ECE 353 Introduction to Microprocessor Systems

1. ECE 353Introduction to Microprocessor Systems Michael G. Morrow, P.E. Week 12

2. Topics • Digital Inputs • Mechanical switches • Keypads and keyboards • Rotary encoders • Displays • LED displays • LCD displays

3. Mechanical Switches • Types of switches • Poles • Throws • Contact action • Converting switch position to a logic level • Mechanical contact bounce • Debouncing in Hardware • RS latch • Integrator/Schmitt-trigger • Dedicated ICs (MAX6816/7/8)

4. Mechanical Switches • Debouncing in Software • How to do? • Exercise • Develop an algorithm for debouncing of a single switch. (Assume 0 when pressed.) • Draw a flowchart to implement your algorithm in an ISR that will be invoked by a periodic timer interrupt. • Implementing repeat and acceleration.

5. Keypads and Keyboards • Switch connections • Linear • Matrix • Matrix scanning algorithm (keypad example) • Drive columns (rows) low one at a time. • Read the rows (columns), any low rows (columns) correspond to pressed switches in the column (row). • Matrix Circuit Requirements • Pull-up resistors to ensure that open rows (columns) are high. • Use open-drain/open-collector driver on each column (row) to avoid contention issues with multiple presses.

6. Switch Matrix Issues • Switch Ambiguities • If more than two switches are pressed, may “see” more switches active than there really are. (phantom switches) • Can be solved in hardware by placing a series diode at each switch. (keypad example) • Handling Roll-Over • Another switch is pressed before the current key is released • This is very common when people type on keyboards – especially good touch typists • A keyboard provides N-key roll-over if N switches can be recognized simultaneously

7. Rotary Encoders • Convert angular position to digital value • Encoding schemes • Single channel incremental encoders • Quadrature Encoders • Absolute Encoders • Indexed Encoders • Interfacing • Software • Using interrupts or periodic polling • Hardware • Design logic or use dedicated devices

8. LED Basics • LEDs are long-life, high-efficiency devices • Temperature/vibration tolerant • Electrical characteristics • Typical small LEDs have IF of 5-20mA • IF in high powered LEDs can be much more • Driver requirements for small LEDs • Current limiting must be provided when driving LEDs from a voltage source • Many logic gates cannot directly drive LEDs • Simple transistor switches with current limiting resistors are the simplest solution

9. Multiplexed LED Displays • Display types • Driving multi-segment displays • Multiplexed displays • Driver circuits • Refresh rate • Duty cycle • Intensity control • Display driver ICs

10. LCD Display Characteristics • Numeric, text and/or graphic displays • Extremely low power • Passive • Temperature sensitive • Complex drivers required to create segment waveforms • Require no net DC offset on segments – on single polarity systems this adds to driver complexity

11. LCD Technology • http://www.sharp.co.jp/sc/library/lcd_e/indexe.htm • Natural state • Molecules are arranged in a loosely ordered fashion with their long axes parallel. • Aligned state • When coming into contact with a finely grooved surface (alignment layer), molecules line up in parallel along the grooves.

12. LCD Technology • When liquid crystals are sandwiched between upper and lower plates, they line up with grooves pointing in directions 'a' and 'b,' respectively. The molecules along the upper plate point in direction 'a' and those along the lower plate in direction 'b,' thus forcing the liquid crystals into a twisted structural arrangement. (figure shows a 90-degree twist) (TN type liquid crystal)

13. LCD Technology • Light passes through liquid crystals, following the direction in which the molecules are arranged. When the molecule arrangement is twisted 90 degrees as shown in the figure, the light also twists 90 degrees as it passes through the liquid crystals.

14. LCD Technology • The molecules in liquid crystals are easily rearranged by applying voltage or another external force. When voltage is applied, molecules rearrange themselves vertically (along the electric field) and light passes straight through.

15. LCD Technology • Light passes when two polarizing filters are arranged with their axes aligned (left). • Light is blocked when two polarizing filters are arranged with their axes perpendicular (right).

16. LCD Technology • A combination of polarizing filters and twisted liquid crystal is used to create a liquid crystal display.

17. LCD Character Modules • Based on Hitachi LCD-II controller protocol • 1 to 4 lines, 8-20 characters per line • 4 or 8 bit parallel interface • Motorola style control signals • 256 character font • 8 user defined characters (CGRAM) • Controllable cursor • 2 read/write registers • Instruction/status • data

18. LCD Character Modules • Standard Hardware Interface • Contrast adjustment • Temperature considerations • Control bus connections • Register addressing • Bus Timing Requirements • Often slower than processor – may require wait states • Sequential operations often require delay • Can drive LCD module bus from I/O pins and manipulate to get proper waveforms (bit-banging) • Backlights • LED • CCFL/EL

19. LCD Commands • Commands • Display Clear • Cursor Home • Entry Mode Set • Display On/Off Control • Cursor/Display Shift • Function Set • Set CGRAM Address • Set DDRAM Address • Using BUSY Flag

20. Designing a Software Interface • What procedures would you write in order to create a higher-level software interface to the LCD display? • Build a hierarchy using low-level procedures as building blocks – • LCD Initialization • Send Command (pass command) • Send Data (pass data) • Check if Busy (return state) • Wait until Ready • Write Character at Location (pass char, location) • Write Character String (pass address) • Write Character String at Location (pass addr, location)

21. Wrapping Up • Homework #6 due Wednesday, April 25th • Quiz #3 on Thursday, May 3rd at 7:15pm in 2255EH • Coverage is over Modules 5 and 6. • Same rules as other quizzes, except calculators will be permitted for numeric calculations.

22. Matrix-Connected Switches • Each switch connected between a row line and a column line. • More complicated circuitry required to poll switches. • Why connect in a matrix? Think about keyboard with 100+ keys... Back

23. 74C923 20-Key Encoder Back

24. Maxim MAX6816/7/8 Back

25. Quadrature Rotary Encoders There are normally many electrical cycles for each mechanical revolution. Back

26. Indexed Rotary Encoders • Rotary encoder A/B outputs typically go through many electrical cycles for a single revolution of the encoder shaft. • The absolute shaft position is unknown. • An indexed rotary encoder is essentially a quadrature encoder that has an additional output that provides a single pulse during each mechanical revolution. • This reference signal provides a way to determine the absolute shaft position. Back

27. Absolute Rotary Encoders The n-bit output is directly proportional to the angular position. Back

28. Rotary Encoder with Interrupts Back

29. Typical LED Characteristics Back

30. LED Displays Back

31. MAX7219Multiplexed Display Driver Back

32. LED current limiting resistors MultiplexedDisplay Back

33. HW Switch Debouncing • RS Latch Back

34. HW Switch Debouncing • RC Integrator with Schmitt-trigger gate • Debounces both press and release Back

35. KeypadExample Back

36. PhantomSwitch Back