74248 (BCD to 8-Line Decoder/Driver) multiplexing or 7-segment display drivers

Description

The 74248 is a BCD (Binary Coded Decimal) to 8-Line Decoder/Driver. It is part of the 7400 series of integrated circuits, and it is used for applications where BCD inputs need to be converted into one of several output lines, such as in multiplexing or 7-segment display drivers.

74248 IC Description:

The 74248 is a BCD to 8-line decoder/driver, meaning it takes a 4-bit BCD input and decodes it to select one of 8 output lines. This IC is specifically designed to provide an active-low output for the corresponding decimal value (0–7) from the BCD input (4-bit binary input).

Key Features:

• BCD to 8-Line Decoder/Driver: The IC accepts a 4-bit BCD input and drives one of eight outputs (active-low) based on that input.
• Active-Low Outputs: The output lines are active-low, meaning the selected output line is pulled low (0), while all other outputs remain high (1).
• Decoding Logic: It decodes BCD values from 0 to 7 and activates the corresponding output (Y0–Y7). For BCD inputs from 8 to 15, none of the outputs are activated (they remain high).
• TTL Compatible: The IC is designed to be TTL (Transistor-Transistor Logic) compatible, meaning it works well with other TTL logic devices in a digital system.
• Driver Capability: The IC can drive TTL-compatible loads directly, making it suitable for use in digital systems where a BCD-to-decimal conversion is required.

Pin Configuration (16-pin package):

Here’s the pinout for the 74248 BCD to 8-line decoder/driver:

1. Pin 1 (A): BCD input bit 1.
2. Pin 2 (B): BCD input bit 2.
3. Pin 3 (C): BCD input bit 3.
4. Pin 4 (D): BCD input bit 4.
5. Pin 5 (Y0): Active-low output 0.
6. Pin 6 (Y1): Active-low output 1.
7. Pin 7 (Y2): Active-low output 2.
8. Pin 8 (Y3): Active-low output 3.
9. Pin 9 (GND): Ground connection (GND).
10. Pin 10 (Y4): Active-low output 4.
11. Pin 11 (Y5): Active-low output 5.
12. Pin 12 (Y6): Active-low output 6.
13. Pin 13 (Y7): Active-low output 7.
14. Pin 14 (Vcc): Supply voltage (typically 5V).
15. Pin 15 (Enable): Active-low enable input.
16. Pin 16 (Reset): Active-high reset input.

How It Works:

• BCD Input: The 74248 accepts a 4-bit BCD input (A, B, C, D), which represents a decimal number between 0 and 15.
  • BCD is a way of representing decimal numbers where each decimal digit is represented by a 4-bit binary value.
  • Input Range: Only BCD inputs representing values from 0000 (0) to 0111 (7) are valid, as the IC has 8 outputs (Y0 to Y7).
  • For inputs 1000 (8) to 1111 (15), none of the outputs will be activated.
• Active-Low Outputs: The IC will activate one of the eight outputs (Y0 to Y7) corresponding to the BCD input. The selected output will be LOW (0), and all other outputs will remain HIGH (1).
• Enable Pin: The Enable (pin 15) is an active-low input. If this pin is held LOW, the IC is enabled and can decode the input. If this pin is held HIGH, all outputs will be HIGH, and the IC will not operate.
• Reset Pin: The Reset (pin 16) is an active-high input. When this pin is held HIGH, it forces all outputs to be HIGH, essentially resetting the IC.

Truth Table:

Note: The outputs will remain HIGH (inactive) for BCD inputs of 8 to 15.

Applications:

1. 7-Segment Display Driver: The 74248 can be used to drive a 7-segment display or any other output device where one of 8 lines needs to be activated based on the BCD input.
2. Multiplexing: The IC can be used in multiplexing systems where one of 8 devices or lines needs to be selected based on the 4-bit BCD input.
3. Decimal-to-Line Conversion: In digital circuits, it’s often necessary to convert a BCD input (e.g., from a counter or sensor) into an active line signal to drive other logic or devices. The 74248 is ideal for this purpose.
4. Binary-to-Decimal Decoding: In systems where a binary number needs to be decoded to activate one of several outputs (for example, for selecting between different memory locations or controlling different components), this IC can serve as the decoder.

Electrical Characteristics:

• Supply Voltage (Vcc): Typically 5V (with a range of 4.5V to 5.5V).
• Input Logic Levels: The inputs are TTL-compatible, meaning they recognize standard logic voltage levels (0V for LOW and 2V to 5V for HIGH).
• Output Logic Levels: The outputs are active-low, meaning they will be LOW (0) when active and HIGH (1) when inactive.

Example Circuit:

Consider using the 74248 in a display application. You could connect a 4-bit BCD input from a counter (such as a 74160), and based on the value of the BCD input (0-7), the IC would activate the corresponding output (Y0-Y7). You could use these active-low outputs to drive a set of 7-segment displays or LEDs, showing the decimal number corresponding to the BCD input.