Lecture 8 – Simple I/O (Input and Output) Devices
Contents
8.1. Interface with Light-emitting Diode (LEDs) ............................................................................................................... 2 8.2. LED Circuit of MC9S12DG256 (the Dragon12-Plus Board) ..................................................................................... 2 1. Example 8.2.1 (Example 4.12) – LEDs....................................................................................................................... 3 8.3. Interfacing with Seven-Segment Displays .................................................................................................................. 4 1. Seven-Segment Displays on MC9S12DG256 ............................................................................................................. 5 1. Example 8.3.1 – Seven-segment LEDs ....................................................................................................................... 7 8.4. Generating a Digital Waveform Using I/O Pins.......................................................................................................... 9 1. Example 8.4.1 (Example 4.16) – Generating Siren ..................................................................................................... 9 8.5. Interfacing with Dual Inline Package (DIP) Switches............................................................................................... 11 1. Example 8.5.1 – DIP Switches .................................................................................................................................. 11 Lecture 8 – Simple I/O Devices
Page 1 of 11
Many embedded systems require simple input or output devices such as switches, light-emitting diode
(LED), keypads, and seven-segment displays
8.1. Interface with Light-emitting Diode (LEDs)
LED is often used to indicate the system operation mode such as power is turned on, the system
is in normal mode. An LED can illuminate when it is forward biased and has sufficient current
flowing through it (1mA ~ 10mA). The forward voltage across LED varies from 1.6V to 2.2V.
There three ways to interface with LEDs: positive direct drive, inverse direct drive, and buffered
drive.
VCC
Port
pin
Port
pin
`
VCC
R1
(a) Positive direct drive
R2
(b) Inverse direct drive
Port
pin
74HC04
(c) Buffered drive
R3
Figure 1 – LED Connection
Methods (a) and (b) are often used for a LED because it needs a small current (1mA ~ 2mA) to
produce enough brightness. Method (c) is for LED with a high current to light up. Resistors R1,
R2, and R3 are called current-limiting resistors because it they limit the maximum current
following through LED. For methods (a) and (b), the current-limiting resistor should be large
(between 1.5kΩ to 3kΩ).
8.2.
LED Circuit of MC9S12DG256 (the Dragon12-Plus Board)
The HCS12 board has 8 LEDs connected to Port B. The cathodes of LEDs are connected to PJ1
through current-limiting resistors (2.7kΩ) as below:
H C S1 2
1.5 K 
PB7
P B6
PB5
PB4
PB3
PB 2
PB 1
PB 0
PJ1
Lecture 8 – Simple I/O Devices
Page 2 of 11
1. Example 8.2.1 (Example 4.12) – LEDs
Write an assembly program that turns on each LED, in turn from left to right and then from
right to left with each LED lighted for about half of a second and repeat.
Solution:
LEDs are driven by pins PB7~PB0 once at a time by sending to Port B the values $80, $40,
$20, $10, $08, $04, $02, …, and $01.
; export symbols
XDEF Entry, _Startup
; export 'Entry' symbol
ABSENTRY Entry
; for absolute assembly: mark this as entry
point
; Include derivative-specific definitions
INCLUDE 'derivative.inc'
ROMStart EQU
$4000
; absolute address to place my code/constant data
;**********************************************************************************
; variables / data section
ifdef _HCS12_SERIALMON
ORG
$3FFF - (RAMEnd - RAMStart)
Else
ORG
RAMStart
endif
;**********************************************************************************
; Variables for the program
;**********************************************************************************
lpcnt
DS.B
1
;**********************************************************************************
; Code section
ORG
ROMStart
;**********************************************************************************
; This table contains values for LEDs
;**********************************************************************************
LED_tab DC.B
$80,$40,$20,$10,$08,$04,$02,$01
DC.B
$01,$02,$04,$08,$10,$20,$40,$80
Entry:
_Startup:
; remap the RAM & EEPROM here. See EB386.pdf
ifdef _HCS12_SERIALMON
; set registers at $0000
CLR
$11
; INITRG= $0
; set ram to end at $3FFF
LDAB
#$39
STAB
$10
; INITRM= $39
; set EEPROM to end at $0FFF
LDAA
#$9
STAA
$12
; INITEE= $9
LDS
#$3FFF+1
; See EB386.pdf, initialize the stack pointer
else
LDS
#RAMEnd+1
; initialize the stack pointer
endif
; Set 24MHz clock bus: PLL code for 24MHz bus speed from a 4/8/16 crystal
LDX
#$00
;
BCLR
CLKSEL, X, mCLKSEL_PLLSEL ; clear bit 7, clock derived from
oscclk
BSET
PLLCTL, X, mPLLCTL_PLLON
; Turn PLL on, bit 6=1 PLL on, bit 6=0
PLL off
LDAA
#$05
; 5+1=6 multiplier
STAA
SYNR, X
;
Lecture 8 – Simple I/O Devices
Page 3 of 11
LDAA
#$01
; divisor=1+1=2, 8*2*6 /2 = 48MHz PLL
STAA
BRCLR
BSET
; for 8 MHz crystal
REFDV, X
CRGFLG, X, mCRGFLG_LOCK, * ; Wait until bit 3 = 1
CLKSEL, X, mCLKSEL_PLLSEL ;
freq,
; Configure Port for LEDs
MOVB
#$FF, DDRB
; configure port B for output
BSET
DDRJ, $02
; configure PJ1 pin for output
BCLR
PTJ, $02
; enable LEDs to light
forever: MOVB
#16, lpcnt
; initialize LED pattern count
LDX
#LED_tab
; Use X as the pointer to LED pattern table
led_lp
MOVB
1,X+, PORTB
; turn on one LED
LDY
#50
; wait for half a second
JSR
Delay10ms
;
"
DEC
lpcnt
; reach the end of the table yet?
BNE
led_lp
BRA
forever
; start from beginning
;**********************************************************************************
; The following subroutine creates a time Delay which is equal to [Y] times 10ms.
; The timer prescaler is 1:8.
;**********************************************************************************
Delay10ms:
PSHD
; save ACCD onto the stack
BSET
TIOS, mTIOS_IOS0; enable OC0
LDD
TCNT
Again_10ms:
ADDD
#30000
; start an output compare operation
STD
TC0
; with 10ms time Delay
BRCLR
TFLG1, mTFLG1_C0F, *
LDD
TC0
DBNE
Y, Again_10ms
;
BCLR
TIOS, mTIOS_IOS0; disable OC0
PULD
; Restore the value of ACCD
RTS
;**********************************************************************************
;*
Interrupt Vectors
*
;**********************************************************************************
ORG
$FFFE
DC.W
Entry
; Reset Vector
END
8.3.
Interfacing with Seven-Segment Displays
Seven-segment displays are often used when the embedded system needs to display only a few
digits. Seven-segment displays are mainly used to display decimal digits and a small subset of
letters. Although an HCS12 has enough current to drive a seven-segment display, it is not
advisable to use the HCS12 to drive the seven-segment display directly because the HCS12 may
need to drive other I/O devices in its system. A buffer chip (such as 74HC244) is used to drive
seven-segment display.
Lecture 8 – Simple I/O Devices
Page 4 of 11
470  each
H CS12
a
74HC244
PB6
PB5
PB4
PB3
PB2
PB1
PB0
b f
c
a
b
g
d
e e
c
f
d
g
com m on cathode
Figure 2 – Driving a Single Seven-segment Display with Buffer Chip 74HC244
In the circuit above, Port B of the HCS12 drives a common-cathode seven-segment display
through the buffer chip 74HC244. Since the output voltage of the buffer chip VOH is about 5V,
470-Ω resistors will limit the current to about 6.4 mA, which is sufficient to light an LED
segment. In this circuit, the light patterns corresponding to 10 BCD digits are shown in the table
below:
Segment
Corresponding
Decimal Digit PB6 PB5 PB4 PB3 PB2 PB1 PB0
Hex Number
a
b
c
d
e
f
g
0
1
1
1
1
1
1
0
$7E
1
0
1
1
0
0
0
0
$30
2
1
1
0
1
1
0
1
$6D
3
1
1
1
1
0
0
1
$79
4
0
1
1
0
0
1
1
$33
5
1
0
1
1
0
1
1
$5B
6
1
0
1
1
1
1
1
$5F
7
1
1
1
0
0
0
0
$70
8
1
1
1
1
1
1
1
$7F
9
1
1
1
1
0
1
1
$7B
1. Seven-Segment Displays on MC9S12DG256
The Dragon Plus board has 4 seven-segment displays. The connection is quite different from
the standard connect. Each seven-segment display on the Dragon Plus board has a decimal
point. The two of the digits (DSP3 and DSP4) at the right are deliberately placed upside down
and the hardware connections compensate for this configuration. The reason for the upside
down digits is to place two decimal pointers on the middle as a colon for a clock display.
Lecture 8 – Simple I/O Devices
Page 5 of 11
Figure 3 – Hardware Connection for 7-Segement LEDs display
The digit 3, 2, 1, and 0 are driven by PP0, PP1, PP2 and PP3, respectively, as shown below. A
zero on pin of Port P will turn on the digit display.
#3
#0
800 
a
b
.
.
.
.
.
. g
800  g
common
cathode
PB6
PB0
PB1
74HC244
HCS12
#2
#1
a
b
74HC367
PP0
A0
Y0
PP1
A1
Y1
PP2
A2
Y2
PP3
A3
Y3
PP4
A4
Y4
PP5
A5
Y5
#1
#0
a. . . a
b. . . b
g. . .
common
cathode
#5
a
#0
#1
b
.
.
g . g
common
cathode
.
.
.
common
common
cathode
cathode
.
.
.
Figure 4 – Four Seven-segment LEDs on the Dragon Plus Board
The segments A, B, C, D, E, F, G and Decimal Point are driven by PB0, PB1, PB2, PB3,
PB4, PB5, PB6, and PB7. The hex value of the segment code is shown in the following
tables:
Table 1 – Hex Values of Digit without Decimal Point
Number DP G F E D C B A Hex Value
0
0 0 1 1 1 1 1 1
$3F
1
0 0 0 0 0 1 1 0
$06
2
0 1 0 1 1 0 1 1
$5B
3
0 1 0 0 1 1 1 1
$4F
4
0 1 1 0 0 1 1 0
$66
5
0 1 1 0 1 1 0 1
$6D
6
0 1 1 1 1 1 0 0
$7C
7
0 0 0 0 0 1 1 1
$07
8
0 1 1 1 1 1 1 1
$7F
9
0 1 1 0 0 1 1 1
$67
Lecture 8 – Simple I/O Devices
Page 6 of 11
Table 2 – Hex Values of Digit with Decimal Point
Number DP G F E D C B A Hex Value
0
1 0 1 1 1 1 1 1
$BF
1
1 0 0 0 0 1 1 0
$86
2
1 1 0 1 1 0 1 1
$DB
3
1 1 0 0 1 1 1 1
$CF
4
1 1 1 0 0 1 1 0
$E6
5
1 1 1 0 1 1 0 1
$ED
6
1 1 1 1 1 1 0 0
$FC
7
1 0 0 0 0 1 1 1
$87
8
1 1 1 1 1 1 1 1
$FF
9
1 1 1 0 0 1 1 1
$E7
1. Example 8.3.1 – Seven-segment LEDs
Write an assembly program to display “1234” on 4 digits of seven-segment LEDs.
Solution:
The digits 1, 2, 3, and 4 are displayed on display #3, #2, #1, and #0. The values to send to
Port B and Port P to display one digit at time are in the table below. Each digit is displayed
for 1ms.
Port P
Seven-segment Displayed Port
Display
BCD Digit
B
PP7 PP6 PP5 PP4 PP3 PP2 PP1 PP0 Hex
#3
1
$06
1
1
1
1
1
1
1
0
$FE
#2
2
$5B
1
1
1
1
1
1
0
1 $FD
#1
3
$4F
1
1
1
1
1
0
1
1 $FB
#0
4
$66
1
1
1
1
0
1
1
1
$F7
; export symbols
XDEF Entry, _Startup
; export 'Entry' symbol
ABSENTRY Entry
; for absolute assembly: mark this as entry
point
; Include derivative-specific definitions
INCLUDE 'derivative.inc'
ROMStart EQU
$4000
; absolute address to place my code/constant data
;**********************************************************************************
; variables / data section
ifdef _HCS12_SERIALMON
ORG
$3FFF - (RAMEnd - RAMStart)
Else
ORG
RAMStart
endif
;**********************************************************************************
; Variables for the program
;**********************************************************************************
lpcnt
DS.B
1
;**********************************************************************************
; Code section
ORG
ROMStart
;**********************************************************************************
; This table contains values for 7-segment LEDs
;**********************************************************************************
DispTab DC.B
$06,$FE
Lecture 8 – Simple I/O Devices
Page 7 of 11
DC.B
DC.B
DC.B
$5B,$FD
$4F,$FB
$66,$F7
Entry:
_Startup:
; remap the RAM & EEPROM here. See EB386.pdf
ifdef _HCS12_SERIALMON
; set registers at $0000
CLR
$11
; INITRG= $0
; set ram to end at $3FFF
LDAB
#$39
STAB
$10
; INITRM= $39
; set EEPROM to end at $0FFF
LDAA
#$9
STAA
$12
; INITEE= $9
LDS
#$3FFF+1
; See EB386.pdf, initialize the stack pointer
else
LDS
#RAMEnd+1
; initialize the stack pointer
endif
; Set 24MHz clock bus: PLL code for 24MHz bus speed from a 4/8/16 crystal
LDX
#$00
;
BCLR
CLKSEL, X, mCLKSEL_PLLSEL ; clear bit 7, clock derived from
oscclk
BSET
PLLCTL, X, mPLLCTL_PLLON
; Turn PLL on, bit 6=1 PLL on, bit 6=0
PLL off
LDAA
#$05
; 5+1=6 multiplier
STAA
SYNR, X
;
LDAA
#$01
; divisor=1+1=2, 8*2*6 /2 = 48MHz PLL
freq,
; for 8 MHz crystal
STAA
REFDV, X
BRCLR
CRGFLG, X, mCRGFLG_LOCK, * ; Wait until bit 3 = 1
BSET
CLKSEL, X, mCLKSEL_PLLSEL ;
MOVB
#$FF, DDRB
; configure port B for output
MOVB
#$0F, DDRP
; configure port P for output (PP0~PP3)
forever: MOVB
#4, lpcnt
; initialize LED pattern count
LDX
#DispTab
; Use X as the pointer to display table
again:
MOVB
1,X+, PORTB
; display segment pattern
MOVB
1,X+, PTP
; select the digit for display
LDY
#1
; wait for half a second
JSR
Delay1ms
;
"
DEC
lpcnt
; reach the end of the table yet?
BNE
again
BRA
forever
; start from beginning
;**********************************************************************************
; The following function creates a time Delay which is equal to the multiple
; of 1ms. The value passed in Y specifies the number of 1 milliseconds to be
; delayed.
;**********************************************************************************
Delay1ms:
PSHD
; save ACCD onto the stack
BSET
TIOS, mTIOS_IOS0 ; enable OC0
LDD
TCNT
Again_1ms:
ADDD
#3000
; start an output compare operation
;
1ms = 3000 * 8 / 24Mhz
STD
TC0
; with 1ms time Delay
BRCLR
TFLG1, mTFLG1_C0F, *
; wait until C0F is set
LDD
TC0
; load TC0 for the next loop
DBNE
Y, Again_1ms
;
BCLR
TIOS, mTIOS_IOS0; disable OC0
PULD
; Restore the value of ACCD
RTS
;**********************************************************************************
;*
Interrupt Vectors
*
;**********************************************************************************
Lecture 8 – Simple I/O Devices
Page 8 of 11
ORG
DC.W
END
$FFFE
Entry
; Reset Vector
8.4. Generating a Digital Waveform Using I/O Pins
A periodic digital waveform can be easily generated by manipulating I/O pin voltage level and
inserting an appropriate delay between the two voltage levels. A 1-kHz periodic square wave can
be generated from the PT5 pin (pin 5 of Port T) using the following steps:
 Step 1 – configure the PT5 pin for output
 Step 2 – Pull the PT5 pin to high
 Step 3 – wait for 0.5 ms (the first half of the period)
 Step 4 – Pull the PT5 pin to low
 Step 5 – wait for 0.5 ms (the second half of the period)
 Step 6 – Go back to Step 2.
The PT5 pin, on the Dragon Plus board, is connected to a small speaker. If the frequency of the
square wave is in the audible range, it can be heard. If we alter the frequency of the generated
square wave from the I/O pin between two different values, a two-tone siren can be generated.
The duration of the siren tone is variable. The siren would sound more urgent if the tone duration
were shorter.
1. Example 8.4.1 (Example 4.16) – Generating Siren
Write an assembly program that generate two-tone siren. It alternates between 250Hz and
500Hz with each siren tone lasting half of a second.
Solution:
 500 Hz: T = 1/500 = 2ms, half of the period = 1ms; lasting for 0.5s  repeating 0.5/2ms
= 250 times.
 250 Hz: T = 1/250 = 4ms, half of the period = 2ms, lasting for 0.5s  repeating 0.5/4ms
= 125 times.
; export symbols
XDEF Entry, _Startup
ABSENTRY Entry
point
; export 'Entry' symbol
; for absolute assembly: mark this as entry
; Include derivative-specific definitions
INCLUDE 'derivative.inc'
ROMStart EQU
$4000
; absolute address to place my code/constant data
;**********************************************************************************
; variables / data section
ifdef _HCS12_SERIALMON
ORG
$3FFF - (RAMEnd - RAMStart)
Else
ORG
RAMStart
endif
;**********************************************************************************
; Code section
ORG
ROMStart
Entry:
_Startup:
; remap the RAM & EEPROM here. See EB386.pdf
Lecture 8 – Simple I/O Devices
Page 9 of 11
ifdef _HCS12_SERIALMON
; set registers at $0000
CLR
$11
; INITRG= $0
; set ram to end at $3FFF
LDAB
#$39
STAB
$10
; INITRM= $39
; set EEPROM to end at $0FFF
LDAA
#$9
STAA
$12
; INITEE= $9
LDS
#$3FFF+1
; See EB386.pdf, initialize the stack pointer
else
LDS
#RAMEnd+1
; initialize the stack pointer
endif
; Set 24MHz clock bus: PLL code for 24MHz bus speed from a 4/8/16 crystal
LDX
#$00
;
BCLR
CLKSEL, X, mCLKSEL_PLLSEL ; clear bit 7, clock derived from
oscclk
BSET
PLLCTL, X, mPLLCTL_PLLON
; Turn PLL on, bit 6=1 PLL on, bit 6=0
PLL off
LDAA
#$05
; 5+1=6 multiplier
STAA
SYNR, X
;
LDAA
#$01
; divisor=1+1=2, 8*2*6 /2 = 48MHz PLL
freq,
; for 8 MHz crystal
STAA
REFDV, X
BRCLR
CRGFLG, X, mCRGFLG_LOCK, * ; Wait until bit 3 = 1
BSET
CLKSEL, X, mCLKSEL_PLLSEL ;
BSET
DDRT, mDDRT_DDRT5 ; configure PT5 pin for output
Forever: LDX
#250
; repeat 500Hz waveform for 250 times
Tone1:
BSET
PTT, mPTT_PTT5
; pull PT5 pin high
LDY
#1
; first half period of 500Hz
JSR
Delay1ms
;
BCLR
PTT, mPTT_PTT5
; pull PT5 pin low
LDY
#1
; second half period of 500Hz
JSR
Delay1ms
;
DBNE
X, Tone1
;
LDX
#125
; repeat 250Hz waveform for 125 times
Tone2:
BSET
PTT, mPTT_PTT5
; pull PT5 pin high
LDY
#2
; first half period of 250Hz
JSR
Delay1ms
;
BCLR
PTT, mPTT_PTT5
; pull PT5 pin low
LDY
#2
; second half period of 250Hz
JSR
Delay1ms
;
DBNE
X, Tone2
;
BRA
Forever
;**********************************************************************************
; The following function creates a time Delay which is equal to the multiple
; of 1ms. The value passed in Y specifies the number of milliseconds to be
; delayed.
;**********************************************************************************
Delay1ms:
PSHD
; save ACCD onto the stack
BSET
TIOS, mTIOS_IOS0 ; enable OC0
LDD
TCNT
Again_1ms:
ADDD
#3000
; start an output compare operation
;
1ms = 3000 * 8 / 24Mhz
STD
TC0
; with 1ms time Delay
BRCLR
TFLG1, mTFLG1_C0F, *
; wait until C0F is set
LDD
TC0
; load TC0 for the next loop
DBNE
Y, Again_1ms
;
BCLR
TIOS, mTIOS_IOS0; disable OC0
PULD
; Restore the value of ACCD
RTS
Lecture 8 – Simple I/O Devices
Page 10 of 11
;**********************************************************************************
;*
Interrupt Vectors
*
;**********************************************************************************
ORG
$FFFE
DC.W
Entry
; Reset Vector
END
8.5.
Interfacing with Dual Inline Package (DIP) Switches
A switch is the simplest input device. A set of eight switches organized as a dual inline package
(DIP) is often used. In the Dragon Plus board, DIP switches are connected to Port H. Each Port H
pin is pulled up to high through 51kΩ when a switch is opened. When a switch is closed, the
associated Port H pin input is 0.
V CC
SW DIP-8
10
K
51 kΩ
HCS12
PA0
PH0
PH1
PA1
PH2
PA2
PH3
PA3
PH4
PA4
PH5
PA5
PA6
PH6
PH7
PA7
In addition, there are pushbutton switches SW5, SW4, SW3, and SW2 that are also connected to
PH0, PH1, PH2, and PH3 accordingly. Similar to DIP, when these pushbutton switches are
opened, the Port H pins are pulled high. When a pushbutton switch is pressed, the associated Port
H pin input is 0.
1. Example 8.5.1 – DIP Switches
Write a sequence of assembly instructions that read the value from an eight-switch DIP that
connected to Port H of MC9S12DG256.
Solution:
…
MOVB
LDAA
Lecture 8 – Simple I/O Devices
#$00, DDRH
PTH
; configure Port H for input
; read the value of DIP switches
Page 11 of 11