OE
ECE 447: Octal Buffer 74HC244
Note: Outputs controlled by two enables, 1OE &
2OE (1OE controls 1A0-1A3, 2OE controls 2A0-2A3)
ECE447: Latches 74HC373
ECE447: Registers 74HC374
ECE 447: Port Multiplexing
• Allow multiple devices to share the input
and output ports of the microcontroller.
• Additional hardware typically needed to
support connection and selection of
peripheral devices.
• Software typically selects the peripheral
devices using additional output pins.
• Some I/O devices have built-in enable
signals.
ECE 447: Port Multiplexing
MSP430
Port 1
.....
OE
DIR
74HC245
I/O device 1
Port 2
. . . . . . .
. . . . . . .
OE
DIR
74HC245
I/O device 8
ECE 447: Port Multiplexing
MSP430
Port 1
Port 2
8
OE
DIR 74HC245
I/O device 1
(e.g., LCD display)
1G 2G
74HC244
I/O device 2
(e.g., keypad)
ECE 447: Port Multiplexing
MSP430
Port 1
Port 2
8
OE
DIR 74HC245
I/O device 1
(e.g., LCD display)
OE CP
74HC373
I/O device 2
(e.g., 7 segment
display)
Summary
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•
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Internal/External Resistors
LEDs
Switch Debouncing
Buffers
Port Multiplexing