【IC每日一题：AMBA总线--APB协议时序及Verilog实现】

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AMBA总线是用于连接微控制器和外围设备的总线协议，由ARM公司提出的一种片上系统SoC互联标准。
AMBA总线协议家族主要包括：APB、AHB、AXI、ACI、CHI协议；其中后两个协议主要是专门为了缓存一致性；

本篇博客主要介绍 APB3协议；

1 APB3协议

APB3协议：APB用来实现与外围设备进行通信–一般用来配置寄存器；APB协议是一主多从，唯一的master一般是apb_bridge模块；
总体特点：低功耗、低带宽、无流水线结构、每次传输至少需要两个时钟周期；

1.1 APB3时序

在这里会分别给出有等待和无等待的时序图做对比，在项目中主要还是用的有等待时序图；
什么是无等待：即slave会直接在maste拉高penable时进行拉高，只维持一个周期；
什么是有等待：即slave会跟自己内部业务进行任意时刻拉高pready，而在此期间以上master的信号都要维持不变；

1.1.1 APB写操作

【描述】
1.T0周期(T0-T1)：IDLE状态；
2.T1周期(T1-T2)：master提前准备好pasel,paddr,pwrite,pwdata;
3.T2周期(T2-T3)：master拉高penable，告知当前数据有效，进行采样传输；
–>T3周期(T3-T4)：是代表数据传输结束，回到IDLE状态；

1.1.2 APB读操作

T0周期：IDLE状态；
T1周期：master提前准备好psel,paddr,pwrite;
T2周期：在无等待模式下：slave准备好prdate并拉高pready；当然在有等待模式下，pready可以继续没准备好，当pready准备好拉高后，prdata同时返回并有效；

2 代码

从前面系列篇可知：对待时序，我们尽量用状态机来设计指导编码；
在这里官方给出状态机：正好对应上面时序图上的周期；

IDLE：deault state;
SetUp:psel立刻拉高，相关信号在setup阶段准备好，并且下一周期就立刻转移到下一状态；
Access:penable在该状态拉高，相关信号paddr,pwrite,psel,pwdata等在这保持；

2.1 apb_master

在这里实现一个apb_master的代码示例：

/*-------------------------------------------------------------
-- modified by xlinxdu, 2022/05/27
-- pclk 50MHz
-- APB3,No pslverr signal
-- cmd_i:56bit;[55:48]:r/w ,8'b0 -> read,8'b1 -> write[47:32]:paddr ,[31:0]:pwdata
-------------------------------------------------------------*/
module apb
#(parameter RD_FLAG        = 8'b0           ,parameter WR_FLAG        = 8'b1           ,parameter CMD_RW_WIDTH   = 8              ,parameter CMD_ADDR_WIDTH = 16             ,parameter CMD_DATA_WIDTH = 32             ,parameter CMD_WIDTH      = CMD_RW_WIDTH   + CMD_ADDR_WIDTH + CMD_DATA_WIDTH
)(
//-- system signalinput                           pclk_i       ,input                           prst_n_i     ,//-- cmd_ininput      [CMD_WIDTH-1:0]      cmd_i        ,input                           cmd_vld_i    ,output reg [CMD_DATA_WIDTH-1:0] cmd_rd_data_o,//-- apb interfaceoutput reg [CMD_ADDR_WIDTH-1:0] paddr_o      ,output reg                      pwrite_o     ,output reg                      psel_o       ,output reg                      penable_o    ,output reg [CMD_DATA_WIDTH-1:0] pwdata_o     ,input      [CMD_DATA_WIDTH-1:0] prdata_i     ,input                           pready_i     ,input                           pslverr_i
);//-- FSM state
parameter IDLE   = 3'b001;
parameter SETUP  = 3'b010;
parameter ACCESS = 3'b100;//-- current state and next state
reg [2:0] cur_state;
reg [2:0] nxt_state;//-- data buf
reg                      start_flag     ;
reg [CMD_WIDTH-1:0]      cmd_in_buf     ;
reg [CMD_DATA_WIDTH-1:0] cmd_rd_data_buf;/*-----------------------------------------------\--             update cmd_in_buf              --
\-----------------------------------------------*/
always @ (posedge pclk_i or negedge prst_n_i) beginif (!prst_n_i) begincmd_in_buf <= {(CMD_WIDTH){1'b0}};endelse if (cmd_vld_i && pready_i) begincmd_in_buf <= cmd_i;end
end/*-----------------------------------------------\--             start flag of transfer         --
\-----------------------------------------------*/
always @ (posedge pclk_i or negedge prst_n_i) beginif (!prst_n_i) beginstart_flag <= 1'b0;endelse if (cmd_vld_i && pready_i) beginstart_flag <= 1'b1;endelse beginstart_flag <= 1'b0;end
end/*-----------------------------------------------\--           update current state             --
\-----------------------------------------------*/
always @ (posedge pclk_i or negedge prst_n_i) beginif (!prst_n_i) begincur_state <= IDLE;endelse begincur_state <= nxt_state;end
end/*-----------------------------------------------\--               update next state            --
\-----------------------------------------------*/
always @ (*) begincase(cur_state)IDLE  :if(start_flag)beginnxt_state = SETUP;endelse beginnxt_state = IDLE;endSETUP :nxt_state = ACCESS;ACCESS:if (!pready_i)beginnxt_state = ACCESS;endelse if(start_flag)beginnxt_state = SETUP;endelse if(!cmd_vld_i && pready_i)beginnxt_state = IDLE;endendcase
end/*-----------------------------------------------\--         update signal of output            --
\-----------------------------------------------*/
always @ (posedge pclk_i or negedge prst_n_i) beginif (!prst_n_i) beginpwrite_o  <= 1'b0;psel_o    <= 1'b0;penable_o <= 1'b0;paddr_o   <= {(CMD_ADDR_WIDTH){1'b0}};pwdata_o  <= {(CMD_DATA_WIDTH){1'b0}};endelse if (nxt_state == IDLE) beginpsel_o    <= 1'b0;penable_o <= 1'b0;endelse if(nxt_state == SETUP)beginpsel_o    <= 1'b1;penable_o <= 1'b0;paddr_o   <= cmd_in_buf[CMD_WIDTH-CMD_RW_WIDTH-1:CMD_DATA_WIDTH];//-- readif(cmd_in_buf[CMD_WIDTH-1:CMD_WIDTH-8] == RD_FLAG)beginpwrite_o <= 1'b0;end//-- writeelse beginpwrite_o  <= 1'b1;pwdata_o  <= cmd_in_buf[CMD_DATA_WIDTH-1:0];endendelse if(nxt_state == ACCESS)beginpenable_o <= 1'b1;end
end/*-----------------------------------------------\--            update cmd_rd_data_buf          --
\-----------------------------------------------*/
always @ (posedge pclk_i or negedge prst_n_i) beginif (!prst_n_i) begincmd_rd_data_buf <= {(CMD_DATA_WIDTH){1'b0}};endelse if (pready_i && psel_o && penable_o) begincmd_rd_data_buf <= prdata_i;end
end/*-----------------------------------------------\--            update cmd_rd_data_o            --
\-----------------------------------------------*/
always @ (posedge pclk_i or negedge prst_n_i) beginif (!prst_n_i) begincmd_rd_data_o <= {(CMD_DATA_WIDTH){1'b0}};endelse begincmd_rd_data_o <= cmd_rd_data_buf;end
endendmodule

2.2 apb_slave

在这里实现一个apb_slave读写寄存器的Demo;

`timescale 1ns/100psmodule apb_slave #(parameter AW = 32,//地址总线宽度parameter DW = 32 //数据总线宽度) (//systeminput               reset_n,//apb_interfaceinput               pclk,     //时钟input [AW-1:0]      paddr,    //地址input               pwrite,   //读/写控制信号input               psel,     //选择信号input               penable,  //使能信号input [DW-1:0]      pwdata,   //写(输入)数据output reg [DW-1:0] prdata,   //读(输出)数据output reg          pready,   //传输完成标志，low未完成，high完成output reg          pslverr   //错误标志，high出现错误);
//apb状态机
parameter IDLE   = 2'b00;
parameter SETUP  = 2'b01;
parameter ACCESS = 2'b10;
reg [1:0] state_now  = 2'b00;
reg [1:0] state_next  = 2'b00;//映射到总线上的寄存器
reg [31:0]   readonly32;     //0x10000000，只可读
reg [15:0]   readonly16;     //0x10000004，只可读
reg [31:0]   readwrite32;    //0x10000008，可读可写
reg [15:0]   readwrite16;    //0x1000000C，可读可写always @(posedge pclk or negedge reset_n) begin: init_and_change_stateif (!reset_n) beginpready  <= 1'b0;pslverr <= 1'b0;prdata  <= 32'h0;readwrite32 <= 32'h0;readwrite16 <= 16'h0;readonly32 <= 32'h12345678;readonly16 <= 16'habcd;state_now <= IDLE;endelse beginstate_now <= state_next;end
end//! fsm_extract
always @(posedge pclk) begin :main_fsm_of_apbcase (state_now)IDLE: begin //IDLE状态case (psel)1'b1: state_next <= SETUP;  //选择本apb外设，进入setup状态1'b0: state_next <= IDLE;default: state_next <= IDLE;endcaseendSETUP: begin//SETUP状态case (psel)1'b1: begincase (penable)1'b1: state_next <= ACCESS;//选中且enable信号拉高，进入ACCESS模式传输数据1'b0: state_next <= SETUP;default: state_next <= SETUP;endcaseend1'b0: state_next <= IDLE;   //未被选中，回到IDLEdefault: state_next <= IDLE;endcasepready <= 1'b0;endACCESS: begin //ACCESS状态，根据寄存器地址读写prdata <= 32'h0;pslverr <= 1'b0;if (pwrite) begin   //wirte信号拉高，写入模式case (paddr)32'h10000008: begin readwrite32 <= pwdata; pready <= 1'b1; end//数据传输完成 32'h1000000C: begin readwrite16 <= pwdata[15:0]; pready <= 1'b1; end//数据传输完成 default: pslverr <= 1'b1;endcaseendelse if(!pwrite) begin  //write信号拉低，读取模式case (paddr)32'h10000000: begin prdata <= readonly32; pready <= 1'b1; end//数据传输完成 32'h10000004: begin prdata <= readonly16; pready <= 1'b1; end//数据传输完成 32'h10000008: begin prdata <= readwrite32; pready <= 1'b1; end//数据传输完成 32'h1000000C: begin prdata <= readwrite16; pready <= 1'b1; end//数据传输完成 default: pslverr <= 1'b1;endcaseendif (pready) begincase (psel)1'b1: state_next <= SETUP;  //不需要传输数据，回到SETUP1'b0: state_next <= IDLE;   //未被选中，回到IDLEdefault: state_next <= IDLE;endcaseendelse state_next <= ACCESS; //需要继续传输数据，则回到ACCESS       enddefault: state_next <= IDLE;endcase
end
endmodule

[ref]
1.https://blog.csdn.net/qq_43244515/article/details/124968189
2.https://www.amghank.cn/2021/08/24/CORE%E5%AD%A6%E4%B9%A0%EF%BC%9AAMBA3–APB%E6%80%BB%E7%BA%BF%E5%8D%8F%E8%AE%AE%E5%8F%8A%E7%AE%80%E5%8D%95%E4%BE%8B%E5%AD%90/