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Interfacing LED Displays and Switch Buttons with Motorola Microcontrollers

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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