Question

Explain the need to demultiplex the bus ad7-ad0

Answer 1

The address bus has 8 signal lines A8-A15. They are unidirectional. The other 8 address bits are multiplexed with the 8 data bits. Therefore the bits AD0-AD7 are bi-directional. They serve as A0-A7 and D0-D7 at the same time. As AD7-AD0 lines serve a dual purpose they have to be demultiplexed to get all the information.

The address's high order bits remain on the bus for 3 clock periods. The low order bits remain for only 1 clock period and may be lost if they are not saved externally. An external latch is used to save the value of AD7-AD0 when it is carrying the address bits so that the entire address remains for the 3 clock cycles. 

Answer 2

Answer:

Explanation:

8085 microprocessor have 16 bit address bus  but its lower order bit multiplexed with 8 bit data bus (as 8085 has 8 bit data bus) to reduce number of pins so AD7-AD0 is bidirectional , whereas higher order bit only carries address so it unidirectional.

                                         As AD7-AD0 lines serve a dual purpose they have to be demultiplexed to get all the information. A15-A8 carries higher order address bit for all 3 machine cycle whereas AD7-AD0 carries lower order address bit only for 1st cycle and next 2 machine cycle it carries data and may be lost if they are not saved externally. So an external latch is used to save the value of AD7-AD0 when it is carrying the address bits so that the entire address remains for the 3 clock cycles. That is why its need to be demultiplexed.