Verilog -- state machine coding method

Contained in the state machine N States usually need to be represented by some coding , Status code , Or status assignment .

Choose the appropriate coding scheme , It will contribute to the utilization of circuit area and resources .

The three most common types of state coding are ： Sequential binary encoding 、 Gray code and Hot code alone 、 Johnson code

Sequence code （Sequential State Machine Encoding）

Binary coding is the most compact coding , Each state encoded with sequential binary numbers .

eg. state0 state1 state2 state3 Expressed as '00'  '01' '10' '11'

The advantage is that it uses the smallest number of bits of the state vector . But when it changes from one state to another , Many bits may change ,[ There are intermediate states ] Many transients , Easy to produce burr , Cause a logical error

Gray code （Gray Code ）

In the transition of adjacent states , Only one bit at a time changes , It can reduce the possibility of burrs and some transients , But it is not suitable for situations with many state jumps .

Recursively generate code table

This method is based on the fact that gray code is reflection code , Use the following rules of recursion to construct ：

1. 1 Bit gray code has two codewords

2. (n+1) The first bit of gray code 2n Codeword equals n Codeword of bit gray code , Write in order , Prefixed 0

3. (n+1) After... In gray code 2n Codeword equals n Codeword of bit gray code , Write in reverse order , Prefixed 1 [4] 

4. n+1 A collection of bit gray codes = n Bit gray code set ( The order ) Prefixed 0 + n Bit gray code set ( The reverse ) Prefixed 1

2 Bit code	3 Bit code	4 Bit code	4 Bit natural binary code
00	000	0000	0000
01	001	0001	0001
11	011	0011	0010
10	010	0010	0011
	110	0110	0100
	111	0111	0101
	101	0101	0110
	100	0100	0111
		1100	1000
		1101	1001
		1111	1010
		1110	1011
		1010	1100
		1011	1101
		1001	1110
		1000	1111

Hot coding alone

namely One-Hot code , Also known as a bit effective code , Only one bit of each symbol value is '1', The others are '0',

Such as S0=3'b001,S1=3'b010,S2=3'b100

namely N A state needs N Trigger . The speed of the hot code state machine is only related to the number of transitions to a particular state, but not to the number of States , fast . And because its state decoding is simple , Therefore, the combinational logic circuit is saved and simplified .FPGA Due to the large number of registers, the logic gate resources of devices are tight , Using heat only coding can effectively improve FPGA Resource utilization and circuit speed . Single hot code also has simple design 、 Flexible modification and easy debugging 、 Easy to integrate 、 Easy to find critical path 、 Easy to carry out static timing analysis and other advantages . Therefore, try to use unique coding .

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Specify the encoding method in the synthesizer