A decoder is a type of circuit that can translate the state of the input binary code into an output signal to express its original meaning. It can be said that the circuit that realizes the specific meaning of the code state is called a decoder.

The following figure 7 is an example of using the enable terminal to convert two 2-input decoders into a 3-input decoder. When I2 is low, the enable terminal of the first decoder is high. When I2 is high, the decoder 2 works and the second decoder does not work at this time, so the output of decoder 1 corresponds to 0~3 of input I3I2I1; when I2 is high, decoder 2 works and decoder 1 is not working, so its output corresponds to 4-7 of the input I2I1I0. So the circuit in the figure below realizes the use of the enable terminal to expand the decoder with 2 inputs to the decoder with 3 inputs.

Standard medium-scale decoder circuit

There are several types of decoders in medium-scale integrated circuits. The most widely used is usually 74138, which is a 3-8 decoder. The following figure 8 shows its logic symbols and pin assignments. The following table lists the logic function of the device. It can be seen from the table that its output is active low, the enable terminal G1 is active high, and /G2, /G3 are active low. When one of them is low, The output terminals are all 1.

Example: Try 74138 to realize the function F(X,Y,Z)=∑m(0,2,4,7)

Using 74138 to realize the function is the same as the method of the decoder to realize the logic function mentioned above, but two points must be paid attention to:

The output of 1.74138 is low-level effective, so when implementing logic functions, the output terminal can not be connected to the OR gate and NOR gate (because each time only one is low level, the rest are high level);

2.74138 differs from the previous one in that it has an enable terminal, so the enable terminal must be processed, otherwise, the required logic function cannot be realized. The following figure shows the final circuit.

Display decoding

The working principle of commonly used display devices

Common digital displays in digital systems usually include light-emitting diode nixie tubes (LED nixie tubes) and liquid crystal display nixie tubes (LCD nixie tubes). The LED digital tube uses light-emitting diodes to form strokes for displaying numbers to display numbers. Because the LEDs emit light, the LED digital tube is suitable for various occasions. The liquid crystal display digital tube uses the liquid crystal material to absorb light under the action of alternating voltage, and there is no stroke without the action of the alternating electric field, so it can display the number. But because the liquid crystal material must have light It can only be used, it cannot be used in situations where there is no outside light (the LCD of portable computers can be used at night under the action of the backlight). One of the biggest advantages of liquid crystal displays is power saving, so they are widely used for digital displays of small devices such as small calculators.

The following figure 9 shows the internal structure of the LED digital tube and the display of numbers. It is a kind of LED digital tube with an anode connected in one. We usually call it a common anode digital tube. Since there is a common anode digital tube, there is a common cathode digital tube.

LED decoding drive circuit

Light-emitting diodes only need to be forward-conducted. According to whether the common electrode of the LED is the anode or the cathode, there are two types of decoders, namely, the low-level effective decoder for the common anode; the high-level effective decoder for the common cathode LED.

4511 is a CMOS display decoder with effective high-level output. Its input is 8421BCD code. The figure and table below are the pin arrangement, logic symbol, and logic function table of 4511 respectively.

In the figure:

/LT: low level is effective, when it is low level, all strokes are all on, if not, it means there is a problem with the stroke;

/BL: low level is active, when it is low level, regardless of the input data state, its output is all low level, that is, all strokes are off;

/ST/LE: Strobe/Latch pole, which is a multiplexed functional terminal. When the input is low, its output is related to the input variable; When the input is high, its output is only the same as the state before the end is high. No matter how the input DCBA terminal changes, its displayed value remains unchanged.

D, C, B, A: 8421BCD code input, the D bit is the most bit;

a～g: The output terminal is high-level effective, so its output should correspond to the digital tube of its cathode.

LCD decoder driver

The LCD decoder driver circuit is different from the LED decoder driver circuit. Its output is not a high level or a low level, but a pulse voltage. When the output is valid, its output is an alternating pulse voltage, otherwise, it is a high level or low level.