Week #4 Parallel IO Interfacing ENG3640 Microcomputer Interfacing.

Week #4 Parallel IO Interfacing

ENG3640 Microcomputer Interfacing

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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Why Parallel I/O? Several embedded applications require interfacing an MCU with

Light Emitting Diodes, Switches, Liquid Crystal Display, Seven Segment Displays.

I/O ports therefore have to be programmed to handle input/output signals.

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I/O Addressing Techniques

If the same address bus is used for both memory and I/O, how should the hardware be designed to differentiate between memory and I/O reads and writes?

CPU MemoryI/O

Interface

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 those that 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 Structure1. Data Register: for data in transit

2. Control Register: Hold commands from processor to port

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

Principle functionality is serve as way

station for data in transit

between the computer and

external world.

Polling

Interrupt Driven

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68HC812A4 Block

Diagram

CPU12

1-KB SRAM

4-KB EEPROM

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Memory Map

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Port Direction Function

Port A I/OSingle-chip modes: general-purpose I/OExpanded modes: external address bus ADDR15–ADDR8

Port B I/OSingle-chip modes: general-purpose I/OExpanded modes: external address bus ADDR7–ADDR0

Port C I/O

Single-chip modes: general-purpose I/OExpanded wide modes: external data bus DATA15–DATA8Expanded narrow modes: external data bus DATA15–DATA8/DATA7–DATA0

Port D I/O Single-chip and expanded narrow modes: general-purpose I/OExternal data bus DATA7–DATA0 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 IADC 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 in either 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 in

high impedance state.

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I/O PORTS AddressesRegister Name Address Functionality

PORTA $0000 Port A Data Register

DDRA $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 cathodesc) 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 reading switches (unconditional transfer)

Steps for using Ports:

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

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

3. The value written to the DDR reflects the appropriate setting for the port (i.e a `0’ will make the 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 to the address of the I/O PORT.

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General Purpose I/O Usage

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

DDRH EQU $25

bset DDRH, $01 ; Set PORTH direction

Once bit 0 of PORTH has been configured as an output, 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 ports are configured as inputs.

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

Can this cause a problem when a port is used as an output in normal operation?

The external device connected to the port may do strange things!

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

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Avoiding Transients and Glitches

1. A pull-up or pull-down resistor can be added to the port so the node will always be pulled to the inactive level instead of floating.

2. The port can be preset to the inactive level before changing its direction to an output (when the port direction is changed a temporary glitch on the output 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 know the specification for the interface parameter.

The first parameter to consider are the input and output voltage levels and corresponding noise margins.

VDD and VSS are supply voltages. The output voltage

parameters are VOL and VOH. The input voltage parameters are VIL and VIH. For a digital system to work correctly, the output high

voltage always must 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 load current 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 the anode side is made higher than the voltage on the cathode side.

2. The forward voltage across the LED is typically about 1.5 to 2 Volts.

MCU produces 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 figure is connected active low so that when the output goes low, the current IL flows through the LED and turns it on.

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Connecting an LED to an Output Port

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

Answer: 5V = 2.0V + IRx x Rx

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

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Connecting an LED to an Output Port

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 the current.

So an inverter is often used as a switch to turn the LED on and off.

Active High since MCU 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

300 300 300 300

green yellow red blue

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PORTH equ $24DDRH equ $25

org $1000 ldaa #$FF ; configure PORTH for output staa DDRH ; “

Forever ldaa#$80 ; turn on green LED and turn off other LEDs staa PORTH ; “ jsr delay_hs ; wait for half of a second ldaa #$40 ; turn on yellow LED and turn off other LEDs staa PORTH ; “ jsr delay_hs ; wait for half of a second ldaa #$20 ; turn on red LED and turn off other LEDs staa PORTH ; “ jsr delay_hs ; wait for half of a second ldaa #$10 ; turn on blue LED and turn off other LEDs staa PORTH ; “ jsr delay_hs ; wait for half of a second jmp forever ; repeat swi

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Seven Segment Displays

Consists of seven LED segments (a, b, c, d, e, f, g)

Alphanumeric characters can be displayed by controlling the segments.

Two types of Seven Segment Displays:

1. Common Cathode

2. Common Anode

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Seven Segment Displays

Common Anode:1. All anodes are tied in

common.

2. Segment will be lit whenever a low voltage is applied.

Common Cathode: 1. all cathodes are tied in

common.

2. Segment will be lit whenever a high voltage is applied.

Current limiting resistors must be included or else you might damage display.

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Seven Segment Displays: Examples

Depending on the type of display used a different hex code is generated by the MCU.

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 buffers to supply required drive currents.

When using MC144495 decoder, no current limiting resistors have to be used since they are built in the MC14495

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Software 7-Segment Code Look-up

If your interface circuit uses only buffers and resistors to connect an output port to a seven segment display, you will have to use software to generate 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 and

type 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 determined by 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 internal pull-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 Register corresponds to a PORT J pin. Each bit selects a pull-up or pull-down device for the

associated PORT J pin. The pull-up or pull-down is active only if enabled by the PULEJ

Register

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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) are

used 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 The key wakeup feature of the MC6812 issues an interrupt that

wakes 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 closed

there are brief oscillations due to mechanical bouncing (switch bounce).

+5V

A

+5V

0V

What is the consequence of such 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 the debouncing 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 feedback internally to shift the switching threshold depending on whether 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 slowly changing input

Output produced by ordinary inverter

Hardware Debouncing: Schmitt Trigger

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A noisy slowly changing input

Output produced by ordinary inverter

Output produced by inverter with 0.8 V

of hysteresis

Hardware Debouncing: Schmitt Trigger

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Integrating Debouncer:

5V

Vout

The RC constant of the integrator determines the rate at which the capacitor charges up towards the supply voltage.

The capacitor value is chosen large enough so that Vout does not exceed the zero threshold value while the switch is bouncing!

R

CThreshold Level

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Calculating the capacitor value

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

Vout = 5 – 5e-t/RC

0.7 >= 5 – 5e -5ms/(1K.C)

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

1/0.86 >= 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 the voltage does not exceed the 0.7-V threshold of the NOT gate while it is bouncing.

5V

Vout

R

C

74LS14MCU

Input Port

Problem?

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Integrating Debouncer:

5V

Vout

1K

C

74LS14

MCU

Input Port22 ohm

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

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

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Software Debouncing

To debounce a switch we can use the following simple approach:

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 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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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, it

will find a short circuit thus it will identify the key to be 11.

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Keypad Decoding

PP7 PP6 PP5 PP4 Selected keys

1110

1101

1011

0111

0,4,8,C,

1,5,9,D,

2,6,A,E,

and 3and 7and Band 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

10K

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 the pressed key.

1

0

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Keypad Decoding

• Write an assembly subroutine that reads a character from the keypad. The subroutine should perform keypad scanning, debouncing, and ASCII 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 bookENG3640 Fall 2012

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Liquid Crystal Displays

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

1. They are used in watches, calculators, instrument panels, consumer electronic 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 the combination of numbers-letters, graphics to be driven with relatively simple interface.

Disadvantage?1. Low response.

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Liquid Crystal Displays: Structure1. An LCD display has two plates

separated by crystal material.

2. The polarizer plates are used to pass light (acts like a capacitor).

3. The liquid crystal can be made to pass or stop light.

4. Unlike LEDs that convert electric power to optical power, an LCD uses AC voltage to change the light.

5. An LCD could be either of reflective type or an absorption type.

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Liquid Crystal Displays: Operation1. The light energy is supplied by

(a) room (b) separate back light.

2. The LCD display requires an alternating excitation wave applied to selected electrodes to charge selected areas.

3. The excitation wave develops an electrostatic field to align the liquid crystal molecules in these selected areas.

4. When the crystals are aligned they allow light to pass through to the mirror.

5. In the charged areas the mirror reflects more light.

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Liquid Crystal Displays: Operation

An LCD display requires an alternating excitation wave applied to selected electrodes to change selected areas.

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

60 Hz Oscillator

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, VLCD will be zero (display is blank).

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

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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 either

sends 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. SetInstruction 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, # of display lines, char font

40 micro sec

Display on/off control

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 2x40

jsr 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 ; wait rts

HD44780: Initialization

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Interfacing: Loads

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

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

2. To do this we need to add a resistorresistor to limit the current or a driver ICdriver IC to protect the MCU from dangerous current levels or transients.

The Maximum Ratings in the electrical specifications for the 68HC12 is +/- 25mA.

This Maximum ratings give the value that if exceeded may destroy the part.

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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.

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Interfacing: Current Parameters

The interface current parameters are the output currents, IOH and IOL, and the input leakage current IIN.

1. IOH is the current flowing out of a high output

2. IOL is the current flowing out of 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 static fanout of a device, that is, the number of inputs that can 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 ]

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Interfacing: Simple Load Model To determine the output

characteristics, we need to model the port output as shown.

Rp is the equivalent resistance of the internal 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 transistor that is off is infinite.

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

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

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Operating Modes and Resource Mapping

The MCU can operate in eight different modes. Each mode has a different default memory map and external bus configuration.

After reset, most system resources can be mapped to other addresses by writing to the appropriate control register.

The states of BKGD, MODB and MODA pins during reset determine the operating mode after reset.

Two basic types of operating modes

1. Normal Mode

2. Special Mode

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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 a

stand-alone device and all program and data resources are on chip.

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

Normal Expanded Wide Mode: The 16-bit external address bus uses port A for the high byte and port B for the low byte. The 16-bit external data bus uses port C for the high byte and port D for 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 testing purposes.

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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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Connecting an LED to an Output Port

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) / IF

3. = (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 the current to less than 25mA under all conditions and assume VDD,max = +5V (+-10%) = 5.5V

1. Conservative approach: set R n(on), min =0 and V F,min = 0

2. Practical approach: use 50% of the maximum values for

R n(on), min and V F,min

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Connecting an LED to an Output Port

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 be destroyed.

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

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Connecting an LED to an Output Port

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

IF,max = (VDD,max – VF,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 the output and the maximum current limit of the LED.

74

PORTJ: Initialization The software initialization sets bits in PUPEJ register to enable

pull-up or pull-down. For each Port J pin that is enabled for pull-up or pull-down, the

corresponding bit in the PUPSJ register determines if it 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-downENG3640 Fall 2012

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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 8x8 matrix keyboard with 64-keys, 74HC138 (3-to-8 Decoder), and 74HC151 (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 then test for the switch still being low.

Approach #2:1. Initialize a counter with a value of 10 and after the first logic low

level is detected, poll the switch every millisecond.

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

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

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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 are low and determine the key code.

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Keypad Decoding

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

2. Checks the corresponding input line.

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

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

5. The combination of both low logic column 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

Understand Keypad Interface*(Note: 68HC12812A4 has internal 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

MCU checks

these lines

MCU checks

these lines

MCU sends low signal to

these lines

ENG3640 Fall 2012