In 8085 microprocessor ALE signal is made high to______.

Question

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Enable the data bus to be used as low order address bus

Explanation: The Address Latch Enable (ALE) signal is used by the 8085 microprocessor to demultiplex the address-data bus. When ALE is high, the AD0-AD7 lines carry the low-order address, which is captured (latched) by an external latch (like the 74LS373).

Working Principle: The 8085 uses a multiplexed bus to save pin count on the IC package. During the T1 state of a machine cycle, ALE goes high to signal that the bus contains a valid address. The external address latch uses the falling edge of the ALE signal to capture the address from the bus, allowing the bus to be reused for data in subsequent T-states.

Key Formulas: $T_{cycle} = \frac{1}{f_{clk}}$ — Determining the timing of the ALE pulse; $Address_{latched} = AD_0 - AD_7 	ext{ at } ALE=1$

Key Points: ALE is active high for the duration of the T1 state.; The 74LS373 latch is transparent when ALE is high.; Demultiplexing is essential for separating address and data lines for memory/IO interfacing.; The falling edge of ALE is the sampling trigger for address capture.

Advantages: Reduces the number of pins required on the 8085 IC package.; Minimizes PCB trace routing complexity.

Disadvantages: Requires an external latch component in every system design.; Adds propagation delay to the address bus access time.

Applications: Interfacing memory devices like EPROM/RAM.; Interfacing peripheral chips like 8255 PPI.

Additional Info: The ALE signal is an output of the 8085.. Option B is incorrect because while the latch captures the low-order address, it does not latch the data (D0-D7) during the ALE high pulse; data transfer occurs during T2/T3.. Option C is incorrect as ALE does not disable the bus; it merely defines the function of the bus lines at a specific time.

Answer: A is correct — ALE is used to latch the lower 8 bits of the address from the multiplexed address-data bus (AD0-AD7) into an external latch.

Exam Tip: Always remember that the 8085 ALE signal corresponds strictly to the T1 state; any question linking ALE to data reading is a distractor.

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