74LS48 Function, Wiring Method, Application and 74LS48 vs 74HC48
12 March 2024
Ⅱ. 74LS48 function and principle
Ⅳ. 74LS48 series logic function and truth table
Ⅴ. Counter example based on 74LS48
Ⅶ. What is the difference between 74LS48 and 74HC48?
74LS48 is a commonly used seven-segment digital tube decoder driver, which is widely used in display systems of various digital circuits and microcontroller systems. It has many excellent features, such as low power consumption, high driving capability and fast response, etc., thus having a wide range of applications in different fields. This article will focus on the functions of 74LS48, introduce its definition, principle, wiring method and application, and compare it with 74HC48.
The 74LS48 is a seven-segment decoder specifically designed to convert binary codes into seven-segment LED digital displays. When it works in conjunction with the 8421BCD encoded counter, it can easily convert the counter's output into a human-readable digital form. This decoder is composed of multiple logic gates and latches. Specifically, it contains 4 input buffers dedicated to receiving BCD codes; a latch to latch data stably; and 4 output drivers to drive a seven-segment LED display to present numbers. These components work together to achieve the conversion process from BCD code to seven-segment LED display.
The main function of 74LS48 is to convert 4-bit binary code into seven-segment digital signal output for easy display on display devices such as digital tubes. Here's how it works:
First, the 74LS48 receives a four-bit binary input code, and then converts it into a corresponding seven-segment digital output through an internal decoding circuit. Depending on the input code, different digital signals will be activated at the output. For example, when 0001 is input, the output a is 0, the output b is 1, the output c is 1, the output d is 0, the output e is 0, and the output f is 0, which represents the display of the number 1 on the digital tube.
In addition to being used as a BCD-7 segment decoder, the 74LS48 can also be used as a BCD-10 segment decoder. To achieve this function, we only need to cascade the 74LS48 with the 74LS49 (BCD-10 segment decoder). In this configuration, the 74LS48 is responsible for decoding the first four input codes, and the 74LS49 is responsible for decoding the last two input codes. By combining the decoding results of the two, we can obtain the complete decimal number, which can then be displayed on a display device such as a digital tube.
In order to correctly connect the 74LS48 chip, the following are several common wiring methods:
1. Single chip wiring
This is the simplest wiring method, just connect pins A, B, C and D to the corresponding pins of the input BCD code, and connect pins a, b, c, d, e, f and g to the corresponding segments of the seven-segment digital tube. By controlling the input BCD code, 74LS48 will activate the corresponding segment selection signal according to the input code value to realize the digital display.
2. Multi-chip cascade connection
When it is necessary to display multiple digits, we can use multiple 74LS48 chips for cascade connection. In cascade wiring, except for the output segments (a, b, c, d, e, f and g) of the first chip which are connected to the segments of the digital tube, the RBI pins of other chips are connected to the RBO pins of the previous chip in order to transfer the feed signal. This can extend the number of bits and realize the display of more digits.
3. Addition of filter capacitors
In order to reduce the ripple and stability of the output signal, we can connect an appropriate size filter capacitor in parallel with the power supply pin of the 74LS48 chip. Filter capacitors can reduce the interference of power supply noise on the chip to provide a more stable working environment.
4. Latch mode wiring
If we need to display a fixed number in a static state, we can use the latch mode to realize it. In latch mode, we should connect the LT pin to a logic high level (usually 5V). In this way, the output of the decoder will be latched and no longer updated in real time. This wiring method is especially suitable for those scenarios where we need to maintain a stable digital display to ensure that the display will not be affected by changes in external signals. During the wiring process, we need to pay attention to factors such as the connection of the input BCD code, the definition of the pins, and the possible cascade and latch modes. At the same time, adding filter capacitors can improve the stability and anti-interference capability of the circuit.
1. 7-segment decoding function (LT=1, RBI=1)
When the lamp test input (LT) and the dynamic zero-off input (RBI) are both connected to the invalid level, the input DCBA will output the driving signal of the 7-segment character display with high level after being decoded by the 7448 decoder. The corresponding characters are displayed. It should be noted that, in addition to when DCBA = 0000, RBI can also be connected to a low level. For specific conditions, please refer to the data in rows 1 to 16 in table 1.
2. Blanking function (BI=0)
At this time, the BI/RBO terminal is used as the input terminal. When a low-level signal is input to this terminal, as shown in the third row from the bottom of table 1, no matter what level signals are input to LT and RBI, and what state the input DCBA is in, the output will all be "0", which means 7 The segment display will go out. This function is mainly used for dynamic display of multiple monitors.
3. Lamp test function (LT=0)
At this time, the BI/RBO terminal is used as the output terminal. When a low-level signal is input to this terminal, as shown in the last row of Table 1, the output will be all "1". At this time, regardless of the input status of DCBA, all 7 fields of the display will light up. This function is mainly used for testing 7-segment displays. By observing the lighting conditions of each field, you can determine whether there are damaged fields.
4. Dynamic zero elimination function (LT=1, RBI=1)
At this time, the BI/RBO terminal also serves as the output terminal. When the LT terminal inputs a high-level signal and the RBI terminal inputs a low-level signal, if the value of DCBA is 0000, according to the second to last row of table 1, the output will be all "0", causing the display to go out and not display the zero. However, if the value of DCBA is not equal to 0, then there will be no effect on the display. This function is mainly used to extinguish the high-order zero when multiple 7-segment displays are displayed simultaneously.
In this example, we will design an automatic counter from 0 to 9. To provide the input data, although a microcontroller is a viable option, here we will use a 74LS90 to generate the four-bit binary data corresponding to 0 to 9. This IC will serve as the input source for 74LS48. We just need to provide a clock pulse to the 74LS90 and it will send the corresponding data to the 74LS48. The 74LS48 then generates a seven-segment display output based on this data. In Proteus, we can easily design and simulate this circuit to see how it works. It is worth mentioning that although in this example we used 74LS90 to design this counter, you can also choose 74LS93 as a four-bit binary counter to complete the same function.
The LT' pin will turn on all the LEDs and when the clock is enabled the circuit will start and start counting until we stop providing the clock pulses. These seven segments do not have any special initial state, but the IC we are using here has an initial state such that the output on the LED is equal to 0. 74LS90 helps here in designing the counter, but while designing other functions, we can design other 7-segment devices using other TTL devices or microcontrollers with clever handling of 74LS49.
The function of the LT' pin is to turn on all LEDs. Once the clock signal is enabled, the circuit fires up and starts counting until we stop providing clock pulses. In the initial state, these seven segments have no specific display content, but the IC we use here has a specific initial state, which causes the output on the LED to display 0. 74LS90 is used in this example to aid in the design of the counter, but when designing other functions, we can design other 7-segment devices using other TTL devices or a microcontroller that cleverly handles the 74LS49.
74LS48 is often used in the following areas:
1. Seven-segment digital tube driver: 74LS48 one of the most common uses is to drive a seven-segment digital tube. By connecting the binary input signal to the input pins on the chip, the chip will activate the corresponding digital tube segment selection pins according to the input binary digits to display the numbers.
2. Timer: In the design of a digital timer, the clock signal usually comes from a stable oscillator, such as a quartz crystal oscillator. This clock signal, after appropriate frequency division and processing, is fed into the input of the 74LS48, which receives these clock pulses and converts them to the logic levels required by the seven-segment digital tube, thus driving the digital tube to display the current timing value.
3. Counter: In the design of digital counters, 74LS48 is usually used in conjunction with a binary counter (such as 74LS90 or 74LS190, etc.). When the binary counter completes the counting operation, its output will be sent to the input of the 74LS48. 74LS48 is able to decode these binary coded decimal (BCD) numbers and convert them to the corresponding seven-segment digital tube to display the required logic levels.
4. Instrument and equipment control: In a variety of instruments and equipment, the 74LS48 is used to control the seven-segment digital tube to ensure that they can accurately display the relevant information of the equipment. When a device needs to be measured or detected, its internal sensors or circuits generate corresponding electrical signals. These signals are converted into BCD-coded digital signals after appropriate processing and fed into the inputs of the 74LS48.
74LS48 and 74HC48 are two common decoder or driver chips, they both have the function of converting input signals into corresponding output signals. We will compare the two in terms of features and performance.
1. 74LS48 and 74HC48 features comparison
(1) Compatibility: 74HC48 is compatible with TTL levels, we can directly replace the 74LS48 without other circuit modifications.
(2) Supply voltage: 74LS48 has a supply voltage of 5V, while 74HC48 has a wider range of supply voltages, which is suitable for more scenarios with different power supply voltage requirements.
(3) Reliability: 74HC48 has better anti-interference capability and electrical stability due to its CMOS technology, making it more reliable in unstable or noisy operating environments.
(4) Power consumption and speed balance: 74LS48 has higher power consumption but slower speed; 74HC48 has lower power consumption and faster speed. Therefore, when choosing a chip, we need to weigh and choose according to the specific application requirements.
2. 74LS48 and 74HC48 performance indicators comparison
(1) Speed: 74HC48 has a higher speed response due to CMOS technology. In contrast, the 74LS48 has a lower speed and is suitable for some applications that do not require high speed.
(2) Operating voltage range: The operating voltage range of 74LS48 is 4.75V to 5.25V, while the operating voltage range of 74HC48 is wider, usually 2V to 6V, and even up to 2V to 12V.
(3) Power dissipation: 74LS48 using TTL technology, its power consumption is relatively high. The 74HC48 adopts CMOS technology, which has lower power consumption and is suitable for application scenarios with higher power consumption requirements.
(4) Open collector output: 74HC48 has a conditional output function, which can realize multi-chip cascade or multi-bit decoder connection through external circuitry. However, 74LS48 does not support conditional output.
(5) Noise immunity: 74HC48 adopts CMOS technology, so it has good noise suppression ability and can work more stably in the electromagnetic interference environment. The 74LS48 is relatively poor, and the anti-interference ability to noise is weak.
Frequently Asked Questions
1. What is 74LS48 IC?
The SN54/74LS48 is a BCD to 7-Segment Decoder consisting of NAND gates, input buffers and seven AND-OR-INVERT gates. Seven NAND gates and one driver are connected in pairs to make BCD data and its complement available to the seven decoding AND-OR-INVERT gates.
2. What is the function of the 74LS48?
The 74LS48 takes a Binary-Coded Decimal (BCD) input and converts it into the appropriate signals to drive a 7-segment display.
3. How does 7-segment display work?
The operation of 7-segment displays is rather simple and involves activation of the appropriate segments so as to form the desired characters. A controller, such as a microcontroller or integrated circuit, controls the current flowing through individual LEDs, depending on the digit or character to be displayed.
