An Article to Help You Understand How the 74HC595D Shift Register Works

In digital circuits, a shift register is a device based on flip-flops that can input data in parallel or serial mode under several identical time pulses and shift one bit to the left or right at each time pulse, eventually outputting it at the output end. Toshiba Semiconductor - The 74HC595D is a high-speed CMOS 8-bit serial shift register with a storage register and low-level and high-impedance outputs, widely used in various digital circuits. This article will introduce the working principle and characteristics of the 74HC595D in detail.

I. Basic Information About the 74HC595D

The 74HC595D is a 16-pin CMOS integrated circuit in an SOIC package, with an operating temperature range of -40°C to +125°C, a power supply voltage range of 2V to 6V, and a maximum total power of 500mW. This device features a high-speed shift clock frequency, capable of reaching over 25MHz and even up to 100MHz in some cases. Its main function is to convert 8-bit serial input into parallel output with tri-state outputs.

II. Working Principle of the 74HC595D

The 74HC595D consists of two main parts: a shift register and a storage register, which operate independently based on separate clock signals.

Shift Register:

Data Input: Data is input through the Serial Data Input (SI) pin.

Clock Signal: The data in the shift register shifts on the rising edge of the Serial Clock Input (SCK). Data enters from the SI pin and shifts sequentially to QA to QH (eight parallel output pins).

Reset Function: The /SCLR (low-level reset) pin is used to clear the data in the shift register.

Storage Register:

Data Transfer: On the rising edge of the Storage Clock Input (RCK), the data in the shift register is transferred to the storage register. The falling edge of RCK keeps the data in the storage register unchanged.

Output Control: The /G (output enable) pin is high for high-impedance output, disabling the output. When /G is low, the data in the storage register is output through QA to QH pins.

III. Characteristics and Applications of the 74HC595D

Characteristics:

High-Speed Performance: Maximum shift clock frequency can reach over 25MHz, suitable for high-speed data transmission.

Cascade Function: Through the QH' (cascade output) and SI (serial data input) pins, multiple 74HC595Ds can be cascaded to expand the number of output bits.

Tri-State Output: The output pins have tri-state functionality, allowing easy control of the output state.

Data Storage Function: During shifting, the data in the storage register remains unchanged, preventing flickering of numeric displays.

Applications:

LED Displays: The 74HC595D is commonly used in constant-voltage LED monochrome display drivers, simplifying circuit design by inputting data serially and outputting it in parallel.

Numeric Display Driving: Utilizing the parallel output function of the 74HC595D, it can directly control the eight segments of numeric displays to achieve digital display.

Data Conversion and Processing: The 74HC595D can also be used for serial-to-parallel data conversion, numerical operations, and data processing.

IV. Conclusion

As a high-speed CMOS 8-bit serial shift register, the 74HC595D, with its high-speed performance, cascade function, tri-state output, and data storage characteristics, has a wide range of applications in digital circuits. By deeply understanding its working principle and characteristics, one can better utilize this device in circuit design to meet various functional requirements.

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