1. Week #4 Parallel IO Interfacing
ENG3640 Microcomputer Interfacing
Week #4 Parallel IO Interfacing

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
ENG3640 Fall 2012

3. Resources Huang, Chapter 4 Sections Huang, Chapter 7 Section
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
ENG3640 Fall 2012

4. 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.
ENG3640 Fall 2012

5. 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?
I/O Interface
Memory
CPU
Data
Address
Control
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6. Memory Mapped I/O vs. Isolated I/O
Memory Mapped I/O (MOTOROLA):
Any instruction that reads or writes memory can read/write I/O Port
Address specifies which module (input, output, RAM, ROM), will communicate with the processor
Ex: LDAA #56 STAA $0024 (copy value to port H)
Isolated I/O (INTEL):
The control bus signals that activate the I/O are separate from those that activate the memory device.
These systems have a separate address space.
Separate instructions are used to access I/O and Memory.
Ex: IN AL, $10 (copy values of port $10 into register AL)
Advantages/Disadvantages?
ENG3640 Fall 2012

7. I/O Port Structure Data Register: for data in transit
Control Register: Hold commands from processor to port
Status Register: Used to monitor I/O activity
Polling
Principle functionality is serve as way station for data in transit between the computer and external world.
Interrupt Driven
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8. 1-KB SRAM
4-KB EEPROM
68HC812A4 Block Diagram
CPU12
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9. Memory Map
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10. Port Details Key wakeup(3) Port H Key wakeup(4) Port J Port S
Direction
Function
Port A
I/O
Single-chip modes: general-purpose I/O
Expanded modes: external address bus ADDR15–ADDR8
Port B
Expanded modes: external address bus ADDR7–ADDR0
Port C
Expanded wide modes: external data bus DATA15–DATA8
Expanded narrow modes: external data bus DATA15–DATA8/DATA7–DATA0
Port D
Single-chip and expanded narrow modes: general-purpose I/O
External 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 signals
General-purpose I/O
Port F
Chip select
Port G
Memory expansion
Port H
Key wakeup(3)
Port J
Key wakeup(4)
Port S
SCI and SPI ports
Port T
Timer port
Port AD I
ADC port
General-purpose input

11. 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.
ENG3640 Fall 2012

12. I/O PORTS Addresses PORTH $0024 Port H Data Register DDRH $0025
Register 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

13. I/O PORTS Usage on EVB
Port B is connected to the Light Emitting Diodes (LEDS)
Each Port B line is monitored by an LED.
In order to turn on Port B LEDs, the PJ1 (Port J pin 1) must be programmed as output and set for logic zero.
If you ignore the status of the LEDs, the Port B can drive any other I/O on the breadboard.
Port P is connected to the Seven Segment Display
There are 4 digits of 7-Segment Displays on the EVB.
Port B is used to drive the 7-segment anodes and PP0-PP3 (Port P) to drive common cathodes
To use the 7-Segments you need to multiplex among them.
Port K is connected to the Liquid Crystal Display (LCD)
Port A is connected to the Hex Key Pad
Port H is connected to the DIP Switches and Push buttons.

14. 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:
Identify the address of an I/O port and its Data Direction Register (DDR)
Program the DDR by writing a value to it.
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).
Load a register with a value and store this value to the address of the I/O PORT.
ENG3640 Fall 2012

15. 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, $ ; 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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16. 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!
ENG3640 Fall 2012

17. Avoiding Transients and Glitches
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.
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, $ ; preset PORTH bit-0 low
bset DDRH, $ ; PORTH bit-0 an output
ENG3640 Fall 2012

18. 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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19. Interfacing: Applications
Like most digital logic devices, the output of MCUs can sink more current than they can source.
Consequently, devices that require significant load current like an LED should be connected active-low.
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20. Light Emitting Diodes (LED)
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.
The forward voltage across the LED is typically about 1.5 to 2 Volts.
MCU produces low
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21. 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.
ENG3640 Fall 2012

22. 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.
ENG3640 Fall 2012

23. Active High since MCU produced a 1 to turn the LED on
Connecting an LED to an Output Port
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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24. Use the 68HC12 Port H to drive green, yellow, red, and blue LEDs
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.
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25. ldaa #$FF ; configure PORTH for output staa DDRH ; “
PORTH equ $24
DDRH 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 ; “
ldaa #$20 ; turn on red LED and turn off other LEDs
ldaa #$10 ; turn on blue LED and turn off other LEDs
jmp forever ; repeat
swi
ENG3640 Fall 2012

26. 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:
Common Cathode
Common Anode
ENG3640 Fall 2012

27. Seven Segment Displays
Common Anode:
All anodes are tied in common.
Segment will be lit whenever a low voltage is applied.
Common Cathode:
all cathodes are tied in common.
Segment will be lit whenever a high voltage is applied.
Current limiting resistors must be included or else you might damage display.
ENG3640 Fall 2012

28. Seven Segment Displays: Examples
Depending on the type of display used a different hex code is generated by the MCU.
Common Anode: sending a ``0” will illuminate the segment.
Common Cathode: sending a ``1” will illuminate the segment.
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29. Seven Segment Displays: Decoders
Some ICs are specially designed to drive 7-segment displays.
They contain buffers to supply required drive currents.
When using MC decoder, no current limiting resistors have to be used since they are built in the MC14495
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30. 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?
ENG3640 Fall 2012

31. 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?
+5V 1K
switch
Output
Open
+5V
Closed 0V
I/O
Port
ENG3640 Fall 2012

32. 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.
+5V
I/O
Port
switch
Output
Open 0V
Closed
+5V 1K
ENG3640 Fall 2012

33. 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
ENG3640 Fall 2012

34. 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
ENG3640 Fall 2012

35. PORTJ: Pull-up/Pull-down Registers
DDRJ
PUPSJ bit
PULEJ bit
Port J mode 1 X
Regular output
Regular input
Input with passive pull-down
Input with passive pull-up
ENG3640 Fall 2012

36. 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!
ENG3640 Fall 2012

37. 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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38. What is the consequence of such bouncing for interfacing?
Switch Bouncing
When mechanical switches are opened or closed there are brief oscillations due to mechanical bouncing (switch bounce).
+5V
+5V A 0V
What is the consequence of such bouncing for interfacing?
ENG3640 Fall 2012

39. 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”
ENG3640 Fall 2012

40. Switch Debouncing: Techniques
Several techniques exist to solve the debouncing problem:
Hardware Techniques:
RS-flip flop
Integrating debouncer (capacitor)
Schmitt Trigger Circuit
Software Techniques:
Delay loops
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41. Hardware Debouncing: Flip Flop
NAND
An SR latch can be used to debounce a switch I1 I2 F 1
+5V S Q S R R S R Q 1 X Q
+5V
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42. 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.
VOUT
V T-
V T+
5.0
VIN
Example: 74LS14
2.1
2.9
5.0
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43. Hardware Debouncing: Schmitt Trigger
A noisy slowly changing input
Output produced by ordinary inverter
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44. Hardware Debouncing: Schmitt Trigger
A noisy slowly changing input
Output produced by ordinary inverter
Output produced by inverter with 0.8 V of hysteresis
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45. Integrating Debouncer:
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! 5V R
Vout
Threshold Level C
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46. 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.
ENG3640 Fall 2012

47. 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 R
74LS14
MCU
Input Port
Vout C
Problem?
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48. Integrating Debouncer:5V
74LS14 1K
Vout
MCU
Input Port
22 ohm C
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.
ENG3640 Fall 2012

49. Software Debouncing
To debounce a switch we can use the following simple approach:
A software time delay is used that provides a time delay (usually ms) longer than the duration of the switch bouncing action.
So if switch goes low, wait for longer than 10ms or 20ms and then test for the switch still being low.
ENG3640 Fall 2012

50. 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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51. 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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52. Keypad Decoding
To detect a short circuit, the MCU drives one of the output lines low
Checks the corresponding input line, if it is low, key was pressed.
If the key was not pressed, the open circuit allows the resistor to pull up the input line to logic high.
The combination of both low logic column and row will identify the pressed key. 1

53. 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:
Configure Port H such that 4 MSB are output and 4 LSB are input.
Select the row containing keys 0,1,2,3
If Key 0 pressed then
debounce key 0 (i.e. jump to subroutine wait 10ms)
get the ASCII code of 0
else if key 1 pressed then
debounce key 1 (i.e. jump to subroutine wait 10ms)
See page 265, Example 7.9 in your text book
ENG3640 Fall 2012

54. Liquid Crystal Displays
Liquid Crystal Displays (LCDs) are widely used in microcomputer systems (Embedded Systems).
They are used in watches, calculators, instrument panels, consumer electronic displays (VCRs)
Advantages over LEDs and 7-Segment Displays?
Low power consumption with respect to LEDs. This allows the display and even the computer system to be battery operated.
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?
Low response.
ENG3640 Fall 2012

55. Liquid Crystal Displays: Structure
An LCD display has two plates separated by crystal material.
The polarizer plates are used to pass light (acts like a capacitor).
The liquid crystal can be made to pass or stop light.
Unlike LEDs that convert electric power to optical power, an LCD uses AC voltage to change the light.
An LCD could be either of reflective type or an absorption type.
ENG3640 Fall 2012

56. Liquid Crystal Displays: Operation
The light energy is supplied by (a) room (b) separate back light.
The LCD display requires an alternating excitation wave applied to selected electrodes to charge selected areas.
The excitation wave develops an electrostatic field to align the liquid crystal molecules in these selected areas.
When the crystals are aligned they allow light to pass through to the mirror.
In the charged areas the mirror reflects more light.
ENG3640 Fall 2012

57. 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.
Control FP
Front Plane
XOR
Liquid Crystal Material
VLCD
60 Hz Oscillator BP
Back Plane
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58. 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).
ENG3640 Fall 2012

59. 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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60. HD44780 LCD Controller Instr. Set
Instruction
Command Code
Description
Time
Clear Display
Clears Display
1.64 ms
Cursor Home x
Returns cursor to home position
Function Set
001 DL N F * *
Sets interface data length, # of display lines, char font
40 micro sec
Display on/off control
D C B
Set on/off of all display (D), cursor on/off (C), and blink (B)
D: display on/off =off, 1=on
F: font size =5x7 dots, 1=5x10 dots
B: cursor blink on/off =off, 1=on
N: number of lines =1 line, 1=2 lines
ENG3640 Fall 2012

61. HD44780: Initialization LCD Initialization Routine
initlcd ldaa #$3C ; configure display format to 2x40
jsr lcdcmd ; ”
ldaa #$0f ; turn on display and cursor
jsr lcdcmd ; “
ldaa #$ ; shift cursor right
ldaa #$ ; clear display and return cursor to home
rts
; Send a command in A to the LCD command register
lcdcmd staa cmd_reg ; write command
jsr delay40 ; wait
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62. Extra Slides
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63. Interfacing: Loads
The second case is to loads which will require more current than 10mA
We might want to connect an LED or small relay to an output of the MCU.
To do this we need to add a resistor to limit the current or a driver 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.
ENG3640 Fall 2012

64. Interfacing: No-Loads
The data sheet for a digital device lists two values for the voltage output levels.
The first is for small loads, typically 10mA (use for the same family, i.e HCMOS devices)
This represents about 10 HCMOS logic gates.
ENG3640 Fall 2012

65. Interfacing: Current Parameters
The interface current parameters are the output currents, IOH and IOL, and the input leakage current IIN.
IOH is the current flowing out of a high output
IOL is the current flowing out of a low output
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.
Static fanout for a low output is: nL = | IOL,max |/| IIn |
Static fanout for a high output is: nH = | IOH,max |/| IIn |
n = min [ nH, nL ]
ENG3640 Fall 2012

66. 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 |
ENG3640 Fall 2012

67. 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
Normal Mode
Special Mode
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68. Special expanded narrow Normal expanded narrow
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
Mode
Port A
Port B
Port C
Port D
Special single-chip
G.P.(1) I/O
G.P. I/O 1
Special expanded narrow
ADDR
DATA
Special peripheral
Special expanded wide
Normal single chip
Normal expanded narrow
Reserved
(forced to peripheral) —
Normal expanded wide
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69. 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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70. 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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71. Connecting an LED to an Output Port
Step 1. Determine RL such that IL is at least 5mA.
IL flows through RL the LED, and through R n(on) . R p(on) is infinite because PMOS is off.
RL = (VDD - IF.R n(on) – VF) / IF
= (5V – 5mA.250W – 2V) / 5mA = 150
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
Conservative approach: set R n(on), min =0 and V F,min = 0
Practical approach: use 50% of the maximum values for
R n(on), min and V F,min
ENG3640 Fall 2012

72. Connecting an LED to an Output Port
Conservative Approach:
IF,max = (VDD,max) / (RL,min)
= 5.5V/ = 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
ENG3640 Fall 2012

73. 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.
ENG3640 Fall 2012

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-down
ENG3640 Fall 2012

75. Hardware Debouncing: Flip Flop
An SR latch can be used to debounce a switch R
+5V S Q
+5V
ENG3640 Fall 2012

76. Software Flowcharts for debouncing
Wait for press
Wait for release
Switch
Switch
not pressed
pressed
not pressed
pressed
Wait 10 ms
Wait 10 ms
RTS
RTS
Polling vs Interrupt Driven?
ENG3640 Fall 2012

77. Using Hardware Decoding Chips
8x8 matrix keyboard with 64-keys, 74HC138 (3-to-8 Decoder), and HC151 (multiplexer).
PC0, PC1, PC2 are used to send values to rows
The MCU will scan the columns one by one via PC3, PC4, PC5
ENG3640 Fall 2012

78. Software Debouncing
To debounce a switch we can use several approaches:
Approach #1:
A software time delay is used that provides a time delay (usually ms) longer than the duration of the switch bouncing action.
So if switch goes low, wait for longer than 10ms or 20ms and then test for the switch still being low.
Approach #2:
Initialize a counter with a value of 10 and after the first logic low level is detected, poll the switch every millisecond.
If the switch output is low, decrement the counter. If the switch output is high increment the counter.
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.
ENG3640 Fall 2012

79. 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.
ENG3640 Fall 2012

80. Keypad Decoding
To detect a short circuit, the MCU drives one of the output lines low
Checks the corresponding input line.
If it is low, the key was pressed.
If the key was not pressed, the open circuit allows the resistor to pull up the input line to logic high.
The combination of both low logic column and row will identify the pressed key.
For example, to check key code 5 the MCU drives PC1 (terminal J) low and checks the input at PC5 (terminal E) .
ENG3640 Fall 2012

81. 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.
ENG3640 Fall 2012

82. Keypad Decoding: 4x4 keypad
MCU sends low signal to these lines
MCU checks these lines
MCU checks these lines
ENG3640 Fall 2012