ece 372 – microcontroller design parallel io ports - outputs

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ECE 372 – Microcontroller Design Parallel IO Ports - Outputs

Parallel IO Ports E.g. Port T, Port AD Used to interface with many devices

Switches LEDs LCD Keypads Relays Stepper Motors

Interface with parallel IO requires us to connect the devices correctly and write code to interface with the devices

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HCS12 – Where can we connect external hardware?

Unused ports (every port has DIO functionality) Expand through the serial interface SPI serial peripheral interface SCI serial communication interface RS232

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HCS12 – Where can we connect external hardware?

Port B and A are not connected except PB4 and PA0 Port S0,1 is RxD and TxD for SCI Port M2,3,4,5 are used for SPI Port M0,1 are used for CAN Port P is cross connected with Port T except PP5 Port T0..7 can be sued for timer I/O and PWM Port A/D 0...7 is used for A/D input

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Directly Controlling LEDs HCS12 can directly control LED from output ports Why would we NOT want to do this?

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5V5V

5V5V

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Simple LED Output Port is High - LED is On

7405 inverter output will be low Current flows through 7405 to ground

Output Port is Low – LED is Off 7405 inverter output will be high impedance No current flows through 7405

Resistor used to control brightness: LED is about 2V, 10-20mA

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LED – Open Collector NPN transistor is On if Vb is High LED will be On is Vb is High Can support up to 250mA

NPN

CMicro-

controller

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LED – Open Emitter PNP transistor is On if Vb is Low LED will be On is Vb is Low Can support up to 250mA

PNP

E

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Directly Controlling LEDs We can use external IC to drive the LEDs

Reduce current driven by microcontroller and protects it 74LS244IC Tri-state Buffer/Line Driver/Line Receiver

Used to provide power for LEDs Has OE signals that can be used to disable the output

OE1

A8A7A6A5A4A3A2A1

A8A7A6A5A4A3A2A1

OE2

74LS244

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Directly Controlling LEDs Limitation - Can only drive as many LEDs as available pins Solutions:

Latching Scanning Multiplexing

OE1

A8A7A6A5A4A3A2A1

A8A7A6A5A4A3A2A1

OE2

74LS244

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LEDs - Latching Control LED by using a latch hardware Use additional hardware to control when to latch values

Similar to memory mapped IO

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

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Address=0 Address=1

LED Data

LED Address

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LEDs - Latching Control LED by using a latch hardware Use additional hardware to control when to latch values

Similar to memory mapped IO

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

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Address=0 Address=1

Always writing last LED value!

Any problems??Addr

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LEDs - Latching Add enable signal from microcontroller

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

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Address=0 Address=1

AddrEn

What did we build?Is there another way to implement the address

decoding?

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LEDs - Latching Use decoder for addressing LED banks

Decoder - Converts input binary number to single high output

2-input decoder: four possible input binary numbers Thus, four outputs, one for each possible input binary number

Enable signal Outputs all 0 if e=0 Regular behavior if e=1

i0

d0

d1

d2

d3

i1

i0

i1

d0

d1

d2

d3e

i1’i0’

i1’i0

i1i0’

i1i0


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LEDs - Latching Use decoder for addressing LED banks

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

Latch

OE

A8

A7

A6

A5

A4

A3 A2

A1

A8

A7

A6

A5

A4

A3 A2

A1

LE

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Address=0 Address=1

AddrEn i0

d0d1

en

1x2 Decoder

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LEDs - Scanning Use software executing on the microcontroller to control LEDs row

by row, col by col, or bank by bank (depending on organization) Operation

Continually scan LEDs fast enough that human eye cannot detect it Similar to refresh rate of TVs and Monitors

Enable one column of LEDs by writing 0 to output port controlling desired column

Write LEDs value for select row Cannot access all LEDs simultaneously

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Column Select

Row Data

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LEDs - Multiplexed Use software executing on the microcontroller to control LEDs

one at a time Operation

Continually scan LEDs fast enough that human eye cannot detect it Similar to refresh rate of TVs and Monitors

Enable LED column by outputting column address to column decoder Enable individual LEDs by outputting row address to row decoder

Cannot access all LEDs simultaneously

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dec

4x16

dec

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How many LEDs can I control using only eight output pins using the following schemes?

Latched:

Scanned

Multiplexed:

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How many LEDs can I control using only eight output pins using the following schemes?

Latched: 64 LEDs – Individual latch used for each LED 1 Enable Output, 6 Address Lines, and 1 Data Line Six address lines allow for access to 26 latches (or LEDs)

Scanned 16 LEDs – 4 columns by 4 rows

Multiplexed: 256 LEDs – 16 columns by 16 rows Enabled by using 4x16 decoder to select both row and

columns

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Mill Game How many pins do we need to connect the 48 LEDs in the

following diagram?

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Mill Game

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7-Segment LEDs

Common Cathode

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7-Segment LED – Design Example Control three 7-segment LED displays Scanning through the entire display must happen faster

than the human eye can detect (> 40 Hz) The LED must look like it is continuously on at 10 mA

Average current = 10 mA

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7-Segment LED – Design Example Use scanning to control 7-segment LEDs

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7447 – BCD to 7-Segment Display

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7-Segment LED – Design Example Use scanning to control 7-segment LEDs

15 30

Time (ms)

Current (mA)

30

10 mA DC

1/15 ms > 66 Hz

ece 372 – microcontroller design parallel io ports - outputs

Documents

timer io

unused ports

high led

latchingcontrol led

low led

led value

groundoutput port

port adused