1 Transceivers
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1 Transceivers
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Engineering 4862 Microprocessors
Lecture 21Cheng [email protected]
Engr 4862 Microprocessors
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8086/88 uPro and Supporting Chips8086/88: Microprocessor8237: DMA controller to transfer data8284A: Clock generator, provide critical timing for the microprocessor8288: Provide control signals
8253/8254: Timer8255: Port expansion8259: Interrupt controller
Engr 4862 Microprocessors
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8088 / 8086 CPU in Min Mode
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8086/88 uPro and Supporting ChipsData Bus:Pins AD0 AD15 for 8086; Pins AD0 AD7 for 8088Great effort to minimize the number of pins for external connection Multiplexed address and data bussesALE (Address Latch Enable)Signals whether the information is address or dataWhen address is sent, ALE = HIGHWhen data is to be sent out or in, ALE = LOWBidirectional bus go through 74LS245 transceiverDT/R and DEN: two signals to activate the transceiverDT/R = HIGH: transmit information from uPro: A BDT/R = LOW: receive information from outside: B A
Engr 4862 Microprocessors
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Role of ALE in Addr/Data Demux
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ALE Timing in 8088 Based System
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D Latch (74LS373)
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BidirectionalBuffer(74LS245)
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8086/88 uPro and Supporting ChipsAddress Bus:To demultiplex the address signals from the address/data pins A latch must be used74LS373 is commonly used to grab the addressTwo purposes: 1) Latch the address from the 8088 and provide address to the entire computerControlled by signal AEN and ALEWhen 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 slots2) To isolate the system address busses from local address bussesSystem busses could be used by DMA or other boards through the expansion slots. Must not disturb CPU. Achieved by AEN.
Engr 4862 Microprocessors
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Local Bus v.s. System BusLocal Bus: to the left of the 8288, 74LS373, 74LS245System Bus: everything to the right side of those chipsWhy 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 locatedThe more pins a signal is connected to, the stronger the signal must be to drive them all Thus we need bufferingBus buffering is nothing more than boosting the signals traveling on the busses commonly used 74LS244, 245Signals 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
Engr 4862 Microprocessors
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8088 Connection and Bus in IBM/XT
Engr 4862 Microprocessors
