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ECE 447: Lecture 6

ECE 447: Lecture 6. Input/Output Interfacing LEDs, Button Switches & Port Multiplexing. ECE 447: 68HC11 Current Limits. V DD =5 V. Maximum sink current. V OHmin =4.5 V. I OLmax = 1.6 mA. Maximum source current. I OHmax = 0.8 mA. V OLmax = 0.4 V. GND=0 V.

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ECE 447: Lecture 6

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  1. ECE 447: Lecture 6 Input/Output Interfacing LEDs, Button Switches & Port Multiplexing

  2. ECE 447: 68HC11 Current Limits VDD=5 V Maximum sink current VOHmin=4.5 V IOLmax = 1.6 mA Maximum source current IOHmax = 0.8 mA VOLmax= 0.4 V GND=0 V Sufficient to drive one input of a TTL device and about 10 inputs of an LSTTL device

  3. 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

  4. ECE 447: LED Interfacing VDD=5 V R=? VDD – VF – VOL Forward current VF Forward voltage I ≈ IF VOL Low Level Output Voltage VOL Low Level Output Current (sink current) IOL buffer I ≈ IF  IOLmax R ≈ (VDD – VF – VOL) / IF

  5. ECE 447: LED Interfacing VDD=5 V R1=? VDD – VF – VCE Forward current I ≈ IF Forward voltage VF 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

  6. ECE 447: MAN-72: 7-Segment LED

  7. ECE 447: LED

  8. ECE 447: Single Switch/Push Button

  9. ECE 447: Switch Key Bouncing key bounce, tBOUNCE key bounce, tBOUNCE typically, tBOUNCE < 10 ms

  10. ECE 447: Key Debounce in Software key bounce, tBOUNCE key bounce, tBOUNCE typically, tBOUNCE < 10 ms debouncing delay debouncing delay update display

  11. ECE 447: Key Debounce in Hardware

  12. 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.

  13. OE OE 74LS245 74LS245 DIR DIR ECE 447: Port Multiplexing MC68HC11 Port C Port B . . . . . . . . . . . . I/O device 1 I/O device 8 . . . . . . .

  14. OE ECE 447: Port Multiplexing MC68HC11 Port C Port B 8 1G 2G 74LS245 74LS244 DIR I/O device 2 (e.g., keypad) I/O device 1 (e.g., LCD display)

  15. ECE 447: Octal Buffer 74LS245 Note: All 8 outputs controlled by one enable, OE

  16. ECE 447: Octal Buffer 74LS244 Note: Outputs controlled by two enables, 1G & 2G (1G controls 1A1-1A4, 2G controls 2A1-2A4)

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