1. Engineering 4862 Microprocessors Lecture 21
Cheng Li
EN-4012

2. 8086/88 uPro and Supporting Chips
8086/88: Microprocessor
8237: DMA controller to transfer data
8284A: Clock generator, provide critical timing for the microprocessor
8288: Provide control signals
8253/8254: Timer
8255: Port expansion
8259: Interrupt controller
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3. 8088 / 8086 CPU in Min Mode
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4. 8086/88 uPro and Supporting Chips
Data Bus:
Pins AD0 – AD15 for 8086; Pins AD0 – AD7 for 8088
Great effort to minimize the number of pins for external connection  Multiplexed address and data busses
ALE (Address Latch Enable)：Signals whether the information is address or data
When address is sent, ALE = HIGH
When data is to be sent out or in, ALE = LOW
Bidirectional bus go through 74LS245 transceiver
DT/R and DEN: two signals to activate the transceiver
DT/R = HIGH: transmit information from uPro: A  B
DT/R = LOW: receive information from outside: B  A
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5. Role of ALE in Addr/Data Demux
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6. ALE Timing in 8088 Based System
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7. D Latch (74LS373)
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8. Bidirectional Buffer (74LS245)
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9. 8086/88 uPro and Supporting Chips
Address Bus:
To demultiplex the address signals from the address/data pins  A latch must be used
74LS373 is commonly used to grab the address
Two purposes: 1) Latch the address from the 8088 and provide address to the entire computer
Controlled by signal AEN and ALE
When AEN (connect to OE) is LOW, 8088 provides address busses to the system. The 8288 provide ALE (connect to G) to enable to latch the address from the CPU. Thus providing a 20-bit stable address to all memory, peripheral and expansion slots
2) To isolate the system address busses from local address busses
System busses could be used by DMA or other boards through the expansion slots. Must not disturb CPU. Achieved by AEN.
Engr 4862 Microprocessors

10. Local Bus v.s. System Bus
Local Bus: to the left of the 8288, 74LS373, 74LS245
System Bus: everything to the right side of those chips
Why Bus buffering (boosting)?
When a pulse leaves an IC, it can lose source of its strength, depending on how far away the receiving IC chip is located
The more pins a signal is connected to, the stronger the signal must be to drive them all  Thus we need buffering
Bus buffering is nothing more than boosting the signals traveling on the busses  commonly used 74LS244, 245
Signals provided by the CPU need boosting since 8086/88 is a CMOS chip and MOS has a much lower driving capability than that of TTL
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11. 8088 Connection and Bus in IBM/XT
Engr 4862 Microprocessors