Demultiplexing Address/Data Bus AD7- AD0

Pin 30 of 8085 is the ALE pin which stands for ‘Address Latch Enable’. ALE signal is used to demultiplex the lower order address bus (AD0 – AD7). Pins 12 to 19 of 8085 are AD0 – AD7 which is the multiplexed address-data bus. Multiplexing is done to reduce the number of pins of 8085. Lower byte of address (A0 – A7) are available from AD0 – AD7 (pins 12 to 19) during T1 of machine cycle. But the lower byte of address (A0 – A7), along with the upper byte A8 – A15 (pins 21 to 28) must be available during T2 and rest of the machine cycle to access memory location or I/O ports. Now ALE signal goes high at the beginning of T1 of each machine cycle and goes low at the end of T1 and remains low during the rest of the machine cycle. This high to low transition of ALE signal at the end of T1 is used to latch the lower order address byte (A0 – A7) by the latch IC 74LS373, so that the lower byte A0 – A7 is continued to be available till the end of the machine cycle. The situation is explained in the following figure

Ø  From the above description, it becomes obvious that the AD7–AD0 lines are serving a dual purpose and that they need to be demultiplexed to get all the information.

Ø  The high order bits of the address remain on the bus for three clock periods. However, the low order bits remain for only one clock period and they would be lost if they are not saved externally. Also, notice that the low order bits of the address disappear when they are needed most.

Ø  To make sure we have the entire address for the full three clock cycles, we will use an external latch to save the value of AD7–AD0 when it is carrying the address bits. We use the ALE signal to enable this latch.

Ø  Given that ALE operates as a pulse during T1, we will be able to latch the address. Then when ALE goes low, the address is saved and the AD7–AD0 lines can be used for their purpose as the bi-directional data lines.

Ø  The high order address is placed on the address bus and hold for 3 clk periods.

Ø  The low order address is lost after the first clk period, this address needs to be hold however we need to use latch

Ø  The address AD7 –AD0 is connected as inputs to the latch 74LS373.

Ø  The ALE signal is connected to the enable (G) pin of the latch and the OC –Output control –of the latch is grounded

Data flow from memory to microprocessor unit (MPU)

Step1:- The microprocessor places the 16-bit memory address from the pc on address bus.

Step 2 :- The control unit send the control signal  RD to enable the memory chip

Step3 :- The byte from the memory location is placed on the data bus

Step 4 :- The byte is placed in the instruction decoder of the microprocessor and the task is carried out according to the instruction

REFERENCES

1. R. S. Gaonkar, Microprocessor Architecture, Programming, and Applications with the 8085, Fifth Edition, Penram International Publishing (India) Private Limited.
2. S Ghoshal, Microprocessor Based System Design, Macmillan India Limited, 1996
3. M. Mano, Digital Logic and Computer Design, Prentice – Hall India
4. B. Ram - Fundamentals of Microprocessor and Microcontrollers
5. “Microprocessors: Principles and Applications” by A Pal
6. “Microprocessors and Microcontrollers : Architecture, Programming and Interfacing Using 8085, 8086 and 8051” by Soumitra Kumar Mandal
7. “Introduction to Microprocessors and Microcontrollers” by Crisp John Crisp
8. “Microprocessors And Microcontrollers” by A Nagoor Kani
9. “Microprocessors And Microcontrollers : Architecture, Programming and System Design 8085, 8086, 8051, 8096” by KRISHNA KANT
10. “8 - Bit Microprocessor” by Vibhute