Shift Registers
Logic shift registers are sequential logic devices used in digital circuits to store and shift binary data bit by bit. Their core function is to control the direction of data movement in the register chain (left or right) through the clock signal. They are widely used in scenarios such as data buffering, serial-to-parallel conversion, and pseudo-random sequence generation.
1. What are Logic Shift Registers?
1) Basic Structure
It consists of cascaded triggers (such as D triggers), each of which stores 1 bit of data. When the clock edge arrives, the data moves to the adjacent trigger in the set direction to achieve serial transmission.
Example: When the enable signal (ena) is valid for a 4-bit right shift register, the data shifts right, the high bit is filled with 0, and the low bit is output 1.
2) Key Control Signals
Asynchronous Reset: Immediately clear the register to the all-0 state.
Synchronous Load: Load parallel data into the register at the clock edge.
Shift Direction Control: Select left shift, right shift, or hold through the enable signal (such as ena[1:0]).
2. What are the Main Types and Functions of Logic Shift Registers?
1) Basic Shift Register
Right/Left Shift Register: discard the shifted bits when shifting, and fill the empty bits with 0 (or sign bit).
Rotator Register: circular shift, the shifted bits are filled in from the other end (such as 100-bit rotator).
2) Arithmetic Shift Register
Supports signed shift (such as 64-bit arithmetic shift), retains the sign bit (MSB) when shifting right, and is used for signed number operations.
3) Linear Feedback Shift Register (LFSR)
Generates pseudo-random sequences through XOR feedback of specific tap bits, which are used for encryption or testing.
4) General Shift Register
Integrates multiple modes (hold, load, left shift, right shift), and dynamically switches functions through selection signals (such as s1, s0).
3. Hardware Implementation and Optimization of Logic Shift Registers
FPGA Dedicated Resources
The CLB of the 7 series FPGA contains dedicated shift register units (SRL16/SRL32), which can efficiently implement 16/32-bit shifts and reduce logic resource usage.
Design Flexibility
Support parameterized bit width (such as parameter word_size) to adapt to different data length requirements.
4. What are Logic Shift Registers Used for?
Data Serialization/Deserialization: Convert parallel bus and serial interface data.
Delay Line: Signal synchronization or time delay control.
Pseudo-random Number Generator: Random sequence based on LFSR.
Numerical Operation: Arithmetic shift to achieve multiplication/division (such as the power of 2 operations).
5. Summary
As a key component of digital systems, the logic shift register achieves efficient data processing and conversion through flexible shift operations and multi-mode control. Its hardware implementation relies on trigger cascading, and the performance is further optimized through an FPGA-specific structure.