The 74LS138 is a 3-to-8 line decoder that allows for the decoding of three binary input signals into eight output lines. It is commonly used in digital circuits to expand the number of available output lines from a microcontroller or other digital system. The pin configuration of the 74LS138 includes address inputs, enable pins, and active-low outputs. Here’s a detailed breakdown of its functionality:
- **A0–A2**: These are the address input pins, which receive binary signals to select one of the eight output lines.
- **STA (E1)**: This is the primary enable pin; it must be active high for the decoder to function.
- **/STB (/E2) and /STC (/E3)**: These are the secondary enable pins, and they must be active low for the chip to operate.
- **/Y0–/Y7**: These are the active-low output lines, meaning only one of them will be low at any given time based on the input combination.
- **VCC**: Positive power supply, typically 5V.
- **GND**: Ground connection.
When A0, A1, and A2 are set to a binary value, the corresponding Y output becomes low, while all others remain high. For example, if A0=0, A1=0, A2=0, then Y0 will be low. This makes the 74LS138 ideal for selecting specific devices in a multi-chip system.
To expand the I/O port of a single-chip microcontroller like the AT89C51, two 74LS138 chips can be cascaded. In this configuration, the microcontroller’s P1.0 to P1.3 pins are connected to the address inputs of both decoders. The first 74LS138 is enabled by a high signal on G1 and a low signal on G2b, while the second 74LS138 is controlled by P1.3, allowing for more complex selection logic.
By combining the outputs of both chips, the system can generate up to 16 unique output lines, effectively expanding the I/O capabilities of the microcontroller. This setup is particularly useful when interfacing with multiple peripheral devices, such as the AD9835 waveform generator, where each device requires a separate chip-select signal.
The truth table for the two 74LS138 chips shows how the various combinations of address inputs result in specific output lines being activated. Logical expressions derived from this table help in designing the circuit and ensuring proper operation.
In practice, the first 74LS138 is used to control the lower 8 outputs (Y0–Y7), while the second controls the upper 8 (Y8–Y15). This allows for precise control over multiple devices connected to the microcontroller. The enable pins are carefully managed to ensure that only one decoder is active at a time, preventing conflicts and ensuring reliable operation.
Overall, using two 74LS138 chips provides an efficient way to expand the I/O ports of a microcontroller, making it easier to interface with a wide range of digital components.
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