SPI协议介绍
spi是serial peripheral interface的缩写，即串行扩展总线。SPI是单主设备通信，总线中只有一个主设备发起通信，能发起通信的设备称为主设备。当SPI主设备想读写从设备时，首先拉低对应从设备的ss线（低电平有效）。然后发送工作麦种到时钟线上，在相应的脉冲时间上，主设备把信号发送到MOSI实现读写，同时又可以对MISO采样实现读。一般SPI通信涉及到一下术语：
| SCLK | serial clock (来自主设备) |
|---|---|
| MOSI | Master Output Slave Input(来自主设备) |
| MISO | Master Input Slave Output(来自从设备) |
| SS | Slave Select（低电平有效，来自主设备） |
主设备和从设备的两种链接方式
一主一从
一主一从模式表明有一个主机和一个从机，如下所示：（其中还有SCK信号，由主机到从机）
一主多从
一主多从表示有一个主机和一个从机，唯一不同的点是要为每个从机配备一个选择信号。
SPI协议的工作模式
SPI有四种工作模式，主要由时钟极性CPOL（Clock Polarity），时钟相位CPHA（Clock Phase）的组合决定。
- CPOL为0，表示SCK在空闲状态为0，为1，则表示SCK在空闲状态为1
- CPHA为0，表示在SCK的第一个边沿时输出输出数据有效，CPHA为1时，表示在SCK的第二个边沿输入输出数据有效
CPOL = 0， CPHA = 0
CPOL = 0， CPHA = 1
CPOL = 1. CPHA = 0
CPOL = 1， CPHA = 1
SPI MASTER 的verilog设计思路
设计的引脚说明：
| 信号名 | 方向 +解释 |
|---|---|
| clk | 输入，时钟信号 |
| rst_n | 输入，复位信号 |
| miso | 输入，从机输入到主机 |
| data_i | 输入，主机发送从机的数据（一定比特位宽） |
| start | 输入，开始的使能信号 |
| mosi | 输出，主机到从机 |
| sclk | 输出，时钟信号 |
| ss_n | 输出，从机的选择信号 |
| finish | 一次传输完成信号 |
首先将需要的宏定义收录在一个文件defines.v中，其中的代码如下所示：
`define CPOL 0 //clock polarity
`define CPHA 0 //clock phase
`define CLK_FREQ 50_000_000 // input clk frequency
`define SCLK_FREQ 5_000_000 // sclk frequency
`define DATA_WIDTH 8 // a word width
`define CLK_CYCLE 20
其次时SPI MASTER的设计思路，总体思路时采用一个状态机，首先状态机是在IDLE状态，然后接收开始信号后，会将寄存器data_I中数据一个一个的发送出去，当指定宽距的比特位发送完成后。此时有两种选择，一种是跳转到FINISH状态，另一种是跳转到EXTRA状态，在跳转到FINISH状态。主要是由于当数据发送完成之后要判断此时的SCLK状态是不是空闲下的默认状态，如果不是，则需要跳转到EXTRA状态。结束状态下一个状态回到IDLE状态，得到start命令。
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| IDLE | 空闲状态 |
|---|---|
| DATA | 发送数据状态 |
| EXTRA | 额外状态 |
| FINISH | 结束状态 |
verilog的代码如下：
`include "defines.v"
module SPI_MASTER(
input wire clk ,
input wire rst_n ,
input wire miso ,
input wire [`DATA_WIDTH-1:0] data_i ,
input wire start ,
output wire mosi ,
output reg sclk ,
output reg ss_n ,
output wire finish
);
parameter IDLE = 5'b00001 ,
//CHOOSE = 5'b00010 ,
DATA = 5'b00100 ,
EXTRA = 5'b01000 ,
FINISH = 5'b10000 ;
parameter CNT_MAX = `CLK_FREQ / `SCLK_FREQ - 1;
reg [31:0] cnt ; //sclk的时钟周期的计数器
reg [4:0] state ;
reg [4:0] nx_state ;
wire [3:0] cnt_data ; //输出的数据计数器
reg sclk_dly ; //sclk的打一拍信号
reg [3:0] cnt_sclk_pos ; //sclk的上升沿计数器信号
reg [3:0] cnt_sclk_neg ; //sclk的下降沿计数器信号
reg start_dly ; //start的打一拍信号
reg [3:0] cnt_data_dly ;
wire cnt_max_flag ; //计数器cnt达到最大值的信号
wire dec_pos_or_neg_sample ; //1 posedge sample, 0 negedge sample
wire sclk_posedge ; //sclk的上升沿
wire sclk_negedge ; //sclk的下降沿
assign dec_pos_or_neg_sample = (`CPOL == `CPHA) ? 1'b1 : 1'b0;
assign cnt_max_flag = (cnt == CNT_MAX ) ? 1'b1 : 1'b0;
assign sclk_posedge = ((sclk == 1'b1) && (sclk_dly == 1'b0)) ? 1'b1 : 1'b0;
assign sclk_negedge = ((sclk == 1'b0) && (sclk_dly == 1'b1)) ? 1'b1 : 1'b0;
assign cnt_data = dec_pos_or_neg_sample ? cnt_sclk_pos : cnt_sclk_neg;
always @(posedge clk or negedge rst_n) begin
sclk_dly <= sclk;
start_dly <= start;
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
cnt_sclk_pos <= 4'd0;
end
else if(state == FINISH) begin
cnt_sclk_pos <= 4'd0;
end
//else if((sclk_posedge) && (cnt_sclk_pos == `DATA_WIDTH - 1)) begin
// cnt_sclk_pos <= `DATA_WIDTH - 1;
//end
else if(sclk_posedge) begin
cnt_sclk_pos <= cnt_sclk_pos + 1'b1;
end
else begin
cnt_sclk_pos <= cnt_sclk_pos;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
cnt_sclk_neg <= 4'd0;
end
else if(state == FINISH) begin
cnt_sclk_neg <= 4'd0;
end
//else if((sclk_negedge) && (cnt_sclk_neg == `DATA_WIDTH - 1)) begin
// cnt_sclk_neg <= `DATA_WIDTH - 1;
//end
else if(sclk_negedge) begin
cnt_sclk_neg <= cnt_sclk_neg + 1'b1;
end
else begin
cnt_sclk_neg <= cnt_sclk_neg;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
state <= IDLE;
end
else begin
state <= nx_state;
end
end
always @(*) begin
nx_state <= IDLE;
case(state)
IDLE: nx_state <= start_dly ? DATA : IDLE;
DATA: nx_state <= (cnt_data == `DATA_WIDTH) ? (`CPHA == 0) ? EXTRA : FINISH : DATA;
EXTRA: nx_state <= cnt_max_flag ? FINISH : EXTRA ;
FINISH: nx_state <= IDLE;
default:nx_state <= IDLE;
endcase
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
cnt <= 'd0;
end
else if((state == DATA) && (nx_state == FINISH) && (cnt == CNT_MAX)) begin
cnt <= 'd0;
end
else if((state == DATA) && (nx_state == EXTRA) && (cnt == CNT_MAX)) begin
cnt <= 'd0;
end
else if((state == DATA) && (cnt == CNT_MAX)) begin
cnt <= 'd0;
end
else if((state == EXTRA) && (cnt == CNT_MAX)) begin
cnt <= 'd0;
end
else if(state == DATA) begin
cnt <= cnt + 1'b1;
end
else if(state == EXTRA) begin
cnt <= cnt + 1'b1;
end
else begin
cnt <= 'd0;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
sclk <= (`CPOL) ? 1'b1 : 1'b0;
end
else if(start_dly) begin
sclk <= ~sclk;
end
else if((state == DATA) && (cnt_max_flag) && (cnt_data < `DATA_WIDTH) ) begin
sclk <= ~sclk;
end
else if((state == DATA) && (cnt_max_flag) && (cnt_data == `DATA_WIDTH) && (nx_state == EXTRA)) begin
sclk <= ~sclk;
end
else if((state == EXTRA) && (cnt_max_flag) && (nx_state == FINISH)) begin
sclk <= ~sclk;
end
else begin
sclk <= sclk;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
ss_n <= 1'b1;
end
else if(start) begin
ss_n <= 1'b0;
end
else if(state == FINISH) begin
ss_n <= 1'b1;
end
else begin
ss_n <= ss_n;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
cnt_data_dly <= 'd0;
end
else begin
cnt_data_dly <= cnt_data;
end
end
assign finish = (state == FINISH) ? 1'b1 : 1'b0;
assign mosi = (state == DATA) ? ((cnt_data_dly < `DATA_WIDTH) ? data_i[cnt_data_dly] : data_i[`DATA_WIDTH-1]) : data_i[0];
endmodule
仿真的testbench如下：
`timescale 1ns/1ns
`include "defines.v"
module tb_master_slave();
reg clk ;
reg rst_n ;
//reg mosi ;
reg [`DATA_WIDTH-1:0] data_i ;
reg start ;
reg miso ;
wire mosi ;
wire sclk ;
wire finish ;
wire ss_n ;
wire [`DATA_WIDTH-1:0] data_o ;
wire r_finish ;
SPI_MASTER u_spi_master (
.clk (clk) ,
.rst_n (rst_n) ,
.miso (miso) ,
.data_i (data_i) ,
.start (start) ,
.mosi (mosi) ,
.sclk (sclk) ,
.finish (finish) ,
.ss_n (ss_n)
);
SPI_SLAVE u_spi_slave(
.clk (clk) ,
.rst_n (rst_n) ,
.mosi (mosi) ,
.sclk (sclk) ,
.tx_finish(finish),
.start (start) ,
.ss_n (ss_n) ,
.data_o (data_o) ,
.r_finish(r_finish)
);
initial begin
clk = 1'b0;
rst_n = 1'b0;
start = 1'b0;
data_i = 8'h35;
miso = 1'b0;
#30
rst_n = 1'b1;
#10;
@(posedge clk);
start <= 1'b1;
@(posedge clk);
start <= 1'b0;
@(negedge finish);
data_i = 8'h44;
repeat(2) @(posedge clk);
start = 1'b1;
@(posedge clk);
start = 1'b0;
end
always #(`CLK_CYCLE / 2) clk = ~clk;
endmodule
仿真波形如下，可以通过mosi成功发出了数据。
SPI SLAVE设计思路
SPI SPLAVE的设计思路大体如master类似，端口说明如下：
| 信号名 | 方向 +解释 |
|---|---|
| clk | 输入，时钟信号 |
| rst_n | 输入，复位信号 |
| miso | 输入，从机输入到主机 |
| data_i | 输入，主机发送从机的数据（一定比特位宽） |
| start | 输入，开始的使能信号 |
| mosi | 输出，主机到从机 |
| sclk | 输入，时钟信号 |
| ss_n | 输入，从机的选择信号 |
| r_finish | 一次传输完成信号 |
| data_o | 输出，收集到的数据 |
依然采用的是状态机思路，首先在IDLE状态，当开始信号使能之后，会跳转到RV_DATA接收到数据状态。RV_DATA数据接收完成，之后回跳转到FINISH状态，表明此次读取完成。
| IDLE | 空闲状态 |
|---|---|
| RV_ DATA | 接收数据状态 |
| FINISH | 结束状态 |
| spi slave的代码如下所示： |
`include "defines.v"
module SPI_SLAVE(
input wire clk ,
input wire rst_n ,
input wire mosi ,
input wire sclk ,
input wire tx_finish ,
input wire start ,
input wire ss_n ,
output wire [`DATA_WIDTH-1:0] data_o ,
//output wire miso ,
output wire r_finish
);
parameter IDLE = 4'b0001 ,
RV_DATA = 4'b0010 ,
FINISH = 4'b0100 ;
wire sclk_posedge ;
wire sclk_negedge ;
wire dec_pos_or_neg_sample;
//wire sclk_posedge ;
//wire sclk_negedge ;
reg sclk_dly ;
reg [`DATA_WIDTH-1:0] data_shift_pos ;
reg [`DATA_WIDTH-1:0] data_shift_neg ;
reg [3:0] state ;
reg [3:0] nx_state ;
reg [3:0] cnt_sclk_pos ;
reg [3:0] cnt_sclk_neg ;
wire [3:0] num_sample_data ;
assign sclk_posedge = ((sclk == 1'b1) && (sclk_dly == 1'b0)) ? 1'b1 : 1'b0;
assign sclk_negedge = ((sclk == 1'b0) && (sclk_dly == 1'b1)) ? 1'b1 : 1'b0;
assign dec_pos_or_neg_sample = (`CPOL == `CPHA) ? 1'b1 : 1'b0;
//assign sclk_posedge = ((sclk == 1'b1) && (sclk_dly == 1'b0)) ? 1'b1 : 1'b0;
//assign sclk_negedge = ((sclk == 1'b0) && (sclk_dly == 1'b1)) ? 1'b1 : 1'b0;
assign num_sample_data = (dec_pos_or_neg_sample) ? cnt_sclk_pos : cnt_sclk_neg;
always @(posedge clk or negedge rst_n) begin
sclk_dly <= sclk;
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
state <= IDLE;
end
else begin
state <= nx_state;
end
end
always @(*) begin
nx_state <= IDLE;
case(state)
IDLE: nx_state <= start ? RV_DATA :IDLE;
RV_DATA: begin
if((num_sample_data == 7) && (dec_pos_or_neg_sample) && (sclk_posedge) && (!ss_n)) begin
nx_state <= FINISH;
end
else if((num_sample_data == 7) && (~dec_pos_or_neg_sample) && (sclk_negedge) && (!ss_n)) begin
nx_state <= FINISH;
end
else begin
nx_state <= RV_DATA;
end
end
FINISH: nx_state <= IDLE;
endcase
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
cnt_sclk_pos <= 4'd0;
end
else if((state == FINISH)) begin
cnt_sclk_pos <= 4'd0;
end
else if(sclk_posedge) begin
cnt_sclk_pos <= cnt_sclk_pos + 1'b1;
end
else begin
cnt_sclk_pos <= cnt_sclk_pos;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
cnt_sclk_neg <= 4'd0;
end
else if (state == FINISH) begin
cnt_sclk_neg <= 4'd0;
end
else if (sclk_negedge) begin
cnt_sclk_neg <= cnt_sclk_neg + 1'b1;
end
else begin
cnt_sclk_neg <= cnt_sclk_neg;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
data_shift_pos <= {
`DATA_WIDTH{
1'b0}};
end
else if((state == RV_DATA) && (sclk_posedge)) begin
data_shift_pos <= {
mosi, data_shift_pos[`DATA_WIDTH-1:1]};
end
else if (state == FINISH) begin
data_shift_pos <= {
`DATA_WIDTH{
1'b0}};
end
else begin
data_shift_pos <= data_shift_pos;
end
end
always @(posedge clk or negedge rst_n) begin
if(!rst_n) begin
data_shift_neg <= {
`DATA_WIDTH{
1'b0}};
end
else if((state == RV_DATA) && (sclk_negedge)) begin
data_shift_neg <= {
mosi, data_shift_neg[`DATA_WIDTH-1:1]};
end
else if(state == FINISH) begin
data_shift_neg <= {
`DATA_WIDTH{
1'b0}};
end
else begin
data_shift_neg <= data_shift_neg;
end
end
//assign data_o = dec_pos_or_neg_sample ? data_shift_pos : data_shift_neg;
assign data_o = (state == FINISH) ? (dec_pos_or_neg_sample ? data_shift_pos : data_shift_neg): {
`DATA_WIDTH{
1'b0}};
assign r_finish = (state == FINISH);
endmodule
SPI MASTRT 和 SLAVE联合仿真
testbench如下所示：
`timescale 1ns/1ns
`include "defines.v"
module tb_master_slave();
reg clk ;
reg rst_n ;
//reg mosi ;
reg [`DATA_WIDTH-1:0] data_i ;
reg start ;
reg miso ;
wire mosi ;
wire sclk ;
wire finish ;
wire ss_n ;
wire [`DATA_WIDTH-1:0] data_o ;
wire r_finish ;
SPI_MASTER u_spi_master (
.clk (clk) ,
.rst_n (rst_n) ,
.miso (miso) ,
.data_i (data_i) ,
.start (start) ,
.mosi (mosi) ,
.sclk (sclk) ,
.finish (finish) ,
.ss_n (ss_n)
);
SPI_SLAVE u_spi_slave(
.clk (clk) ,
.rst_n (rst_n) ,
.mosi (mosi) ,
.sclk (sclk) ,
.tx_finish(finish),
.start (start) ,
.ss_n (ss_n) ,
.data_o (data_o) ,
.r_finish(r_finish)
);
initial begin
clk = 1'b0;
rst_n = 1'b0;
start = 1'b0;
data_i = 8'h35;
miso = 1'b0;
#30
rst_n = 1'b1;
#10;
@(posedge clk);
start <= 1'b1;
@(posedge clk);
start <= 1'b0;
@(negedge finish);
data_i = 8'h44;
repeat(2) @(posedge clk);
start = 1'b1;
@(posedge clk);
start = 1'b0;
end
always #(`CLK_CYCLE / 2) clk = ~clk;
endmodule
仿真的波形如下，可知成功的读取到了8’h35和8’h44。验证成功。
总结
通过这次spi的协议的编写，感觉自己对于状态机的掌握，以及脑子里可以浮现波形。加油加油！！！