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F09_447_6_IO_Interfacing

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					ECE 447 Fall 2009

 Lecture 6: TI MSP430
     IO Interfacing
    Agenda

•   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
                                  IOL Low Level Output Current
                                      (sink current)
           buffer
                         I ≈ IF  IOLmax

                    R ≈ (VDD – VF – VOL) / IF
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
Base-Emitter          R1 ≈ (VDD – VF – VCE) / IF
Voltage               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                     debouncing delay

update display
ECE 447: Key Debounce in Hardware
Schmidt Trigger Debounce
Schmidt Trigger Performance
  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
                  OE
                        74HC245
                  DIR

                            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             Port 2
           .....



   OE                 . . . . . . .    OE
         74HC245                             74HC245
   DIR                                 DIR




     I/O device 1                            I/O device 8
                       . . . . . . .
ECE 447: Port Multiplexing
               MSP430



      Port 1                 Port 2


                                      8


            OE                            1G 2G
         DIR 74HC245                         74HC244


           I/O device 1                     I/O device 2
       (e.g., LCD display)                 (e.g., keypad)
ECE 447: Port Multiplexing
               MSP430



      Port 1                 Port 2


                                      8


            OE                            OE CP
         DIR 74HC245                        74HC373


           I/O device 1                      I/O device 2
       (e.g., LCD display)                 (e.g., 7 segment
                                                display)
    Summary

•   Internal/External Resistors
•   LEDs
•   Switch Debouncing
•   Buffers
•   Port Multiplexing

				
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posted:8/5/2012
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