本文是上篇文章《Zynq 7000 自定义IP实验》的扩展，
在Zynq 7000 的ip设计应用中，为了实现PS-PL 互动，我们需要选择 AXI 4 协议， 而Axi4 Lite 是其中简化版本，耗用资源比较少，也比较好理解。大多数情况，我们选择它。
为了设计好自定义ip 包，我们需要熟悉和掌握 Axi 4 Lite。
首先使用ILA 采集读写的波形图，另存工程，然后添加ila。我简单介绍其操作过程，如果不能掌握，也关系不大。主要是看波形。
写时序波形图(ILA 实测）
写时序波形图(ILA 实测）
现在我们转到我们分析的文件 myip_led_v1_0_S00_AXI.v
在文件的前部分 ，是模块的输入输出定义，这就是
module myip_led_v1_0_S00_AXI #
(
// Users to add parameters here
// User parameters ends
// Do not modify the parameters beyond this line
// Width of S_AXI data bus
parameter integer C_S_AXI_DATA_WIDTH = 32,
// Width of S_AXI address bus
parameter integer C_S_AXI_ADDR_WIDTH = 4
)
(
// Users to add ports here
output wire [3:0]LED,
// User ports ends
// Do not modify the ports beyond this line
// Global Clock Signal
input wire S_AXI_ACLK,
// Global Reset Signal. This Signal is Active LOW
input wire S_AXI_ARESETN,
// Write address (issued by master, acceped by Slave)
input wire [C_S_AXI_ADDR_WIDTH-1 : 0] S_AXI_AWADDR,
// Write channel Protection type. This signal indicates the
// privilege and security level of the transaction, and whether
// the transaction is a data access or an instruction access.
input wire [2 : 0] S_AXI_AWPROT,
// Write address valid. This signal indicates that the master signaling
// valid write address and control information.
input wire S_AXI_AWVALID,
// Write address ready. This signal indicates that the slave is ready
// to accept an address and associated control signals.
output wire S_AXI_AWREADY,
// Write data (issued by master, acceped by Slave)
input wire [C_S_AXI_DATA_WIDTH-1 : 0] S_AXI_WDATA,
// Write strobes. This signal indicates which byte lanes hold
// valid data. There is one write strobe bit for each eight
// bits of the write data bus.
input wire [(C_S_AXI_DATA_WIDTH/8)-1 : 0] S_AXI_WSTRB,
// Write valid. This signal indicates that valid write
// data and strobes are available.
input wire S_AXI_WVALID,
// Write ready. This signal indicates that the slave
// can accept the write data.
output wire S_AXI_WREADY,
// Write response. This signal indicates the status
// of the write transaction.
output wire [1 : 0] S_AXI_BRESP,
// Write response valid. This signal indicates that the channel
// is signaling a valid write response.
output wire S_AXI_BVALID,
// Response ready. This signal indicates that the master
// can accept a write response.
input wire S_AXI_BREADY,
// Read address (issued by master, acceped by Slave)
input wire [C_S_AXI_ADDR_WIDTH-1 : 0] S_AXI_ARADDR,
// Protection type. This signal indicates the privilege
// and security level of the transaction, and whether the
// transaction is a data access or an instruction access.
input wire [2 : 0] S_AXI_ARPROT,
// Read address valid. This signal indicates that the channel
// is signaling valid read address and control information.
input wire S_AXI_ARVALID,
// Read address ready. This signal indicates that the slave is
// ready to accept an address and associated control signals.
output wire S_AXI_ARREADY,
// Read data (issued by slave)
output wire [C_S_AXI_DATA_WIDTH-1 : 0] S_AXI_RDATA,
// Read response. This signal indicates the status of the
// read transfer.
output wire [1 : 0] S_AXI_RRESP,
// Read valid. This signal indicates that the channel is
// signaling the required read data.
output wire S_AXI_RVALID,
// Read ready. This signal indicates that the master can
// accept the read data and response information.
input wire S_AXI_RREADY
);
这部分先是参数定义，接下来就是所有信号的列表。每个信号上面都有一行注释，说明功能。归类一下
1：写地址通道： AWVALID, AWADDR, AWREADY(O)，AWPROT
2：写数据通道： WVALID, WDATA, WSTRB, WREADY(O)
3：写应答通道： BVALID(O)， BRESP(O)， BREADY
4：读地址通道： ARVALID, ARADDR，ARREADY(O)，ARPROT
5：读数据通道： RVALID,RDATA，RREADY， RRESP
6：系统通道： ACLK， ARESETN
接下来的代码是：
// AXI4LITE signals
reg [C_S_AXI_ADDR_WIDTH-1 : 0] axi_awaddr;
reg axi_awready;
reg axi_wready;
reg [1 : 0] axi_bresp;
reg axi_bvalid;
reg [C_S_AXI_ADDR_WIDTH-1 : 0] axi_araddr;
reg axi_arready;
reg [C_S_AXI_DATA_WIDTH-1 : 0] axi_rdata;
reg [1 : 0] axi_rresp;
reg axi_rvalid;
// Example-specific design signals
// local parameter for addressing 32 bit / 64 bit C_S_AXI_DATA_WIDTH
// ADDR_LSB is used for addressing 32/64 bit registers/memories
// ADDR_LSB = 2 for 32 bits (n downto 2)
// ADDR_LSB = 3 for 64 bits (n downto 3)
localparam integer ADDR_LSB = (C_S_AXI_DATA_WIDTH/32) + 1;
localparam integer OPT_MEM_ADDR_BITS = 1;
//----------------------------------------------
//-- Signals for user logic register space example
//------------------------------------------------
//-- Number of Slave Registers 4
reg [C_S_AXI_DATA_WIDTH-1:0] slv_reg0;
reg [C_S_AXI_DATA_WIDTH-1:0] slv_reg1;
reg [C_S_AXI_DATA_WIDTH-1:0] slv_reg2;
reg [C_S_AXI_DATA_WIDTH-1:0] slv_reg3;
wire slv_reg_rden;
wire slv_reg_wren;
reg [C_S_AXI_DATA_WIDTH-1:0] reg_data_out;
integer byte_index;
// I/O Connections assignments
assign S_AXI_AWREADY = axi_awready;
assign S_AXI_WREADY = axi_wready;
assign S_AXI_BRESP = axi_bresp;
assign S_AXI_BVALID = axi_bvalid;
assign S_AXI_ARREADY = axi_arready;
assign S_AXI_RDATA = axi_rdata;
assign S_AXI_RRESP = axi_rresp;
assign S_AXI_RVALID = axi_rvalid;
基本上，所有的输出都定义一个对应的reg，然后都有一个assign。
接下来的代码是：
// Implement axi_awready generation
// axi_awready is asserted for one S_AXI_ACLK clock cycle when both
// S_AXI_AWVALID and S_AXI_WVALID are asserted. axi_awready is
// de-asserted when reset is low.
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_awready <= 1'b0;
end
else
begin
if (~axi_awready && S_AXI_AWVALID && S_AXI_WVALID)
begin
// slave is ready to accept write address when
// there is a valid write address and write data
// on the write address and data bus. This design
// expects no outstanding transactions.
axi_awready <= 1'b1;
end
else
begin
axi_awready <= 1'b0;
end
end
end
正如第一行的注释，实现 axi_awready， AWVALID和WVALID都为1， axi_awready 就为1
接下来代码是：
// Implement axi_awaddr latching
// This process is used to latch the address when both
// S_AXI_AWVALID and S_AXI_WVALID are valid.
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_awaddr <= 0;
end
else
begin
if (~axi_awready && S_AXI_AWVALID && S_AXI_WVALID)
begin
// Write Address latching
axi_awaddr <= S_AXI_AWADDR;
end
end
end
这段代码正如第一行的注释，实现写地址的锁存。锁存的条件是axi_awready为0 ， AWVALID，WVALID 都为1
接下来的代码是：
// Implement axi_wready generation
// axi_wready is asserted for one S_AXI_ACLK clock cycle when both
// S_AXI_AWVALID and S_AXI_WVALID are asserted. axi_wready is
// de-asserted when reset is low.
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_wready <= 1'b0;
end
else
begin
if (~axi_wready && S_AXI_WVALID && S_AXI_AWVALID)
begin
// slave is ready to accept write data when
// there is a valid write address and write data
// on the write address and data bus. This design
// expects no outstanding transactions.
axi_wready <= 1'b1;
end
else
begin
axi_wready <= 1'b0;
end
end
end
这段代码正如第一行的注释，实现axi_wready。为1的条件是：axi_wready为0，WVALID，AWVALID都为1
接下来的代码是
// Implement memory mapped register select and write logic generation
// The write data is accepted and written to memory mapped registers when
// axi_awready, S_AXI_WVALID, axi_wready and S_AXI_WVALID are asserted. Write strobes are used to
// select byte enables of slave registers while writing.
// These registers are cleared when reset (active low) is applied.
// Slave register write enable is asserted when valid address and data are available
// and the slave is ready to accept the write address and write data.
assign slv_reg_wren = axi_wready && S_AXI_WVALID && axi_awready && S_AXI_AWVALID;
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
slv_reg0 <= 0;
slv_reg1 <= 0;
slv_reg2 <= 0;
slv_reg3 <= 0;
end
else begin
if (slv_reg_wren)
begin
case ( axi_awaddr[ADDR_LSB+OPT_MEM_ADDR_BITS:ADDR_LSB] )
2'h0:
for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
if ( S_AXI_WSTRB[byte_index] == 1 ) begin
// Respective byte enables are asserted as per write strobes
// Slave register 0
slv_reg0[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
end
2'h1:
for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
if ( S_AXI_WSTRB[byte_index] == 1 ) begin
// Respective byte enables are asserted as per write strobes
// Slave register 1
slv_reg1[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
end
2'h2:
for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
if ( S_AXI_WSTRB[byte_index] == 1 ) begin
// Respective byte enables are asserted as per write strobes
// Slave register 2
slv_reg2[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
end
2'h3:
for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
if ( S_AXI_WSTRB[byte_index] == 1 ) begin
// Respective byte enables are asserted as per write strobes
// Slave register 3
slv_reg3[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
end
default : begin
slv_reg0 <= slv_reg0;
slv_reg1 <= slv_reg1;
slv_reg2 <= slv_reg2;
slv_reg3 <= slv_reg3;
end
endcase
end
end
end
这段代码正如第一行的注释，实现写。其实上面都是为这写做辅助。
assign slv_reg_wren = axi_wready && S_AXI_WVALID && axi_awready && S_AXI_AWVALID;
这一行就是说写发生的条件。
WSTRB[byte_index] 控制对应字节的写操作，比如数据是4字节宽度，0x1111，就控制4个字节都要写入。 0x011就只低2字节写入。
写入的过程就这样完成了，每一个关键信号都有一段代码。
接下来的代码是：
// Implement write response logic generation
// The write response and response valid signals are asserted by the slave
// when axi_wready, S_AXI_WVALID, axi_wready and S_AXI_WVALID are asserted.
// This marks the acceptance of address and indicates the status of
// write transaction.
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_bvalid <= 0;
axi_bresp <= 2'b0;
end
else
begin
if (axi_awready && S_AXI_AWVALID && ~axi_bvalid && axi_wready && S_AXI_WVALID)
begin
// indicates a valid write response is available
axi_bvalid <= 1'b1;
axi_bresp <= 2'b0; // 'OKAY' response
end // work error responses in future
else
begin
if (S_AXI_BREADY && axi_bvalid)
//check if bready is asserted while bvalid is high)
//(there is a possibility that bready is always asserted high)
begin
axi_bvalid <= 1'b0;
end
end
end
end
这段代码是写完成的响应信号控制。
接下来的代码就是读的过程了。
接下来的代码是：
// Implement axi_arready generation
// axi_arready is asserted for one S_AXI_ACLK clock cycle when
// S_AXI_ARVALID is asserted. axi_awready is
// de-asserted when reset (active low) is asserted.
// The read address is also latched when S_AXI_ARVALID is
// asserted. axi_araddr is reset to zero on reset assertion.
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_arready <= 1'b0;
axi_araddr <= 32'b0;
end
else
begin
if (~axi_arready && S_AXI_ARVALID)
begin
// indicates that the slave has acceped the valid read address
axi_arready <= 1'b1;
// Read address latching
axi_araddr <= S_AXI_ARADDR;
end
else
begin
axi_arready <= 1'b0;
end
end
end
这段代码实现axi_arready。 为1的条件是： axi_arready为0，ARVALID为1。为1只是保留1个ACLK
变1的同时锁定读地址：axi_araddr <= S_AXI_ARADDR。
接下来的代码是：
// Implement axi_arvalid generation
// axi_rvalid is asserted for one S_AXI_ACLK clock cycle when both
// S_AXI_ARVALID and axi_arready are asserted. The slave registers
// data are available on the axi_rdata bus at this instance. The
// assertion of axi_rvalid marks the validity of read data on the
// bus and axi_rresp indicates the status of read transaction.axi_rvalid
// is deasserted on reset (active low). axi_rresp and axi_rdata are
// cleared to zero on reset (active low).
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_rvalid <= 0;
axi_rresp <= 0;
end
else
begin
if (axi_arready && S_AXI_ARVALID && ~axi_rvalid)
begin
// Valid read data is available at the read data bus
axi_rvalid <= 1'b1;
axi_rresp <= 2'b0; // 'OKAY' response
end
else if (axi_rvalid && S_AXI_RREADY)
begin
// Read data is accepted by the master
axi_rvalid <= 1'b0;
end
end
end
这段代码实现axi_arvalid，读地址有效。
接下来的代码就是实现读：
// Implement memory mapped register select and read logic generation
// Slave register read enable is asserted when valid address is available
// and the slave is ready to accept the read address.
assign slv_reg_rden = axi_arready & S_AXI_ARVALID & ~axi_rvalid;
always @(*)
begin
// Address decoding for reading registers
case ( axi_araddr[ADDR_LSB+OPT_MEM_ADDR_BITS:ADDR_LSB] )
2'h0 : reg_data_out <= slv_reg0;
2'h1 : reg_data_out <= slv_reg1;
2'h2 : reg_data_out <= slv_reg2;
2'h3 : reg_data_out <= slv_reg3;
default : reg_data_out <= 0;
endcase
end
// Output register or memory read data
always @( posedge S_AXI_ACLK )
begin
if ( S_AXI_ARESETN == 1'b0 )
begin
axi_rdata <= 0;
end
else
begin
// When there is a valid read address (S_AXI_ARVALID) with
// acceptance of read address by the slave (axi_arready),
// output the read dada
if (slv_reg_rden)
begin
axi_rdata <= reg_data_out; // register read data
end
end
end
这段代码实现寄存器映像和读数据输出。根据地址axi_araddr[ADDR_LSB+OPT_MEM_ADDR_BITS:ADDR_LSB]，reg_data_out时刻映像其地址。但还不是输出到axi_rdata。
输出的条件是 assign slv_reg_rden = axi_arready & S_AXI_ARVALID & ~axi_rvalid;
这就是说， axiready 和 ARVALID 都为1 而 axi_rvalid 为0， 数据输出了：axi_rdata <= reg_data_out;。
这就是我对这段代码的理解。再回过头对照看看采集的读写时序图。
仿真还在测试中，放在下一文章中。