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Khaled A. Al-Utaibi  Buffering and Latching  8086 Bus System − Bus Timing − Bus Write Cycle − Read Cycle − Ready and Wait States.

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Presentation on theme: "Khaled A. Al-Utaibi  Buffering and Latching  8086 Bus System − Bus Timing − Bus Write Cycle − Read Cycle − Ready and Wait States."— Presentation transcript:

1 Khaled A. Al-Utaibi

2  Buffering and Latching  8086 Bus System − Bus Timing − Bus Write Cycle − Read Cycle − Ready and Wait States  8086 Bus System Design − Objective − Requirements − Procedure

3  Before starting with 8086 bus system design, one needs to understand the following concepts: − (1) Fan-out limitations of logic gates − (2) Buffering − (3) Latching

4  If the output of some gate A is connected to the input of another gate B, gate A is said to be driving gate, while gate B is said to be the load gate.  The fan-out of a gate is the largest number of gate inputs this gate can drive.  The fan-out for TTL is 10 loads.  The fan-out for CMOS is 8 loads A B Driver Gate Load Gate

5  A buffer is a single input single output gate that provides the necessary drive capability which allows driving larger loads.

6  The outputs of 2 logic gates can not be directly connected because this causes a short circuit that results in huge current flow may result in damaging the circuit  The solution to this problem is to use tri-state buffers.  A tri-state buffer is a special gate with 3 states controlled by an enable input (E) :  Logic 0 (Low Voltage),  Logic 1 (High Voltage), and  Hi-Impedance (Open Circuit)

7  74LS245 3-State Bidirectional Octal Buffers:

8  74LS244 3-State Unidirectional Octal Buffers:

9  The D-Latch is a memory device which can store one bit of data for as long as the device is powered.  The D-Latch has one data input (D), one control input (C) and one data output (Q/Q’). − When C = 0, the output Q keeps its value (No Change) − When C = 1, the output Q follows the input D (Q = D)

10  74LS374 3-State Octal D-Type Transparent Latches :

11  The 8086 has three buses − (1) Address Bus: provides memory and I/O with the memory address or the I/O port number − (2) Data Bus: transfers data between the microprocessor and the memory and I/O − (3) Control Bus: provides control signals to the memory

12  8086 access memory and I/O devices in periods called bus cycles.  Each cycle equals 4 system-clocking periods (T states).  If the clock is operated at 5 MHz, one 8086 bus cycle is complete in 800 ns.



15  The READY input causes wait states for slower memory and I/O components.  A wait state (Tw) is an extra clocking period inserted between T2 and T3.  The READY input is sampled (checked) at the end of T2 and in the middle of Tw.  If READY is a logic 0 at the end of T2, then T3 is delayed and Tw is inserted between T2 and T3  READY is next sampled at the middle of Tw to determine if the next state is Tw or T3.  The READY signal is synchronized with clock using the 8284A clock generator.

16  Objective: to provide the system the three main buses required to access memory and I/O devices − (1) Address Bus − (2) Data Bus − (3) Control Bus

17  Requirements: bus design must satisfy two main requirements − (1) Bus Buffering: the system must be buffered if the number of devices interfaced to it is more than its fan-out − (2) Bus Demultiplexing (Latching): All time multiplexed lines of the processor must be first demultiplexed (latched) to split address, data, status and control lines before interfacing them to memory and I/O devices.

18  Designing a bus system involves the following steps: − (1) Identify input, output and input/output pins of the processor. Note that only output and input/output pins are considered for designing the bus system (i.e. input pins are not part of the bus system). − (2) Identify time multiplexed pins of the processor. These pins need to be demultiplexed. − (3) Use latches to demultiplex time multiplexed pins. − (4) Use buffers to buffer all data, address and control lines to be connected to memory and I/O devices.  Latched lines are already buffered.  Input/output lines require bidirectional buffers  Output lines require unidirectional buffers

19  Design a fully buffered and de-multiplexed bus system for a minimum mode 8086-based microcomputer system.

20  Inputs: these are not part of bus system design − VCC − GND − NMI − INTR − CLK − RESET − READY − TEST’ − HOLD − MN/MX’

21  Outputs: − A16/S3 − A17/S4 − A18/S5 − A19/S6 − RD’ − HOLDA (not required, why?) − WR’ − M/IO’ − DT/R’ − DEN’ − ALE − INTA’ (not required, why?)

22  Inputs/Output: − AD0-AD15

23  Time Multiplexed Pins: − AD0-AD15 − A16/S3-A19/S6 − BHE’/S7

24  As shown in Step(2), the 8086 has 21 multiplexed lines: − AD0-AD15 − A16/S3-A19/S6 − BHE’/S7  To latch (demultiplex) these lines using 74LS373 octal latches, we need  21/8  = 3 chips.  The input controls of the 74LS373 octal latches should be connected as follows : − Output Control (OE’) connected to GND (Why?) − Latch Enable (G’) connected to ALE (Why?)


26  The data lines D15-D0 should buffered using bidirectional buffers (74LS245). Why? − We have 16 data lines. So, we need 16/8 = 2 chips. − The Buffer Enable (G’) should be connected to DEN’ (Why?) − The Direction Control (DIR) should be connected to DT/R’ (Why?)  The Control Lines M/IO’, RD’ and WR’ should be buffered using unidirectional buffers (74LS244). Why? − The Buffer Enable (G’) should be connected to GND (Why?)

27  Since address lines A19-A0 & BHE’ are latched using 74LS373 octal latches, they are already buffered.  The following control lines are will not be buffered: − DT/R’ & DEN’ because they are will not be connected to memory or I/O devices. − HOLDA because the direct memory access is not supported by our system. − INTA’ because interrupts are not supported by our system.


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