The Address Latch Enable (ALE) pin is a crucial control signal in microprocessors like the 8085, primarily used to demultiplex the address/data bus. Its main purpose is to signal when the shared bus lines are carrying address information, allowing external latches to capture and hold this address for memory or I/O operations.
Understanding ALE's Core Function
In many microprocessor architectures, especially older ones like the Intel 8085, pin count was a significant design constraint. To reduce the number of pins, some pins were designed to serve a dual purpose, carrying both address and data information at different times. This is known as a multiplexed address/data bus.
The ALE pin acts as a time-multiplexing signal, providing synchronization for external hardware. Specifically:
- Address Indication: When the ALE pin goes HIGH (logic 1), it indicates that the information currently present on the multiplexed address/data lines (e.g., AD0-AD7 in the 8085) is the lower byte of the memory or I/O address.
- Latching Enable: This HIGH signal is used by external address latches (like a 74LS373) to "capture" or "latch" this address information. Once the address is latched, it remains stable on the latch's output pins, even after the microprocessor changes the information on the multiplexed bus.
- Bus Demultiplexing: After the address is latched, the ALE pin goes LOW, signaling that the same bus lines are now available to carry data (either read from or written to memory/I/O). This effectively separates the address phase from the data phase of a machine cycle.
ALE in the 8085 Microprocessor
The 8085 microprocessor is a classic example that extensively uses the ALE signal. The ALE pin is the 30th pin of the 8085.
As mentioned, the address bus is enabled during the first clock cycle when the ALE pin is highest. This is critical for the proper functioning of memory and peripheral interfacing. For instance, the 8085's lower address lines (A0-A7) are multiplexed with the data lines (D0-D7) on the AD0-AD7 pins.
The following table illustrates the role of ALE during a typical machine cycle:
| ALE Signal State | Bus Function (AD0-AD7 pins) | Purpose |
|---|---|---|
| HIGH (Active) | Lower Order Address (A0-A7) | Indicates that the current information on AD0-AD7 is the address. |
| LOW (Inactive) | Data (D0-D7) | After the address is latched, these pins are used for actual data transfer. |
Why is ALE Necessary?
The necessity of the ALE signal stems from the design choices made to optimize resources in microprocessor systems:
- Pin Optimization: By multiplexing address and data lines, manufacturers can reduce the total number of pins on the microprocessor package. This leads to smaller chips, simpler circuit boards, and potentially lower manufacturing costs.
- System Simplicity (with External Logic): While it requires an external latch, it simplifies the microprocessor's internal design and allows for a more compact pinout.
- Precise Timing: ALE ensures that external memory and I/O devices receive the correct address at the precise moment it's available, preventing data corruption or incorrect addressing.
Practical Implications
In practical microprocessor system design, the ALE pin is directly connected to the latch enable input of an external octal latch (e.g., a 74LS373 or 74HC573). The AD0-AD7 lines from the microprocessor are connected to the data inputs of this latch. When ALE goes high, the address on AD0-AD7 is captured and appears on the output pins of the latch, becoming the stable lower address lines for the entire memory or I/O access cycle.
This demultiplexed address, combined with the non-multiplexed higher address lines (e.g., A8-A15 in 8085), forms the full 16-bit address that points to a specific memory location or I/O port. Without ALE, it would be impossible for external devices to distinguish between address and data on the shared bus.
