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Interfacing LED Displays and Switch Buttons with Motorola Microcontrollers

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Learn about input and output interfacing, LED characteristics, drive currents for Motorola 68HC11 and 68HC24, TTL device compatibility, key debouncing techniques, and a class exercise designing a system with MC68HC11.

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Interfacing LED Displays and Switch Buttons with Motorola Microcontrollers

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  1. ECE 447 – Lecture 6 Input and Output Interfacing LED displays Switch buttons

  2. Maximum currents at the outputs of Motorola 68HC11and 68HC24 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. Light Emitting Diode - LED + Forward current Forward voltage IF VF – Typical values for most common LEDs IF = 10-20 mA VF = 1.7-2.4 V

  4. 68HC11 - LED interface (1) VDD=5 V R=? VDD – VF – VOL Forward current IF VF Forward voltage VOL Low Level Output Voltage VOL Low Level Output Current (sink current) IOL buffer IF  IOLmax R = (VDD – VF – VOL) / IF

  5. 68HC11 - LED interface (2) VDD=5 V R1=? VDD – VF – VCE Forward current IF Forward voltage VF VCE Collector-Emitter Voltage R2=? IOH ICE Collector-Emitter Current IF  ICEmax VOH VBE R1 = (VDD – VF – VCE) / IF R2 = (VOH – VBE) / IOH Base-Emitter Voltage

  6. Key Debouncing key bounce, tBOUNCE key bounce, tBOUNCE typically, tBOUNCE < 10 ms debouncing delay debouncing delay update display

  7. Key Debouncing - Auto-repeat debouncing delay debouncing delay update display update display update display Auto-repeat delay, typically 0.5 s

  8. Class Exercise 4 Design a system based on MC68HC11 that increments a digit displayed by a 7-segment LED display each time a button is pressed.

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