Introduction
The 74HC154 is a 4-to-16 decoder integrated circuit (IC) that converts 4 binary inputs into 16 mutually exclusive outputs. Decoders like the 74HC154 are commonly used in digital logic circuits to reduce the number of chips and connections needed to implement functions with multiple outputs. In this article, we will explore the features and operation of the 74HC154 decoder.
How the 74HC154 Works
The 74HC154 contains 16 outputs labeled Y0 through Y15. Only one output is active (low) at a time, depending on the logic levels applied to the 4 inputs labeled A, B, C, and D.
Logic symbol for 74HC154 – Image credit: Wikimedia
The decoder translates the 4-bit binary input code into a 1-of-16 output code using the following logic:
| A | B | C | D | Output |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | Y0 |
| 0 | 0 | 0 | 1 | Y1 |
| 0 | 0 | 1 | 0 | Y2 |
| 0 | 0 | 1 | 1 | Y3 |
| 0 | 1 | 0 | 0 | Y4 |
| 0 | 1 | 0 | 1 | Y5 |
| 0 | 1 | 1 | 0 | Y6 |
| 0 | 1 | 1 | 1 | Y7 |
| 1 | 0 | 0 | 0 | Y8 |
| 1 | 0 | 0 | 1 | Y9 |
| 1 | 0 | 1 | 0 | Y10 |
| 1 | 0 | 1 | 1 | Y11 |
| 1 | 1 | 0 | 0 | Y12 |
| 1 | 1 | 0 | 1 | Y13 |
| 1 | 1 | 1 | 0 | Y14 |
| 1 | 1 | 1 | 1 | Y15 |
Table 1 – 74HC154 input/output logic table
This table shows how each possible combination of inputs A through D activates a different output pin.
The 74HC154 contains an on-chip oscillator that can be enabled through the Mode Select (MS) input. When enabled, the oscillator sequentially activates each output in order from Y0 to Y15.
Applications
The 74HC154 is commonly used in:
- Memory Address Decoding – To select 1 of 16 memory locations based on a 4-bit address
- 7-Segment Display Driving – To decode a 4-bit BCD value to drive a 7-segment display
- Keyboard Scanning – To scan a 16-key matrix keyboard
- Multiplexing – To multiplex 16 inputs/outputs over 4 selection lines
- State Machines – As a simple state machine with 16 states
By using a 74HC154 as a memory address decoder or display driver, for example, only 5 chips are needed (4-bit counter + decoder) versus 16 chips to drive each output separately. This greatly reduces the cost and complexity of circuit implementations.
Specifications
- Supply Voltage – 2V to 6V
- Input Voltage – 2V to 6V
- Output Current (per pin) – 25mA (sink), 15mA (source)
- Propagation Delay – 15ns
- Operating Temperature – Industrial (-40C to 85C)
- Package – 16-pin DIP, SOIC
Frequently Asked Questions
What is the difference between the 74HC154 and 74LS154?
The main difference is that the 74HC154 uses CMOS technology which results in lower power consumption, higher speed, and better noise immunity compared to the 74LS154 TTL logic device. HC CMOS devices are more modern replacements for LS TTL.
How do I enable the built-in oscillator on the 74HC154?
To enable the oscillator, connect the Mode Select (MS) input pin to ground. The oscillator will cycle through all 16 outputs sequentially. Leaving MS unconnected disables the oscillator.
Why does the 74HC154 need a power-on reset?
The oscillation mode starts at an undefined output on power-up. Adding a reset with RC circuit ensures the decoder starts at Y0 every time to avoid erratic operation.
Can I extend the 74HC154 to more outputs?
Yes, 74HC154 decoders can be cascaded to extend the number of outputs. For example, two cascaded chips can decode 8 inputs into 256 outputs. The previous chip’s Carry output connects to the next chip’s Enable input.
How do I activate multiple outputs at once on the 74HC154?
The 74HC154 outputs are mutually exclusive – only one can be active at a time. To drive multiple outputs, external gates can combine the decoder outputs into the desired logic.
