74HC138

The 74HC138 is a 3-to-8 line decoder and demultiplexer integrated circuit. It translates a 3-bit binary input (A0, A1, A2) into one of eight active-LOW outputs, keeping all unselected outputs HIGH. It operates between 2.0V and 6.0V and is used for memory mapping and peripheral selection.

Core Functionality
• Address Decoding: When enabled, the IC accepts a 3-bit binary address and drives exactly one of its eight outputs (Y0 through Y7) LOW. For example, a binary input of 010 (2 in decimal) sets the Y2 pin LOW while Y0, Y1, and Y3-Y7 remain HIGH.
• Enable Logic: The chip has three enable pins (two active-LOW: {E1}, {E2}; one active-HIGH: E3). All conditions must be met (i.e., {E1} and {E2} are LOW, and E3 is HIGH) for the chip to be active. If disabled, all eight outputs stay HIGH.

Common Applications
• Memory Chip-Select: In microprocessor systems, it activates specific memory chips without requiring multiple individual selection lines.
• Demultiplexing: By utilizing one of the active-LOW enable pins as a data line, the chip routes serial data from a microcontroller to one of eight designated output channels.
• Parallel Expansion: By tying the enable pins of multiple units together, you can easily cascade chips to decode larger address spaces (e.g., creating a 4-to-16 or 5-to-32 decoder).
Technical Specifications
• Logic Family: High-Speed CMOS (HC)
• Propagation Delay: Typically around 15 ns at \(V_{CC} = 4.5V for full timing parameters).
• Output Drive: Can sink or source 8 mA at \(V_{CC} = 4.5V\), making it capable of directly driving multiple standard logic loads.

Package Options: Available through-hole (DIP-16) or surface-mount (SOIC-16, TSSOP-16)