Engineering 4862 Microprocessors Lecture 21 Cheng Li EN-4012 licheng@engr.mun.ca

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 Engr 4862 Microprocessors

8088 / 8086 CPU in Min Mode Engr 4862 Microprocessors

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 Engr 4862 Microprocessors

Role of ALE in Addr/Data Demux Engr 4862 Microprocessors

ALE Timing in 8088 Based System Engr 4862 Microprocessors

D Latch (74LS373) Engr 4862 Microprocessors

Bidirectional Buffer (74LS245) Engr 4862 Microprocessors

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

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 Engr 4862 Microprocessors

8088 Connection and Bus in IBM/XT Engr 4862 Microprocessors