Understand chisel language. 23. Chisel sequential circuit (III) -- detailed explanation of chisel shift register

Chisel Sequential circuits （ 3、 ... and ）——Chisel shift register （Shift Register） Detailed explanation

The last article introduced Chisel Counters and some advanced uses , It's a lot of content , You will definitely gain a lot from learning . In addition to the counter , Another kind of register is widely used , That is the shift register . This article will introduce shift registers and Chisel Shift register in , It also includes parallel output 、 Shift register loaded in parallel .

shift register

The shift register is a set of flip flops connected in sequence , Every register （ trigger ） The output of is used as the input of the next register . Here is a 4 Schematic diagram of stage shift register ：

The shift register circuit will be on each clock tick Move data from left to right , So... In the diagram above 4 Level shift register circuit realizes from din To dout Four beat delay output . It should be noted that , The above diagram uses four registers , But in fact, the implementation is equivalent to a shift register , The same is true of the latter two shift registers .

Below Chisel The code implements a shift register ：

val shiftReg = Reg(UInt(4.W))
shiftReg := Cat(shiftReg(2, 0), din)
val dout = shiftReg(3)

This code needs to be explained , It has done so many things ：

1. Created a 4 Bit register shiftReg;
2. Set the low of the shift register 3 Bit and input din use Cat Splice up , As the next input of the register ;
3. The most significant bit of the value of the register （Most Significant Bit,MSB） As dout Output .

It's easy to understand . Shift registers are usually used to convert serial data to parallel data or parallel data to serial data , These two shift registers are introduced in the following two sections .

Parallel output shift register

The shift register configured with serial input and parallel output can convert a serial input stream into parallel words , This shift register may be used for serial port （UART） Receiving function of . The following figure is a 4 Bit parallel output shift register , The output of each trigger is connected to a single bit output ：

After four clock cycles , The circuit will put a 4 Bit serial data word is converted into a 4 Bit parallel data word q. In this case , Let's assume that No 0 position （ Its lowest ） Input first, so when reading the complete word, it reaches the last register . The following code implements such a 4 Bit parallel output shift register ：

val outReg = RegInit(0.U(4.W))
outReg := Cat(serIn, outReg(3, 1))
val q = outReg

This shift register outReg Initialize to 0, Then we go from MSB Start moving in , That is, the register value is shifted to the right ,4 After a period , The result of parallel output q Just register outReg Value .

Parallel input shift register

The shift register of parallel input serial output configuration can put a parallel input word （ byte ） The stream is converted to a serial output stream , This shift register may be used for serial port （UART） Sending function . Here is a 4 Schematic diagram of shift register with bit parallel input ：

You can see , Each shift register has an input Mux To choose which data to load , If load The signal is set , Then load d, Otherwise load 0 Or the value of the previous register , That is, displacement , In a nutshell load Load when valid ,load Shift output when invalid . use Chisel The code implementation is as follows ：

val loadReg = RegInit(0.U(4.W))
when(load) {
    
    loadReg := d
} .otherwise {
    
    loadReg := Cat(0.U, loadReg(3, 1))
}
val serOut = loadReg(0)

Look for the otherwise In statement block Cat call , During the shift, the most significant bit will be filled 0, The shifted least significant bit is connected as an output to serOut On .

Conclusion

This article introduces the implementation of shift register and two commonly used shift registers , It will be very useful in realizing serial communication , It may have some inspiration for the later high-speed interface design . The register related content ends here , But in digital design , There are not only registers that can store the state signal of the circuit , Memory （Memory） You can also save status information , In processor design, memory is also used for storing program code and data , So it's absolutely important . In the next article, we will talk about it in detail in a long length Chisel Memory implementation in , Coming soon .