Question

What is the function of 74HC138?

Answer 1

In the LED display industry, 74HC138 is a commonly used 3-to-8-line decoder chip. 

The main function of this chip is to convert the input multi-bit binary code into an independent output signal.

Specifically, when the three-bit binary code is input into the 74HC138 chip, the chip will automatically determine the current input code and convert it into the corresponding output signal.

Application of 74HC138 in LED display

1. Select display row:

In the driving circuit of the LED display, 74HC138 is used to select which column of LEDs needs to be lit, thereby controlling the display pattern. Since 74HC138 has 8 output ports (Y0 to Y7), it can be used to select one row (or one column) of 8 rows (or columns, depending on the circuit design) for display. In a complex LED display, multiple 74HC138 chips may be required to expand the number of display rows or columns.

2. Reduce system complexity and cost:

By connecting the 74HC138 chip in series with multiple LED lights, a large number of LED lights can be controlled using very few pins, thereby reducing the complexity and cost of the system. This feature makes the 74HC138 particularly useful in large LED displays and matrix displays.

Pin Functions of 74HC138

Binary Input Pins (A, B, C): Pins 1 to 3 are binary input pins that receive three-bit binary coded signals.

Enable Pins (G1, G2A, G2B): Pins 4 to 6 are enable pins used to control the enable state of the chip. Usually, only under a specific enable pin combination (such as G1 is high, G2A and G2B are low), the chip will be selected and output signals controlled by A, B, and C signals.

Output Pins (Y0 to Y7): Pins 7 to 14 are output pins used to output decoded signals. At any time, only one output pin is low (valid level), and the rest are high.

Features of 74HC138

High or low output: The output port of 74HC138 has two states of high or low, so it can be used to control switches, drive LED lights, etc.

Wide power supply voltage range: 74HC138 chip can work normally in the power supply voltage range of 2.0 to 6.0 volts, suitable for various voltage environments.

High-speed performance: With fast transmission speed, it ensures efficient data processing and response.

Output current capability: It can provide a certain current (such as -5.2 mA) at the maximum when outputting high level, and can withstand a larger current (such as 20 mA) when outputting low level, which is suitable for a variety of load conditions.

In summary, 74HC138 plays an important role in the LED display industry. 

It selects specific LED rows (or columns) for display through the decoding function, thereby realizing the effective control and management of a large number of LED lights.

Answer 2

Function of 74HC138: 8-bit binary decoder

The function of 74HC138 is to select the display line. One 74HC138 can select one of the 8 lines, so there are 2 74HC138s on the unit board/module, so that one line can be selected from 16 lines for display

Pin 8 GND, power ground.
Pin 15 VCC, power positive pole
Pins 1~3 A, B, C, binary input pins.
Pins 4~6 are controlled by the bit selection signal. Only when pins 4 and 5 are "0" and pin 6 is "1", it will be selected, and the output is controlled by the A, B, C signals. Any other combination will not be selected, and the output of Y0~Y7 is all "1".
By controlling the selection pin to cascade, it can be expanded to sixteen bits.
For example: G2A=0, G2B=0, G1=1, A=1, B=0, C=0, then Y0 is "0" and Y1~Y7 are "1", see the truth table for details.
Function of 74HC595: LED driver chip, 8-bit shift latch
Used to drive display columns, each 74HC595 can drive 8 columns, multiple 74HC595s are connected in series
Together, the serial column data signal RI (DATA), latch signal STB, and serial clock signal CLK are all on this chip
Pin 8 GND, power ground.
Pin 16 VCC, power positive
Pin 14 DATA, serial data input port, display data enters from here, must have the cooperation of the clock signal to shift in.
Pin 13 EN, enable port, when this pin is "1", QA~QH ports are all "1", when it is "0", the output of QA~QH is controlled by the input data.
Pin 12 STB, latch port, after the input data is transferred to the register, only a latch signal can be supplied to send the shifted data to QA~QH ports for output.
Pin 11 CLK, clock port, each clock signal will shift one bit of data into the register.
Pin 10 SCLR, reset port, as long as there is a reset signal, the data shifted into the register will be cleared, the display screen does not use this pin, usually connected to VCC.
Pin 9 DOUT, serial data output port, transfers data to the next one.
Pins 15, 1~7, parallel output port, also known as drive output port, drives LED.
Function of 4953: row driver tube, power tube
The current required for each display row is relatively large, so row driver tubes are used. Each 4953 can drive 2 display rows
Its interior contains two CMOS tubes, pins 1 and 3 are VCC, pins 2 and 4 are control pins, pin 2 controls the output of pins 7 and 8, and pin 4 controls the output of pins 5 and 6. Only when pins 2 and 4 are "0", 7, 8, 5, and 6 will output, otherwise the output is in high impedance state.
Function of TB62726: LED driver chip, 16-bit shift latch
Some unit boards/modules use TB62726 instead of 74HC595. One TB62726 can drive 16 columns, and that's it.
Pin 1 is GND, power ground.
Pin 24 VCC, positive power supply
Pin 2 DATA, serial data input
Pin 3 CLK, clock input
Pin 4 STB, latch input
Pin 23 output current adjustment terminal, connected to resistor adjustment
Pin 22 DOUT, serial data output
Pin 21 EN, enable input
Other functions are similar to 74HC595, except that TB62726 is a 16-bit shift latch with output current adjustment function, but no high level will appear on the parallel output port, only high impedance state and low level state. 74HC595 parallel output port has high level and low level output. TB62726 has the same pin function as 5026 and similar structure.

Answer 3

74HC138 is a high-speed CMOS device, and its functions are mainly reflected in the following aspects:

1. Basic functions

Decoding function: 74HC138 is a 3-line to 8-line decoder that accepts 3-bit binary weighted address input (A0, A1 and A2) and provides 8 mutually exclusive low-valid outputs (Y0 to Y7) when enabled. This means that it can convert the input 3-bit binary code into 8 different output states, and these 8 outputs are mutually exclusive, that is, only one output is valid at the same time.

2. Enable control

Compound enable input: 74HC138 has 3 unique enable inputs, including two low-valid (E1 and E2) and one high-valid (E3). Only when E1 and E2 are set low and E3 is set high, the decoder will output a valid decoding result; otherwise, all outputs will remain high. 

This compound enable feature makes 74HC138 more flexible in application, and precise control of output can be achieved by controlling the enable terminal.

3. Expansion and Application

Expansion capability: Parallel expansion can be easily achieved by utilizing the composite enable feature of 74HC138. For example, by combining 4 74HC138 chips and 1 inverter, a 1-32 (5-line to 32-line) decoder can be constructed. This expansion capability makes 74HC138 have a wide range of application prospects in applications that require more output lines.

Multiple application scenarios: 74HC138 plays an important role in high-performance memory systems and can improve the efficiency of decoding systems. At the same time, due to its low power consumption and high-speed transmission characteristics, it is also suitable for battery-powered applications and data transmission systems that require short transmission delay times. In addition, 74HC138 can also be used as an 8-output demultiplexer, and in demodulator applications, the enable input can be used as a data input.

4. Pins and Interfaces

Pin Configuration: The pin configuration of 74HC138 includes input terminals (A0, A1, A2), output terminals (Y0~Y7), enable terminals (E1, E2, E3), power terminals (VDD) and ground terminals (GND). This simple pin configuration makes 74HC138 easy to integrate and use.

In summary, as a high-speed CMOS decoder chip, 74HC138 has a wide range of applications in digital logic circuits. 

Its decoding function, enable control, expansion capability and simple pin configuration make it a key component in many digital circuit designs.