ECE 353Introduction to Microprocessor Systems

Michael Schulte

Week 12

Administrative MattersHomework #6 due Friday, May 9th

Reading for week 12 (switches, keypads, and displays) Textbook 7.6-7.10 Supplement #5 (Learn@UW)

Final exam on Saturday, May 17th from 7:45am to 9:45am in 3418 EH Cumulative exam covering Modules 1-

6 More details coming soon

TopicsDigital Inputs Mechanical switches Keypads and keyboards Rotary encoders

Displays LED displays LCD displays

Mechanical SwitchesTypes of switches Poles Throws Contact action

Converting switch position to a logic levelMechanical contact bounceDebouncing in Hardware RS latch Integrator/Schmitt-trigger Dedicated ICs (MAX6816/7/8)

Mechanical SwitchesDebouncing in Software

How to do?

Exercise1. Develop an algorithm for

debouncing of a single pressed switch. (Assume 0 when pressed.)

2. Draw a flowchart to implement your algorithm in an ISR that will be invoked by a periodic timer interrupt.

Keypads and KeyboardsSwitch connections Linear Matrix

Keypad scanning Row/column scanning

Algorithm Dedicated keypad encoders (74C923)

Keypad issues Phantom keys Roll-over

Rotary EncodersConvert angular position to digital valueEncoding schemes Quadrature Encoders Absolute Encoders Indexed Encoders

Interfacing Software

Using interrupts Hardware

Design logic or use dedicated devices

LED DisplaysCharacteristicsDriver requirementsDisplay types Drivers

Multiplexed displays Driver circuits Refresh rate Duty cycle Intensity control Display Driver ICs

LED ISR algorithm and routine

LCD Display Characteristics

Numeric, text and/or graphic displaysExtremely low powerPassiveTemperature sensitiveComplex drivers required to create segment waveforms Require no net DC offset on segments –

on single polarity systems this adds to driver complexity

LCD Technologyhttp://en.wikipedia.org/wiki/Liquid_crystal_display

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.

LCD TechnologyWhen 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)

LCD TechnologyLight 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.

LCD TechnologyThe 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.

LCD TechnologyLight 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).

LCD TechnologyA combination of polarizing filters and twisted liquid crystal is used to create a liquid crystal display.

LCD Character ModulesBased 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 (Character

Generator RAM) Controllable cursor 2 read/write registers

Instruction/status data

LCD Character ModulesStandard 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

LCD CommandsCommands 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

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)

Wrapping UpHomework #6 due Friday, May 9thth.Final exam on Saturday, May 17th from 7:45am to 9:45am in 3418 EH Cumulative exam covering Modules 1-6 More details coming soon

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...

1 2 3

4 5 6

7 8 9

* 0 #

Back

KeypadExample

Back

1 2 3

4 5 6

7 8 9

* 0 #

D-FF

BUFFER

D0

D1

D2

D3

/ROW

/COL

Vdd

OE

CLK

D Q

Matrix Scanning AlgorithmAlgorithm Drive columns (or rows) low one at a time. Read the rows (or columns), any low rows (columns)

correspond to pressed switches.

Circuit Requirements Pull-up resistors to ensure that open rows (or columns)

are high. Use open-drain / open-collector driver on each column

(row) to avoid contention issues with multiple presses.

Switch Ambiguities If more than two switches are pressed, may “see”

more switches active than there really are. (phantom paths)

Can be solved by placing a series diode at each switch.Back

74C923 20-Key Encoder

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Maxim MAX6816/7/8Back

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Quadrature Rotary Encoders

There are normally many electrical cycles for each mechanical revolution.

Back

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.

Indexed Rotary Encoders

Absolute Rotary Encoders

Back

Position(+/- 22.5°)

Output

Decimal BinaryChanged

BitsGray Code

0° 0 000 - 000

45° 1 001 1 001

90° 2 010 2 011

135° 3 011 1 010

180° 4 100 3 110

225° 5 101 1 111

270° 6 110 2 101

315° 7 111 1 100

0 ° 0 000 3 000

The n-bit output is directly proportional to the angular position.

Rotary Encoder with Interrupts

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Typical LED Characteristics

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LED Displays

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MAX7219Multiplexed Display Driver

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LED current limiting resistors

MultiplexedDisplay

HW Switch DebouncingRS Latch

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VCC

VCC

HW Switch Debouncing

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RC Integrator with Schmitt-trigger gate Debounces both press and releaseV CC V CC

0V