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Lecture 6: TI MSP430 IO Interfacing
ECE 447 Fall 2009 Lecture 6: TI MSP430 IO Interfacing
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Agenda Internal/External Resistors LEDs Switch Debouncing Buffers
Port Multiplexing
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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
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Internal Pull Up Resistor
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ECE 447: Light Emitting Diode (LED)
+ Forward current Forward voltage IF VF – Typical values for common LEDs IF = mA VF = V
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ECE 447: LED
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ECE 447: LED Interfacing VDD=3 V R=? VDD – VF – VOL Forward current
I ≈ IF VF Forward voltage VOL Low Level Output Voltage VOL Low Level Output Current (sink current) IOL buffer I ≈ IF IOLmax R ≈ (VDD – VF – VOL) / IF
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ECE 447: LED Interfacing VDD=3 V R1=? VDD – VF – VCE Forward current
I ≈ IF VF Forward voltage VCE Collector-Emitter Voltage R2=? ICE Collector-Emitter Current I ≈ IF ICEmax IOH IOHmax VBE R1 ≈ (VDD – VF – VCE) / IF R2 (VOH – VBE) / IOHmax Base-Emitter Voltage
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ECE 447: MAN-72: 7-Segment LED
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ECE 447: Single Switch/Push Button
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ECE 447: Switch Key Bouncing
key bounce, tBOUNCE key bounce, tBOUNCE typically, tBOUNCE < 10 ms
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ECE 447: Key Debounce in Software
key bounce, tBOUNCE key bounce, tBOUNCE typically, tBOUNCE < 10 ms debouncing delay debouncing delay update display
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ECE 447: Key Debounce in Hardware
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Schmidt Trigger Debounce
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Schmidt Trigger Performance
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MSP430 and Compatible Logic Devices
HCT Family Uses CMOS technology but TTL (Transistor-transistor 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 74HC245 DIR OE Data Out
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ECE 447: Octal Buffer 74HC245 Note: All 8 outputs controlled by one enable, OE
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ECE 447: Octal Buffer 74HC244 Note: Outputs controlled by two enables, 1OE & 2OE (1OE controls 1A0-1A3, 2OE controls 2A0-2A3)
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ECE447: Latches 74HC373
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ECE447: Registers 74HC374
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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.
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ECE 447: Port Multiplexing
MSP430 Port 1 Port 2 74HC245 DIR OE 74HC245 DIR OE I/O device 1 I/O device 8
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ECE 447: Port Multiplexing
MSP430 Port 1 Port 2 8 OE 1G 2G DIR 74HC245 74HC244 I/O device 1 (e.g., LCD display) I/O device 2 (e.g., keypad)
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ECE 447: Port Multiplexing
MSP430 Port 1 Port 2 8 OE OE CP DIR 74HC245 74HC373 I/O device 1 (e.g., LCD display) I/O device 2 (e.g., 7 segment display)
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Summary Internal/External Resistors LEDs Switch Debouncing Buffers
Port Multiplexing
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