The 74139 IC chip is a dual 2-to-4 line decoder that converts 2 binary inputs into 4 binary outputs. It is commonly used in digital logic circuits to decode addresses and select 1 of 4 output lines based on the input code. Cracking or reverse engineering this chip can help understand its internal logic and workings. This article provides a step-by-step guide on how to crack the 74139 IC chip.
What is IC 74139?
The 74139 IC is a 16-pin dual 2-line to 4-line decoder chip used in various digital logic applications. Some key features of this chip are:
- Dual 2-to-4 line decoder
- Active LOW outputs
- Outputs source up to 20 mA per output
- Inputs compatible with TTL and CMOS logic levels
- Output enable (OE) input for three-state operation
This IC contains two separate decoders labeled A and B. Each decoder has 2 inputs and 4 outputs. Based on the binary code applied to the inputs, 1 of the 4 outputs is activated LOW while the rest remain HIGH.
Truth Table
The truth table for one decoder in the 74139 IC is:
| A (MSB) | B (LSB) | Y0 | Y1 | Y2 | Y3 |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 1 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 | 1 |
| 1 | 0 | 1 | 1 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 | 0 |
As seen from the truth table, a unique output line is activated LOW based on the input code. This facilitates decoding a binary number into 1 of 4 outputs.
How to Identify Pins
Before cracking the IC, it is important to first identify the pins and their functions. This can be done by referring to the pin diagram in the datasheet.
The key pins in 74139 are:
- Pin 1 and 2: GND
- Pin 3 and 4: Input A
- Pin 5 and 6: Input B
- Pin 7: Output Enable (OE)
- Pin 8 to 11: Outputs Y0 to Y3
- Pin 14 and 15: Positive Supply (+Vcc)
Identifying the pins properly is crucial for understanding the internal connections during reverse engineering.
Steps to Crack 74139
The steps involved in successfully cracking the 74139 IC are:
1. Remove the plastic packaging
- First, remove the black epoxy packaging of the IC carefully using a sharp knife.
- Slowly scrape away the epoxy resin around the edges.
- Gradually the chip will be uncovered. Be careful not to damage the tiny bonding wires.
2. Identify the die area
- Once the epoxy is removed, clean the chip with acetone to remove any residue.
- Under a microscope, the silicon die area is visible with the aluminum bonding pads.
- Compare with the pin diagram and identify the pads of each pin.
3. Trace the connections
- Now trace the connections between the various circuit blocks inside the IC.
- Use a multimeter in continuity testing mode to identify connected pads.
- Draw a schematic of the traced connections.
4. Analyze the logic gates
- The chip will contain various logic gate combinations.
- Identify the individual gates by shape and connections.
- Note down the logic for each block.
5. Understand the workings
- Combine the traced schematics and logic gates identified.
- The internal circuitry and logic can now be understood.
- Correlate with the functional table to see how the IC works.
Internal Circuitry
By following the cracking steps above, we can derive the following internal circuitry of the 74139 IC:
The 74139 contains:
- Input buffers to condition the input signals
- NAND gates to generate internal logic
- Inverters at the output
- Wired OR connection on the output lines
This implements the logic for the 2-to-4 line decoding as per the truth table.
Tips for Cracking ICs
Here are some useful tips when reverse engineering integrated circuits:
- Work under a microscope for better visibility.
- Use sharp and fine tipped tools to slowly scrape away the epoxy.
- Identify the die area properly before tracing connections.
- Be very careful not to damage any thin bonding wires.
- Draw the schematic as you trace the connections.
- Correlate with datasheet and pin diagrams.
- Try to identify the logic gates and components from their shapes and placements.
- Relate the traced schematic with known functioning to understand the internal logic.
Applications of Cracked 74139 IC
Once the 74139 IC is successfully cracked, the knowledge can be applied to:
- Understand the internal logic gates utilization
- Learn how basic decoding is implemented
- Modify the IC by tapping intermediate signals
- Attach additional circuits at input and output pins
- Help in debugging circuits using 74139
- Evaluate if the IC can be substituted with different logic
Overall the cracking process provides insight into the IC functionality and how decoding logic is structured.
Frequently Asked Questions
1. Can I crack the IC without damaging it?
Yes, it is possible to crack ICs without causing internal damage by gently removing the epoxy packaging using fine tipped scraping tools under a microscope. Care should be taken not to apply excessive force or heat which can break the tiny bonding wires.
2. What tools are required?
Basic tools like fine tipped scrapers, tweezers, acetone, microscope and multimeter are sufficient. Additionally, a micromanipulator probe station can help trace connections on the die surface.
3. Is the cracking process reversible?
The IC cannot be reassembled into its original packaging after cracking open. But if done carefully, the IC can still be used for experimentation by connecting wires to the bonding pads.
4. Does this void the IC warranty?
Reverse engineering typically voids the warranty of the IC from the manufacturer. The process should only be done for experimental and educational purposes.
5. Is the cracking process legal?
In most cases, reverse engineering commercial ICs may violate copyright laws or license agreements. However, cracking for academic and research purposes under fair use exemptions is generally permitted. Proper legal guidance should be taken as needed.
