eng364 week4 parallel-io

7/28/2019 Eng364 Week4 Parallel-IO

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Week #4Parallel IO Interfacing

ENG3640Microcomputer Interfacing


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ENG3640 Fall 2012 2

Topics

I/O Addressing Techniques I/O Port Structure CPU12 I/O Ports

Programming I/O Ports:

Driving LEDs/7-Segment Displays Interfacing to Switches Switch Debouncing Keypad Interfacing Techniques/Issues Liquid Crystal Displays


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Resources

Huang, Chapter 4 Sections 4.10 Intro to Parallel I/O Ports

4.11 Simple I/O Devices

Huang, Chapter 7 Section 7.2 I/O Related Issues

7.3 I/O Addressing Issues

7.5 The HCS12 Parallel Ports

7.6 Electrical Characteristics

7.7 Liquid Crystal Displays

Interfacing Parallel Ports to a keypad


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I/O Addressing Techniques

If the same address bus is used for bothmemory and I/O, how should the hardware

be designed to differentiate betweenmemory and I/O reads and writes?

CPU Memory I/OInterface

Data

Address

Control


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Memory Mapped I/O vs. Isolated I/O

Memory Mapped I/O (MOTOROLA):1. Any instruction that reads or writes memory can read/write

I/O Port

2. Address specifies which module (input, output, RAM, ROM),will communicate with the processor

3. Ex: LDAA #56 STAA $0024 (copy value to port H)

Isolated I/O (INTEL):

1. The control bus signals that activate the I/O are separate from thosethat activate the memory device.

2. These systems have a separate address space.

3. Separate instructions are used to access I/O and Memory.

4. Ex: IN AL, $10 (copy values of port $10 into register AL)

Advantages/Disadvantages?


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I/O Port Structure

1. Data Register: for data in transit2. Control Register: Hold commands from processor to port

3. Status Register: Used to monitor I/O activity

Principlefunctionality isserve as way

station for datain transit

between thecomputer andexternal world.

Polling

InterruptDriven


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68HC812A4

Block

Diagram

CPU12

1-KB SRAM

4-KB EEPROM


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MemoryMap


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Port Direction Function

Port A I/OSingle-chip modes: general-purpose I/OExpanded modes: external address bus ADDR15ADDR8

Port B I/OSingle-chip modes: general-purpose I/OExpanded modes: external address bus ADDR7ADDR0

Port C I/O

Single-chip modes: general-purpose I/OExpanded wide modes: external data bus DATA15DATA8Expanded narrow modes: external data bus DATA15DATA8/DATA7DATA0

Port D I/OSingle-chip and expanded narrow modes: general-purpose I/OExternal data bus DATA7DATA0 in expanded wide mode(1)

Port E I/O and I(2) External interrupt request inputs, mode select inputs, bus control signalsGeneral-purpose I/O

Port F I/OChip selectGeneral-purpose I/O

Port G I/OMemory expansionGeneral-purpose I/O

Port H I/OKey wakeup(3)General-purpose I/O

Port J I/OKey wakeup(4)General-purpose I/O

Port S I/OSCI and SPI portsGeneral-purpose I/O

Port T I/OTimer portGeneral-purpose I/O

Port AD I ADC portGeneral-purpose input

Port Details


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General Purpose I/O: Bidirectional

Most GPIO pins on the 68HC12 MCU can be programmed for use ineither direction.

Two registers: the data register PORT and data direction register DDR.

The DDR determines the direction of the port pin.

If the DDR = 1 then the port is an output and

if the DDR = 0 the data register output is disabled and the port pin is placed inhigh impedance state.


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I/O PORTS AddressesRegister Name Address Functionality

PORTA $0000 Port A Data RegisterDDRA $0002 Port A Data Direction Register

PORTB $0001 Port B Data Register

DDRB $0003 Port B Data Direction Register

PORTC $0004 Port C Data Register

DDRC $0006 Port C Data Direction Register

PORTD $0005 Port D Data Register

DDRD $0007 Port D Data Direction Register

PORTE $0008 Port E Data Register

DDRE $0009 Port E Data Direction Register

PORTF $0030 Port F Data Register

DDRF $0032 Port F Data Direction Register

PORTG $0031 Port G Data Register

DDRG $0033 Port G Data Direction Register

PORTH $0024 Port H Data Register

DDRH $0025 Port H Data Direction Register

PORTJ $0028 Port J Data Register

DDRJ $0029 Port J Data Direction Register


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I/O PORTS Usage on EVB

1. Port B is connected to the Light Emitting Diodes (LEDS)a) Each Port B line is monitored by an LED.b) In order to turn on Port B LEDs, the PJ1 (Port J pin 1)

must be programmed as output and set for logic zero.c) If you ignore the status of the LEDs, the Port B can drive

any other I/O on the breadboard.2. Port P is connected to the Seven Segment Display

a) There are 4 digits of 7-Segment Displays on the EVB.b) Port B is used to drive the 7-segment anodes and PP0-PP3

(Port P) to drive common cathodes

c) To use the 7-Segments you need to multiplex among them.3. Port K is connected to the Liquid Crystal Display (LCD)4. Port A is connected to the Hex Key Pad5. Port H is connected to the DIP Switches and Push buttons.


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General Purpose I/O Usage Parallel ports are often used for simple I/O such as

turning on LEDs, 7-segment displays or readingswitches (unconditional transfer)

Steps for using Ports:

1. Identify the address of an I/O port and its DataDirection Register (DDR)

2. Program the DDR by writing a value to it.

3. The value written to the DDR reflects theappropriate setting for the port (i.e a `0 will makethe corresponding pin an input and a `1 will force

the pin in the port to be an output).

4. Load a register with a value and store this value tothe address of the I/O PORT.


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General Purpose I/O Usage

To make bit 0 of PORTH (PH0) an output, wewould use the following instruction:

DDRH EQU $25

bset DDRH, $01 ; Set PORTH direction

Once bit 0 of PORTH has been configured as anoutput, we can make the pin go to one by

writing a one to bit0 of PORTH as follows:PORTH EQU $24

bset PORTH,$01 ; Set PORTH bit-0 high


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Avoiding Transients and Glitches

When the MCU is powered up or reset, all GPIO portsare con f igured as inputs.

The port will remain an input until the software changes thedata direction register.

Can th is cause a prob lemwhen a port is used as anoutput in normal operation?

The external device connected to the port may do strangethings!

For example, if an output is connected to a motor or solenoiddriver, the motor or solenoid may run intermittently duringthis time and have serious consequences!


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Avoiding Transients and Glitches

1. A pull-up orpull-down resistorcan be addedto theport so the node will always be pulled to the inactivelevel instead of floating.

2. The port can be preset to the inact ive levelbeforechanging its direction to an output (when the portdirection is changed a temporary glitch on theoutput will occur!)

To avoid the glitch, we can write to the port data register

before the direction is changed.bset PORTH, $00 ; preset PORTH bit-0 low

bset DDRH, $01 ; PORTH bit-0 an output


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Interfacing: Voltage Parameters

Like any digital device, before we can connect

something to an input or output, we need to knowthe specification for the interface parameter.

The first parameter to consider are the input andoutput voltage levels and corresponding noisemargins.

VDD and VSS are supplyvoltages. The output voltageparameters are VOL and VOH.

The input voltage parametersare V

ILand V

IH.For a digital

system to work correctly, theoutput high voltage alwaysmust be between VIH,min and

VDD.

Noise Margin?


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Interfacing: Applications

1. Like most digital logic devices, the output of MCUs

can sink more current than they can source.

2. Consequently, devices that require significant loadcurrent like an LED should be connected active-low.


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Light Emitting Diodes (LED)

1. An LED emits light when current flows through it in

the positive direction i.e. when the voltage on theanode side is made higher than the voltage on thecathode side.

2. The forward voltage across the LED is typically about

1.5 to 2 Volts.

MCU prod uces

low


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Connecting an LED to an Output Port

Determine whether we can drive an active-low LED

from an M68HC12 output? Assume that a high-efficiency LED with IF,min = 10mA

and VF,max = 2.0 V is used

The LED in the figureis connected activelowso that when theoutput goes low, the

current IL flowsthrough the LED andturns it on.


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Using a 5-volt supply and assuming that theLED has a 2.0 V drop across it, what resistorvalue will limit the current to 10mA?

Answer:

5V = 2.0V + IRx x Rx

Setting IRx to 10mA the resistor Rx is solvedto be 300 Ohm.


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74HC04

VCC

Figure 7.9 An LED connected to a CMOS inverter through a current-

limiting resistor.

An I/O port pin of a microcontroller generally

does not have enough drive to supply thecurrent.

So an inverter is often used as a switch to

turn the LED on and off.

Active High sinceMCU produced a 1 to

turn the LED on


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Use the 68HC12 Port H to drive green, yellow, red, and blue LEDs.

Light each of them for half of a second in turn and repeat. The 68HC12

uses a 16-MHz crystal oscillator to generate internal clock signals.

Solution:The upper four pins of the Port H can be used for this purpose.

74HC04

5V

Figure 7.10 Circuit connection for example 7.3

74HC04

5V

74HC04

5V

74HC04

5V

PP7

PP6

PP5

PP4

68HC12

300W 300W 300W 300W

green yellow red blue


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PORTH equ $24DDRH equ $25org $1000ldaa #$FF ; configure PORTH for outputstaa DDRH ;

Forever ldaa #$80 ; turn on green LED and turn off other LEDsstaa PORTH ; jsr delay_hs ; wait for half of a second

ldaa #$40 ; turn on yellow LED and turn off other LEDsstaa PORTH ; jsr delay_hs ; wait for half of a secondldaa #$20 ; turn on red LED and turn off other LEDsstaa PORTH ; jsr delay_hs ; wait for half of a second

ldaa #$10 ; turn on blue LED and turn off other LEDsstaa PORTH ; jsr delay_hs ; wait for half of a secondjmp forever ; repeatswi


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Seven Segment Displays

Consists of seven LED

segments (a, b, c, d, e, f, g)

Alphanumeric characterscan be displayed bycontrolling the segments.

Two types of Seven SegmentDisplays:

1. Common Cathode

2. Common Anode


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Seven Segment Displays

Common Anode:

1. All anodes are tied incommon.

2. Segment will be litwhenever a low voltage

is applied. Common Cathode:

1. all cathodes are tied incommon.

2. Segment will be lit

whenever a high voltageis applied.

Current limiting resistorsmust be included or elseyou might damage

display.


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Seven Segment Displays: Examples

Depending on the type ofdisplay used a different hexcode is generated by theMCU.

1. Common Anode: sending a``0 will illuminate the

segment.

2. Common Cathode:sending a ``1 will

illuminate the segment.


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Seven Segment Displays: Decoders

Some ICs are specially

designed to drive 7-segment displays.

They contain buffersto supply requireddrive currents.

When usingMC144495 decoder, nocurrent limitingresistors have to beused since they are

built in the MC14495


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Software 7-Segment Code Look-up

If your interface circuit uses only buffers andresistors to connect an output port to a sevensegment display, you will have to use software togenerate the character codes.

LAB #2 The program can do this by using a look-up table.

The contents of the table depends on the application andtype of display.

For example, to display hex digits for a common anode

display, entry 4 would have the byte $19. What addressing mode to use?


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Interfacing a Switch: Pull Up

To convert the mechanical signal into an electrical signal, a

resistor pull-up is used. The amount of current this output can source is determined by

the resistor value.

When the switch is open, Output?

When the switch is closed, Output?

switch Output

Open +5V

Closed 0V

+5V

1K

I/O

Port

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Interfacing a Switch: Pull Down

When the switch in pull-down circuit is open the output is

pulled to the ground

The amount of current this output can sink (IOL) is determinedby the resistor value.

switch Output

Open 0V

Closed +5V

+5V

1K

I/O

Port

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Interfacing a Switch: PORTJ MCU

Port J on the MC68HC812A4 supports both internalpull-ups and pull-downs. Either of the two previous circuits could be

implemented on the 6812 without the resistor

+5V

PJ1 with pull-down

PJ0 with pull-up

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PORTJ: Pull-up/Pull-down Registers

Each bit in the PUPSJ Registercorresponds to a PORT J pin.

Each bit selects a pull-up or pull-down device for theassociated PORT J pin.

The pull-up or pull-down is active only if enabled by the PULEJRegister

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PORTJ: Pull-up/Pull-down Registers

DDRJ PUPSJ bit PULEJ bit Port J mode

1 X X Regular output

0 X 0 Regular input

0 0 1 Input with passive pull-down

0 1 1 Input with passive pull-up

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PORTJ: Data Register/Data Direction

RECALL: PORTJ can act as a general purpose I/O

PORTJ (Data Register) and DDRJ (Data Direction Register) areused to setup the port for reading/writing information

PORT J is also used in the key wakeup feature of the MC6812!

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PORTJ : Wakeup Flag/Interrupt Enable


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PORTJ : Wakeup Flag/Interrupt Enable

The key wakeup feature of the MC6812 issues an interrupt thatwakes up the CPU when it is in stop or wait mode.

Wakeups are triggered with falling/rising signal edge An interrupt is generated when a bit in KWIFJ register and its

corresponding KWIEJ bit are both set.

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Switch Bouncing

When mechanical switches are opened or closedthere are brief oscillations due to mechanicalbouncing (switch bounce).

+5V

A

+5V

0V

What is the consequence ofsuch bouncing for

interfacing?

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Switch Debouncing

When the mechanical switch is touched (pressed) or released it

bounces microscopically for a period of milliseconds. The MCU will see many occurrences of ``make and ``break

instead of one occurrence.

The MCU should see only one``break and one ``make

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Switch Debouncing: Techniques

Several techniques exist to solve thedebouncing problem:

Hardware Techniques:

1. RS-flip flop

2. Integrating debouncer (capacitor)

3. Schmitt Trigger Circuit

Software Techniques:

1. Delay loops

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Hardware Debouncing: Flip Flop

I1 I2 F

0 0 1

0 1 1

1 0 1

1 1 0

+5V

+5V

QS

R

S

R

Q

NAND

S R Q

0 1 1

1 1 1

1 0 0

1 1 0

0 0 X

An SR latch can be used to debounce a switch

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Hardware Debouncing: Schmitt Trigger

A Schmitt trigger is a special circuit that uses feedbackinternally to shift the switching threshold depending onwhether the input is changing from low to high or high to low.

The difference between V T+ and V T- is called hysteresis.

A 74LS14 Schmitt Trigger inverter can be used to debounce a

switch.

VIN

VOUT

2.1 2.9 5.0

V T+

V T-

5.0

Example: 74LS14

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A noisy slowlychanging input

Output produced byordinary inverter


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A noisy slowlychanging input

Output produced byordinary inverter

Output produced byinverter with 0.8 V

of hysteresis


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Integrating Debouncer:

5V

Vout

The RC constant of the

integrator determines therate at which the capacitorcharges up towards thesupply voltage.

The capacitor value is

chosen large enough so thatVout does not exceed thezero threshold value whilethe switch is bouncing!

R

CThreshold Level

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C l l ti th it l


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Calculating the capacitor value

Given R = 1K, bounce time = 5ms, what is thevalue of C such that the output voltage seenby the MCU = 0.7V?

Vout = 5 5e-t/RC

0.7 >= 5 5e -5ms/(1K.C)

0.86 = e 5ms/(1K.C)

1.16 > = e 5ms/(1K.C) ln(1.16) >= 5ms/(1K.C)

C >= 5ms/(1K.ln(1.16)) = 33micro Farad.

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Integrating Debouncer: The capacitor value is

chosen such that thevoltage does not exceedthe 0.7-V threshold of theNOT gate while it isbouncing.

5V

Vout

R

C

74LS14

MCU

Input Port

Problem?

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Integrating Debouncer:

5V

Vout1K

C

74LS14

MCU

Input Port22 ohm

Current can be large (causes spark), these sparkswill produce carbon deposits on the switch that willbuild up until switch no longer works.

To limit the current a small resistor is placed inseries with the switch.

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Software Debouncing

To debounce a switch we can use thefollowing simple approach:

1. A software time delay is used that providesa time delay (usually 10-20 ms) longer than

the duration of the switch bouncing action.

2. So if switch goes low, wait for longer than10ms or 20ms and then test for the switch

still being low.

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Keyboard/Keypad Interfacing Keyboards are used to enter input into a computer.

A common type of keyboard is the matrix type.

It saves an amount of I/O wiring because the keys share wires.

Each has its own combination of row and column.

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K d I t f i


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Keypad Interfacing Each key has an identifying number (key code or scan code)

as well as a character or function associated with it.

For example key 11 has the character (0). How does the MCU identify that key 11 was pressed?

By Sending a signal to terminal G and Checking terminal E, itwill find a short circuit thus it will identify the key to be 11.

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K d D di


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Keypad Decoding

PP7 PP6 PP5 PP4 Selected keys

1

1

1

0

1

1

0

1

1

0

1

1

0

1

1

1

0,

4,

8,

C,

1,

5,

9,

D,

2,

6,

A,

E,

and 3

and 7

and B

and F

Table 7.6 Sixteen-key keypad row selections

A

B

C

D

E

F

0

1

2

3

4

5

6

7

8

9

10KW

VCC

PP7

PP6

PP5

PP4

PP3

PP2

PP1

PP0

68HC12

Figure 7.23 Sixteen-key keypad connected to 68HC12

1. To detect a short circuit, the MCU drives one of the output lines low

2. Checks the corresponding input line, if it is low, key was pressed.3. If the key was not pressed, the open circuit allows the resistor to pull up

the input line to logic high.

4. The combination of both low logic column and row will identify thepressed key.

1

0

K d D di


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Keypad Decoding

Write an assembly subroutine that reads a character from the keypad.

The subroutine should perform keypad scanning, debouncing, andASCII code generation.

Steps:

1. Configure Port H such that 4 MSB are output and 4 LSB are input.

2. Select the row containing keys 0,1,2,3

3. If Key 0 pressed then

4. debounce key 0 (i.e. jump to subroutine wait 10ms)

5. get the ASCII code of 0

6. else if key 1 pressed then

7. debounce key 1 (i.e. jump to subroutine wait 10ms)

See page 265, Example 7.9 in your text book

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Liquid Crystal Displays


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Liquid Crystal Displays

Liquid Crystal Displays (LCDs) are widely used inmicrocomputer systems (Embedded Systems).

1. They are used in watches, calculators, instrument panels, consumerelectronic displays (VCRs)

Advantages over LEDs and 7-Segment Displays?1. Low power consumption with respect to LEDs. This allows the

display and even the computer system to be battery operated.

2. LCDs are more flexible in their sizes and shapes. So this permits thecombination of numbers-letters, graphics to be driven withrelatively simple interface.

Disadvantage?

1. Low response.

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Liquid Crystal Displays: Structure


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Liquid Crystal Displays: Structure

1. An LCD display has two platesseparated by crystal material.

2. The polarizer plates are usedto pass light (acts like acapacitor).

3. The liquid crystal can be made

to pass or stop light.

4. Unlike LEDs that convertelectric power to optical power,an LCD uses AC voltage tochange the light.

5. An LCD could be either ofreflective type or an absorptiontype.

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Liquid Crystal Displays: Operation


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1. The light energy is supplied by(a) room (b) separate back light.

2. The LCD display requires analternating excitation waveapplied to selected electrodes tocharge selected areas.

3. The excitation wave develops anelectrostatic field to align theliquid crystal molecules in theseselected areas.

4. When the crystals are aligned

they allow light to pass throughto the mirror.

5. In the charged areas the mirrorreflects more light.

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An LCD display requires an alternating excitation wave applied

to selected electrodes to change selected areas.

A constant (DC) excitation signal will polarize and destroythe crystal.

60 HzOscillator

Control

BP

FP

XOR

VLCD

Front Plane

Liquid Crystal Material

Back Plane

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Liquid Crystal Displays: Operation The oscillator output BP is a square wave with frequency of 60Hz.

When the Control signal is low, FP is in phase with BP.

Therefore, VLCDwill be zero (display is blank).

When control signal is high, VLCD will be an AC square wave andthe display reflects light (display is visible).

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Liquid Crystal Displays: HD44780


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Liquid Crystal Displays: HD44780 The HD44780 is an industry standard LCD controller.

An I/O port can easily be used to interface with the HD44780.

By controlling the value of RS and R/W the MCU can easily eithersends instructions or data to the controller.

Most operations require 40 micro seconds to complete.

To be used in LAB #4.

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HD44780 LCD Controller Instr. Set

Instruction Command Code Description Time

Clear Display 00000001 Clears Display 1.64 ms

Cursor Home 0000001x Returns cursor to home position 1.64 ms

Function Set 001 DL N F * * Sets interface data length, # ofdisplay lines, char font

40 micro sec

Display on/offcontrol 0000 1 D C B Set on/off of all display (D),cursor on/off (C), and blink (B) 40 micro sec

D: display on/off. 0=off, 1=on

F: font size 0=5x7 dots, 1=5x10 dots

B: cursor blink on/off. 0=off, 1=on

N: number of lines. 0=1 line, 1=2 lines

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LCD Initialization Routine

initlcd ldaa #$3C ; configure display format to 2x40jsr lcdcmd ; ldaa #$0f ; turn on display and cursor

jsr lcdcmd ; ldaa #$14 ; shift cursor right

jsr lcdcmd ; ldaa #$01 ; clear display and return cursor to home

jsr lcdcmd ; rts

; Send a command in A to the LCD command registerlcdcmd staa cmd_reg ; write command

jsr delay40 ; waitrts

HD44780: Initialization

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Interfacing: Loads


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Interfacing: Loads

The second case is to loads which willrequire more current than 10mA

1. We might want to connect an LED or small relay toan output of the MCU.

2. To do this we need to add a resistor to limit thecurrent or a driver IC to protect the MCU fromdangerous current levels or transients.

The Maximum Ratingsin the electrical

specifications for the 68HC12 is +/- 25mA. This Maximum ratings give the value that

if exceeded may destroy the part.

Interfacing: No Loads


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Interfacing: No-Loads

The data sheet for a digital device lists two values

for the voltage output levels.1. The first is for small loads, typically 10mA (use for

the same family, i.e HCMOS devices)

2. This represents about 10 HCMOS logic gates.

Interfacing: Current Parameters


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Interfacing: Current Parameters

The interface current parameters are the outputcurrents, IOH and IOL, and the input leakage currentIIN.

1. IOH is the current f low ing ou tof a h igh output

2. IOL is the current f low ing outof a low output

3. IIN is the leakage current that flows into or out of an input pin.

These currents are used to determine the stat icfanout o f a device, that is, the number of inputs thatcan be connected to one output while preserving the

required voltage margins.1. Static fanout for a low output is: nL = | IOL,max |/| IIn |

2. Static fanout for a high output is: nH = | IOH,max |/| IIn |

3. n = min [ nH, nL ]

Interfacing: Simple Load Model


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Interfacing: Simple Load Model

To determine the output

characteristics, we need to model theport output as shown.

Rpis the equivalent resistance of theinternal PMOS, and Rn is the equivalent

resistor of the internal NMOS. When the output is high, the PMOS is

on and the NMOS is off.

Assume that the resistance for a transistorthat is off is infinite.

Rp(on)max = VDD-VOH,min / | IOH |

Rn(on)max = VOL,max / | IOL |


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Operating Modes Normal Modes: Some registers and bits are protected against

accidental changes. Special Modes: Protected control registers and bits are allowed

greater access for special purposes such as testing and emulation.

BKGD MODB MODA ModePort APort B

Port C Port D

0 0 0 Special single-chip G.P.(1) I/O G.P. I/O G.P. I/O

0 0 1 Special expanded narrow ADDR DATA G.P. I/O

0 1 0 Special peripheral ADDR DATA DATA

0 1 1 Special expanded wide ADDR DATA DATA

1 0 0 Normal single chip G.P. I/O G.P. I/O G.P. I/O

1 0 1 Normal expanded narrow ADDR DATA G.P. I/O

1 1 0Reserved

(forced to peripheral)

1 1 1 Normal expanded wide ADDR DATA DATA


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Operating Modes

Normal Single Chip: No external buses. The MCU operates as astand-alone device and all program and data resources are onchip.

Special Single-Chip Mode: This mode can be used to force theMCU to active Background Debug Mode (BDM).

Normal Expanded Wide Mode: The 16-bit external address bususes port A for the high byte and port B for the low byte. The16-bit external data bus uses port C for the high byte and port Dfor the low byte (Factory Configured!)

Special Peripheral Mode: The CPU is not active in this mode.

An external master can control on-chip peripherals for testingpurposes.

Normal Drive Strength Output Characteristics


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Normal Drive Strength Output Characteristics

Parameter 68HC912B32 Value

VOH,min (no load, IOH < 10m A) VDD-0.2V

VOH,min

(IOH

= -0.8mA) VDD

-0.8V

VOL,max (no-load, IOL < 10m A) 0.2V

VOL,max (IOL = 1.6mA) 0.4V

R p(on), max 1000W

R n(on), max 250W



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Step 1. Determine RL such that IL is at least 5mA.1. IL flows through RL the LED, and through R n(on) . R p(on) is

infinite because PMOS is off.

2. RL = (VDD - IF.R n(on) VF)/ IF3. = (5V 5mA.250W 2V) / 5mA = 150

4. The closest 1% standard resistor value is 147

Step 2. Determine if 147 is large enough to limit thecurrent to less than 25mA under all conditions andassume VDD,max = +5V (+-10%) = 5.5V

1. Conservative approach: set R n(on), min =0 and V F,min = 02. Practical approach: use 50% of the maximum values for

R n(on), min andV F,min



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Conservative Approach:IF,max = (VDD,max) / (RL,min)

= 5.5V/145.5 = 37.8mA

This exceed the max current specs (25mA)which means that the MCU could bedestroyed.

Another IC such as a 74AC240 would have tobe used to drive the LED



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Practical Estimation (Cost of adding an extra IC canbe prohibitive for some cost-sensitive designs!)

IF,max = (VDD,maxVF,min)) / (RL,min + R n(on).min)

= (5.5V-1V)/(145.5 ohm 125 ohm) =16.64mA

This is well within the 25mA limit of theoutput and the maximum current limit of theLED.

PORTJ: Initialization


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PORTJ: Initialization

The software initialization sets bits in PUPEJ register to enablepull-up or pull-down.

For each Port J pin that is enabled for pull-up or pull-down, thecorresponding bit in the PUPSJ registerdetermines ifit is pull-up(1) or pull-down (0).

// MC68HC812A4

// Port J bit 1 is connected to a switch to +5, using internal pull-down

// port J bit 0 is connected to a switch to 0, using internal pull-up

Void Initialization(void){

DDRJ &= 0xFC; // PJ1 PJ0 inputs

KPOLJ |= 0x03; // flags set on the rise of PJ1 and PJ0

KWIEJ &= 0xFC; // disarm PJ1, PJ0

KWIFJ = 0x03; // clear flags

PUPSJ = (PUPSJ&0xFC)|0x01; // pull-down on PJ1, pull-up on PJ0

PULEJ |= 0x03;} // enable pull-up and pull-down

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Hardware Debouncing: Flip Flop

An SR latch can be used to debounce a switch

+5V

+5V

R

S

Q

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Wait for press

RTS

Switch

Wait 10 ms

Software Flowcharts for debouncing

Switch

Wait for release

Wait 10 ms

RTS

pressednot pressed not pressedpressed

Polling vs Interrupt Driven?

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Using Hardware Decoding Chips


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Using Hardware Decoding Chips 8x8 matrix keyboard with 64-keys, 74HC138 (3-to-8 Decoder), and74HC151 (multiplexer).

PC0, PC1, PC2 are used to send values to rows

The MCU will scan the columns one by one via PC3, PC4, PC5

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Software Debouncing

To debounce a switch we can use several approaches:

Approach #1:

1. A software time delay is used that provides a time delay (usually 10-20 ms) longer than the duration of the switch bouncing action.

2. So if switch goes low, wait for longer than 10ms or 20ms and thentest for the switch still being low.

Approach #2:

1. Initialize a counter with a value of 10 and after the first logic lowlevel is detected, poll the switch every millisecond.

2. If the switch output is low, decrement the counter. If the switchoutput is high increment the counter.

3. When the counter reaches zero, we know the switch output hasbeen low for at least 10 ms. But if the counter reaches 20, we knowthat the switch has been open for at least 10 ms.

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Keyboard Decoding


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Keyboard Decoding Some keyboards have an extra common terminal. So if a key is

pressed, a short circuit occurs between the common and the

keys row and column line. When PC0 is driven low and one key is pressed, one of the row

inputs and one of the column inputs will be low.

The keyboard software driver routine checks which inputs arelow and determine the key code.

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Keypad Decoding


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Keypad Decoding

1. To detect a short circuit, the MCUdrives one of the output lines low

2. Checks the corresponding inputline.

3. If it is low, the key was pressed.

4. If the key was not pressed, theopen circuit allows the resistor topull up the input line to logic high.

5. The combination of both low logiccolumn and row will identify the

pressed key.6. For example, to check key code 5

the MCU drives PC1 (terminal J)low and checks the input at PC5(terminal E) .

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Keypad Interfacing


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Keypad Interfacing

UnderstandKeypad Interface*(Note: 68HC12812A4 hasinternal pull-up on all pins;

not all are shown here)

To identify the key code,the MCU scans each

contact in sequence.

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Keypad Decoding: 4x4 keypad


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MCUchecks

these lines

MCUchecks

these lines

MCUsends lowsignal to

these lines

eng364 week4 parallel-io

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