Chapter 12
Parallel Interfacing Expected Outcomes Identify the internal registers of MC6821 Design the hardware interface for various I/O devices using MC6821 Develop and write codes using MC6821
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Parallel Interface/Timer I/O interface using a simple I/O device such as latch or buffer has its drawbacks One of them is the circuit design must be reconstructed if a user decide to change its I/O devices A dedicated peripheral such as the 68230 (PI/T - Parallel Interface/Timer) Its primary function is to provide parallel interface Its secondary function is a programmable timer
It provides 4 modes of operation with various handshaking and buffering capabilities Unidirectional 8-bits Unidirectional 16-bits Bidirectional 8-bits Bidirectional 16-bits NMKNYFKEEUMP
Parallel Interface/Timer The programmable timer provides a variety of services Periodic interrupt generation Square wave generator Interrupt after time-out Elapse time measurement System watchdog
Since, it is part of 68xxx family, the PI/T is expensive due to its capability to provide wider functions A simple 8-bit family can be used in certain cases to provide parallel I/O interface PIA 6821 peripheral interface adapter is introduced to give flexibility in I/O system NMKNYFKEEUMP
PIA 6821 PIA was initially designed for use in 6800-based system However, it is widely used in 68000-based system due its flexibility and cost Since it is part of 6800 family, additional pin connections are needed to operate in asynchronous mode The port A and B may be programmed individually to be an input or an output – allowing the software to adjust to a new system requirement Additional signal lines may be programmed to allow interrupts and strobe signals for each port Internal register to allow storage of data temporary thus allowing to interface with a slower devices such as printer
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PIA 6821
PIA has two port with each port may drive two TTL loads Each individual signal line (PA0 through PA7 and PB0 through PB7) can be programmed as an input or an output PA6
PA5
PA4
PA3
PA2
PA1
PA0
Vss
9
8
7
6
5
4
3
2
1
32
33
34
35
36
37
38
39
40
D3
D2
D1
D0
RESET*
RS1
RS0
IRQB*
IRQA*
CA2
CA1
PA7 30
6821
31
10
D5
D4
11
29
PB1 12
D6
PB0
PB2
28
E
27
CS1
13
25
CS2*
14
24
CS0
PB3
23
PB4
17
16
22
D7
PB6 18
R/W*
26
PB7 19
21
15
CB1 CB2 20
PB5
Vcc
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PIA 6821 PIA has 6 internal registers that can be programmed to its need Each port has three registers For Port A For Port B Data Register A (DRA) Data Register B(DRB) To transfer data in or out To transfer data in or out Data Direction Register A (DDRA) Data Direction Register B (DDRB) Determine direction of each line Determine direction of each line Control Register A (CRA) Control Register A (CRB) Control the operation of port A Control the operation of port B
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PIA 6821 Internal structure of PIA consist of six registers and control unit
IRQA IRQB
Data Direction Register A (DDRA)
D7-D0 RS0 RS1 E R/W CS0 CS1 CS2 RESET
Control Register A (CRA)
Data Register A (DRA) Control Unit Control Register B (CRB)
Data Direction Register B (DDRB)
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Data Register B (DRB)
PA0 PA1 PA2 PA3 PA4 PA5 PA6 PA7 CA1 CA2 PB0 PB1 PB2 PB3 PB4 PB5 PB6 PB7 CB1 CB1
Interface With 68000 Since 6821 is operating in synchronous mode, additional pin connections are required 6821
68000
RESET
RESET RS0 RS1
A0 A1
A3-A23
Address Decoder
VPA* R/W* E
R/W* E
UDS*
CS1
VMA*
CS2
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PA7 PA6 PA5 PA4 PA3 PA2 PA1 PA0 CA1 CA2 PB7 PB6 PB5 PB4 PB3 PB2 PB1 PB0 CB1 CB2
PIA Registers The address of 6 internal registers of PIA depends on Address decoder Data Strobe connection (UDS or LDS) A2 A1
RS1 RS0
LDS*
CS1
A2 A1
RS1 RS0
UDS*
CS1
DDRA/DRA
N+1
N+2
CRA
N+3
N+4
DDRB/DRB
N+5
N+6
CRB
N+7
N
DDRA/DRA
N+1
N+2
CRA
N+3
N+4
DDRB/DRB
N+5
N+6
CRB
N+7
N
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PIA Registers Since there is 6 registers and only four locations, two registers are located in the same address
To distinguish these registers, bit2 of control register is used
RS1
RS1 Register
0
0
Data Register A/Data Direction Reg. A
0
1
Control Register A
1
0
Data Register B/Data Direction Reg. B
1
1
Control Register B
RS1
RS0
0
0
0
X
Data Direction Reg. A (DDRA)
0
0
1
X
Data Register A (DRA)
0
1
X
X
Control Register A (CRA)
1
0
X
0
Data Direction Reg. B (DDRB)
1
0
X
1
Data Register B (DRB)
1
1
X
X
Control Register B (CRB)
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CRA2 CRB2 Register
PIA Registers Data Register A (DRA) bit7
bit6
bit5
bit4
bit3
bit2
bit1
bit0
DRA7
DRA6
DRA5
DRA4
DRA3
DRA2
DRA1
DDRA0
Data Direction Register A (DDRA) bit7
bit6
bit5
bit4
bit3
bit2
bit1
bit0
DDRA7
DDRA6
DDRA5
DDRA4
DDRA3
DDRA2
DDRA1
DDRA0
Logic 1 in each DDRAi causes the signal line PAi to become output line Logic 0 in each DDRAi causes the signal line PAi to become input line For example: If DDRA = $4C, PA6, PA3 and PA2 are output and others are input
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Control Register CRA7
CRA6
CRA5
CRA4
CRA3
CRA2
CRA1
CRA0
IRQA
IRQB
CA2
CA2
CA2
DDRA/ DRA
CA1
CA1
Set if transition in CA1 Set if transition in CA2
Refer to data sheet for complete information
0: IRQA disabled 1: IRQA is activated
0: CA1 is recognized on positive -to negative transition
0: CA2 is input 1: CA2 is output If CRA5=0, CRA3 & CRA4 follow
1: CA1 is recognized on positive -to negative transition 0: DDRA is chosen 1: DRA is chosen
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Initialize PIA Initializing PIA is the most crucial steps in programming a system using this I/O device Followings are brief procedure to initialize PIA for simple I/O (Assuming port A) Fill bit 2 of CRA with 0 to access DDRA Fill DDRA with proper value to determine the role of each signal line of the port A (1 for output and 0 for input) Fill bit 2 of CRA with 1 to access DRA Write or read data using DRA
Set CRx2=1
Set direction (DDRx)
Set CRx2=0
Read/Write Data to DRx
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Initialize PIA Example : Initialize PIA assuming port A is an output port and port B is an input port ; Initialize Port A MOVE.B #0,CRA MOVE.B #$FF,DDRA MOVE.B #4,CRA ; Initialize Port B MOVE.B #0,CRB MOVE.B #0,DDRB MOVE.B #4,CRB
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More Examples.. Write a program to switch on the LED as the respective switch is pressed assuming all ports are initialized
+5V
2K2
PB0
PA0
PB1
PA1
PB2
PA2
PB3
PA3
PB4
PA4
PB5
PA5
PB6
PA6
PB7
PA7
330R
2K2 330R
2K2
330R
START MOVE.B DRA,D0 NOT.B D0 MOVE.B DRB BRA START
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2K2
330R 2K2 330R
2K2 330R
2K2 330R 2K2 330R
More Examples.. Write a program to display a pressed number on seven segment display
+5V
2K2
PA0 2K2
PA1 2K2
CLR.B CRA MOVE.B #$F0,DDRA MOVE.B #4,CRA LOOP MOVE.B DRA,D0 ASL.B #4,D0 MOVE.B D0,DRA BRA LOOP
PA2 2K2
PA3 PA4 PA5 PA6 PA7
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a b c B d 7448 e f C g dp D A
a f
e
g
d
b
c dp
More Examples.. Write a program to display an upward counter (0,1,2..8,9,0…) continuously
+5V
2K2
PA0 2K2
PA1 2K2
MAIN CLR.B D0 LOOP MOVE.B D0,DRA BSR DELAY ADDQ.B #1,D0 CMPI.B #10,D0 BNE LOOP BRA MAIN
PA2 2K2
PA3 PA4 PA5 PA6 PA7
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a b c B d 7448 e f C g dp D A
a f
e
g
d
b
c dp
Self-Test Exercise Write as simple program to initialize port A where PA0-PA3 are output and PA4-PA7 are input Exercise If port B is connected to 7-segment display, write a program to initialize port B. Then, write a program to display downward counter on port B Exercise If PB0 is connected to a low current speaker, write a program to generate continuous tone with frequency of 1kHz to the speaker
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Self-Test Exercise Write a program to meet following requirements,
+5V
2K2
If switch 1 is pressed, the segment will display upward counter If switch 2 is pressed, the segment will display downward counter If switch 3 is pressed, the segment will display random value continuously If switch 4 is pressed, the program is terminated
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PA0 2K2
PA1 2K2
PA2 2K2
PA3 PA4 PA5 PA6 PA7
a b c B d 7448 e f C g dp D A
a f
e
g
d
b
c dp
