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Microprocessors
8086/8088 Hardware Specifications
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8088 pin outs and the pin functions
The 8088 microprocessor is housed in a 40-pin DIP chip.
Power is supplied between the Vcc and the GND pins. The
voltage at Vcc should be +5V 10%.
The clock at the CLK pin provides the basic timing to the
microprocessor. The clock must have a 33% duty cycle.
The microprocessor is reset if the RESET pin is held high for
at least four clock periods.Whenever the microprocessor is
reset, it begins executing instructions at memory location
FFFF0H. The READY signal is used to insert wait states, to enable the
communication between the microprocessor and slower
memory or peripheral devices.
Interrupts are supported by the signals NMI (Non-Maskable
Interrupt), INTR (Interrupt Request) and INTA (InterruptAcknowledge).
The HOLD and HLDA (Hold Acknowledge) signals are used
to enable DMA (Direct Memory Access).
A19/S6
A18/S5
A17/S4A16/S3
A15
A8
AD7
AD0
ALE
DEN'
DT/R'
RD'WR'
IO/M'
CLK
RESET
READY
NMI
INTR
INTA'
HOLD
HLDA
MN/MX'
TEST'SS0'
Vcc
GND
8088 CPU
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8088 pin outs and the pin functions
The 8088 can operate in a minimum mode (MN/MX=1) or in a
maximum mode (MN/MX=0). The maximum mode is used in
multiprocessor applications or when a math coprocessor is used.
The 8088 has a 20 bit address bus and an 8-bit data bus.
The address lines A0..A7 are multiplexed with the data lines
D0..D7 on the pins AD0..AD7.
The address lines A16..A19 are multiplexed with status lines.
If the ALE (Address Latch Enable) signal is activated (logic 1),
the AD0..AD7 pins carry the addresses A0..A7. The DEN (Data Enable) signal is used to enable the external
data bus buffers.
The DT/R (Data Transmit/Receive) signal is used to specify the
direction of the external data bus buffers.
The IO/M signal is used to select between I/O and memorydevices.
The RD and WR signals are used in the Read and Write cycles.
A19/S6
A18/S5A17/S4
A16/S3
A15
A8
AD7
AD0
ALE
DEN'
DT/R'
RD'
WR'
IO/M'
CLK
RESET
READY
NMI
INTRINTA'
HOLD
HLDA
MN/MX'
TEST'
SS0'
Vcc
GND
8088 CPU
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8086 pin outs and the pin functions
Most of the 8086 pins/signals function the same way as the
8088 pins/signals.
The main differences between the 8088 and the 8086 are:
The 8086 has a 16-bit data bus.
The address lines A0..A15 are multiplexed with the data
lines D0..D15 on the pins AD0..AD15.
The BHE (Bus High Enable) signal is used to enable the
most significant data bus bits (D8 ..D15) during a read or
write operation.
The IO/M signal is inverted in the 8086 microprocessor,
that is a memory is enabled if the IO/M is high, while an
I/O device is enabled if the IO/M signal is low.
A19/S6
A18/S5A17/S4
A16/S3
AD15
AD0
ALE
DEN'
DT/R'
RD'
WR'
IO'/M
CLK
RESET
READY
NMI
INTRINTA'
HOLD
HLDA
MN/MX'
TEST'
BHE'
Vcc
GND
8086 CPU
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Clock/Reset/Ready Circuit
The 8284 chips serves three
purposes:
Generates the main clock
(CLK) for the processor
(fc/3 with 33% duty cycle)
and the clock for the
peripheral devices (fc/5).
Provides the Reset pulse
according to the state of theRC circuit connected at the
RES input.
Provides the Ready signal to
insert wait states whenever
the processor is accessing
slow memory or peripheral
I/O ports.
100
10K
10uF
+5V
15MHz
On/Off
Reset
8284
X1
X2
RES RESET
CLK
PCLK
OSC
READY
RDY
Wait State Circuit
15MHz
5MHz
3MHz
READY
RESET
CLK
8086/8088
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Operation of the Reset Circuit
Initially the capacitor is uncharged. When power is switched on, the Reset signal is atlogic 1. The capacitor starts charging with time constant (10K*10uF). When the voltageacross the capacitor becomes equal to the minimum High voltage of the 8284 (2V), theReset signal goes to logic 0.
If the Reset button is pressed, the capacitor is discharged through the switch. When the
Button is released, the capacitor starts charging as before. Resistor R1 is used to reduce the current through C1 when the Reset button is pressed,
thus avoid damaging C1. The diode is used to short circuit R1 during switch off, thusdischarge C1 fast.
R2:100
R1
10K
C1
10uF
+5V
On/Off
Reset
VRES
VRES
Reset
Switch ON Reset Button Pressed Reset Button Released Switch OFF
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DC Characteristics and Fan Out
It is essential to examine the DCcharacteristics of any devices involved in amicroprocessor design, before connectinganything on the microprocessors pins.
Failure to do so might result in malfunctionsor even damages on some components.
Fan-Out of a device is the maximum numberof similar devices that can be connected onthe output of that device without anyproblems.
The Fan-Out is limited by the current sink ofthe device (Fan-Out = IOLmax/IILmax)
For example the IOLmax of the 8088 is 2
mA and the IILmax of the 74LS family is
0.4 mA. Thus the fan out is 2.0/0.4 = 5.
The Fan-Out is also limited by the noise
immunity (VILmax- VOLmax) . The noiseimmunity of the 8088 is 0.8-0.45=0.35V.This reduces the maximum fan out to 10.
Input Characteristics of the 8086/8088
Logic Voltage Current
0
1
VILmax
= 0.8 V
VIHmin
= 2.0 V
IILmax
= 10 uA
IIHmax
= 10 uA
Output Characteristics of the 8086/8088
Logic Voltage Current
0
1
VOLmax
= 0.45 V
VOHmin
= 2.4 V
IOLmax
= 2.0 mA
IOHmax
= -400 uA
Recommended Fan-Out of the 8086/8088
Family ISINK
Fan-Out
TTL (74)
TTL (74LS)
-1.6 mA
ISOURCE
TTL (74ALS)
TTL (74F)
CMOS (74HC)
CMOS (CD4)
NMOS
-0.4 mA
-0.1 mA
-0.5 mA
-10 uA
-10 uA
-10 uA
40 uA
20 uA
20 uA
25 uA
10 uA
10 uA
10 uA
1
5
10
10
10
10
10
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Bus Demultiplexing
The processor loads on the address bus (AD0 to AD7 and A8 to A19) the address to be used, andsets the ALE. Thus the address signals A0 to A7 are latched on the 74LS373.
On the next clock the processor resets the ALE and the AD0 to AD7 lines are used to carry data(D0 to D7). The DEN enables the buffers of the 74LS245, while the DT/R specifies the direction
(read/write)
CLK
A8..A15
AD0..AD7
ALE
RD'
DT/R'
DEN'
Float
Read Data
A8..A15
A0..A7 D0..D7
Timing Diagram for a Memory Read Cycle
A7A6A5A4A3A2A1A0
D7D6D5D4
D3D2D1D0
AD7AD6AD5AD4AD3AD2AD1AD0
ALE
DEN'DT/R'
8088CPU
LS373
D Q
EN
EN OE
LS245
GDIR
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Bus Buffering
The 74LS373 and the 74LS245 are used to demultiplex the AD0 to AD7 lines. They also providethe necessary buffering for the A0 to A7 and the D0 to D7 lines.
The rest of the address lines (A8 to A15) as well as control lines (RD, WR, and IO/M) need to bebuffered using the 74LS244 octal buffer.
A15A14A13A12A11A10A9A8
A15A14A13A12A11A10
A9A8
8088CPU
LS244
E2E1
4
4
RDWR
IO/M'
LS244
E2E1
4
4
RDWRIO/M'
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A fully buffered/demultiplexed 8088 system
A19/S6A18/S5A17/S4
A16/S3
A15A14A13A12A11A10A9
A8
AD7AD6AD5
AD4
AD3AD2AD1AD0
ALE
DEN'
DT/R'
RD'
WR'IO/M'
CLK
RESET
READY
NMI
INTR
INTA'
HOLD
HLDA
MN/MX'
TEST'SS0'
Vcc
GND
8088 CPU
100
10K
10uF
+5V
15MHz
On/Off
Reset
15MHz
5MHz
3MHz
D0 . . D7
A0 . .A7
A8 . . A15
A16 . . A19
8284
X1
X2
RES
RESET
CLK
PCLK
OSC
READY
RDY1
LS373
D Q
EN
EN OE
x8
LS373
D Q
EN
EN OE
x8
LS245
GDIR
x8
RD, WR, IO/M
CS from memory devices
RDY2
AEN1
0w
1w
2w
3w
4w
5w
6w7w
Q0
Q1
Q3
Q4
Q5
Q6
Q7
Q2
CLK
CLR SI
LS164 (Shift Reg.)
'1'
LS244
E2E1
4
4
LS244
E2E1
4
4
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A fully buffered/demultiplexed 8086 system
D0..D7
A0..A7
A8..A15
A16..A19
LS373
D
Q
EN
EN
OE
x8
LS373
D
Q
EN
EN
OE
x8
LS245
G
DIR
x8
RD,WR,IO/M
LS244
E2
E1
44
A19/S6
A18/S5
A17/S4
A16/S3
AD15
AD14
AD13
AD12
AD11
AD10
AD9
AD8
AD7
AD6
AD5
AD4
AD3
AD2
AD1
AD0
ALE
DEN'
DT/R'
RD'
WR'
IO'/M
CLK
RESET
READY
NM
I
INT
R
INT
A'
HO
LD
HLDA
MN
/MX'
TEST'
SS0'
Vcc
GN
D8086CPU
LS373
D
Q
EN
EN
OE
x8
LS245
G
DIR
x8
D8..D15
The main difference with the 8086 processor is that it has a 16-bit data bus multiplexed with the16 lower address lines.
Thus the 16-bit data bus (AD0 to AD7 and AD8 to AD15) must be demultiplexed.
BASIC BUS OPERATION
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BASIC BUS OPERATION
The 8086/88 processors use the memory and I/O in periods called bus cycles
Each bus cycle equals four system-clocking periods (T1-T4)
For a 5 MHz clock, one bus cycle lasts 800 ns
SIMPLIFIED 8086/88 WRITE BUS CYCLE
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SIMPLIFIED 8086/88 WRITE BUS CYCLE
During the first clocking period (T1), the address is sent to the address and address/data
connections, and the ALE, DT/R and IO/Mor M/signals are also output
During T2 the WR, DENare asserted, and data appear on the bus
In T4 all bus signals are deactivated in preparation for the next bus cycle, and the WR
signal returns to logic 1.
ADDRESS/DATA
WR
SIMPLIFIED 8086/88 READ BUS CYCLE
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SIMPLIFIED 8086/88 READ BUS CYCLE
1 2 3 4 5 6
VALID ADDRESS
ADDRESS DATA FROM MEMORY
Clock
ADDRESS
ADDRESS/DATA
RD
ONE BUS CYCLE
During the first clocking period (T1), the address is sent to the address and address/data
connections, and the ALE, DT/R and IO/Mor M/signals are also output
During T2 the RD, DENare asserted
In T3 the READY signal is sampled and if low, T3 becomes a wait state, to allow time to the
memory to access data
The bus is sampled at the end of T3
Finally, the RDsignal is deactivated
THE READY SIGNAL AND WAIT STATES
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THE READY SIGNAL AND WAIT STATES
A wait state(Tw) is an extra clocking period, inserted between T2 and T3, to lengthen the
bus cycle, allowing slower memory and I/O components to respond.
The READY input is sampled at the end of T2, and again, if necessary in the middle of
Tw. If READY is 0 then a Tw is inserted.
At the end of T2 is sampled on the falling clock edge, while in the middle of Tw, it is
sampled on the rising clock edge.
1 2 3 4
Clock
READY
Tw
Wait state generator circuit
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Wait state generator circuit
Wait states are extra clock pulsespulses inserted when the processor isaccessing slow memory or I/O
devices. The 8088/8086 allow approximately
3 clock pulses for a memory read ormemory write. If the access time ofthe memory (including the delaysinserted by the bus buffers andaddress decoders) is longer than theaccess time of the processor (3/f)then wait states are needed.
The circuit shown adds 1 wait statein each memory read or write cycle.The number of wait states can bechanged by changing the position ofthe jumper on the outputs of the74LS164 shift register.
0w
1w
2w
3w
4w
5w
6w
7w
Q0
Q1
Q3
Q4
Q5
Q6
Q7
Q2
CLK
CLRSI
LS164 (Shift Reg.)
Ready
Ready
8088
8086
CLK
CLK
RDY1
AEN1
RDY2
RD'
WR'INTA'
'1'
CS from memory devices
8284
Recommended