SN74HC138N

The SN74HC138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer designed for high-performance memory decoding and data routing applications requiring very short propagation delay times. The device accepts a 3-bit binary input (A, B, C) and activates one of eight active-low outputs (Y0-Y7). Three enable inputs—G1 (active-high) and G2A, G2B (active-low)—provide flexible cascading for larger decoding networks. When any enable input is inactive, all outputs are forced HIGH. The active-low output configuration directly interfaces with the active-low chip-select inputs common on memory devices and peripherals, eliminating the need for inverting buffers.

The SN74HC138N decodes a 3-bit binary input to activate one of eight output lines. The three select inputs (A, B, C) are internally decoded through AND-OR logic gates to determine which of the eight outputs (Y0-Y7) should be driven LOW. The enable logic requires G1 = HIGH AND G2A = LOW AND G2B = LOW for the decoder to be active; if any enable condition is not met, all outputs are forced to their inactive HIGH state. This enable structure makes cascading straightforward: two SN74HC138 devices can form a 4-to-16 decoder by connecting the fourth address bit to an enable input. The internal CMOS gate structure uses complementary N-channel and P-channel transistors, providing low static power consumption and balanced output drive capability. The typical propagation delay of 15 ns ensures that the decoder introduces negligible delay in memory access paths.

• Memory decoding: Generating chip-select signals for up to 8 memory devices from a 3-bit address field
• Peripheral selection: Activating one of multiple I/O devices or registers in microprocessor systems
• Cascaded systems: Two devices form a 4-to-16 decoder; four devices form a 5-to-32 decoder for large address spaces
• Data demultiplexing: Routing a single data line to one of eight destinations under select control
• Clock distribution: Distributing a clock signal to one of eight clock domains using an enable input as the clock source