Q6Microprocessor
Question
Q.6. Why are AD0-AD7 lines multiplexed?
Answer
AD0-AD7 lines are multiplexed (shared between address and data functions) to reduce the total pin count of the microprocessor package, since dedicating separate pins for the full address bus and full data bus simultaneously would require significantly more physical pins on the IC.
In microprocessors such as the 8085, the AD0-AD7 pins are multiplexed to serve as the low-order 8 bits of the 16-bit address bus during the early part (T1) of each machine cycle, and then switch to serving as the 8-bit bidirectional data bus for the remainder of the machine cycle — this time-multiplexing scheme is used specifically to reduce the total number of physical pins required on the microprocessor IC package, since providing fully separate, dedicated pins for a complete 16-bit address bus (16 pins) plus a complete 8-bit data bus (8 pins) simultaneously would require 24 pins just for these two bus functions, whereas multiplexing allows the same 8 physical pins to serve double duty, reducing the total pin count (and hence package size and cost) considerably. The low-order address byte present on AD0-AD7 during T1 is captured and held externally by a latch circuit (enabled by the ALE, Address Latch Enable, signal), freeing these same physical pins to then carry the actual data byte for the remainder of the memory/IO read or write operation within that machine cycle.