1. 8086/8088 Hardware Specifications (Chapter 8)
Microprocessors
8086/8088 Hardware Specifications
(Chapter 8)
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2. 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 (Interrupt Acknowledge).
The HOLD and HLDA (Hold Acknowledge) signals are used to enable DMA (Direct Memory Access).
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3. 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 memory devices.
The RD and WR signals are used in the Read and Write cycles.
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4. 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.
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5. 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 the RC 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.
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6. Operation of the Reset Circuit
Initially the capacitor is uncharged. When power is switched on, the Reset signal is at logic 1. The capacitor starts charging with time constant (10K*10uF). When the voltage across the capacitor becomes equal to the minimum High voltage of the 8284 (2V), the Reset 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, thus discharge C1 fast.
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7. DC Characteristics and Fan Out
It is essential to examine the DC characteristics of any devices involved in a microprocessor design, before connecting anything on the microprocessors pins. Failure to do so might result in malfunctions or even damages on some components.
Fan-Out of a device is the maximum number of similar devices that can be connected on the output of that device without any problems.
The Fan-Out is limited by the current sink of the 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 noise immunity of the 8088 is =0.35V. This reduces the maximum fan out to 10.
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8. Bus Demultiplexing
The processor loads on the address bus (AD0 to AD7 and A8 to A19) the address to be used, and sets 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)
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9. Bus Buffering
The 74LS373 and the 74LS245 are used to demultiplex the AD0 to AD7 lines. They also provide the 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 be buffered using the 74LS244 octal buffer.
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10. A fully buffered/demultiplexed 8088 system
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11. A fully buffered/demultiplexed 8086 system
The main difference with the 8086 processor is that it has a 16-bit data bus multiplexed with the 16 lower address lines.
Thus the 16-bit data bus (AD0 to AD7 and AD8 to AD15) must be demultiplexed.
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12. 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
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13. 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/M΄or M/ΙΟ΄ signals are also output
During T2 the WR΄, DEN΄are 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.
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14. SIMPLIFIED 8086/88 READ 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/M΄or M/ΙΟ΄ signals are also output
During T2 the RD΄, DEN΄are 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 RD΄ signal is deactivated
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15. 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.
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16. Wait state generator circuit
Wait states are extra clock pulses pulses inserted when the processor is accessing slow memory or I/O devices.
The 8088/8086 allow approximately 3 clock pulses for a memory read or memory write. If the access time of the memory (including the delays inserted by the bus buffers and address decoders) is longer than the access time of the processor (3/f) then wait states are needed.
The circuit shown adds 1 wait state in each memory read or write cycle. The number of wait states can be changed by changing the position of the jumper on the outputs of the 74LS164 shift register.
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