对SDRAM基本概念的介绍以及芯片手册说明，请参考上一篇文章SDRAM操作说明。
1. 说明
如图所示为状态机的简化图示，过程大概可以描述为：SDRAM（IS42S16320D）上电初始化完成后，进入“空闲”状态，此时一直监控外部控制模块给予的控制信号。初始化完成后，外部定时器开始定时，定时周期为SDRAM刷新周期（7.7us），一旦计数到刷新周期后，向状态机发送auto_ref_req（自动刷新请求），此时状态机进入“刷新”状态，这样就确保在无任何操作时，SDRAM能正常完成刷新。刷新完成后回到“空闲”状态。
当处于空闲状态时，接收到写命令（wr_en），进入“写”状态（有效接收读写命令的时刻有特殊要求，后面再详细说明），在full_page下连续写600个数据（100MHz，恰好耗时6us多一点，这样方便不用考虑定时刷新），写完之后，发送wr_done命令，进入“刷新”状态，相对于每次连续写完成后，提前刷新一次。此时，定时刷新的计数器清零，重新开始计数。
读多过程跟写过程类似，读完600个数据之后，手动完成刷新。
现在就来说一说，“空闲”状态接收读写命令的特殊要求。理论上充电周期为7.8125us，为保证600次读写在充电周期内完成，并且前后预留一些其他命令的时间，所以推荐在0~1us这个时间内接受读写命令，这样读写的时候专注读写就可以了。当然这是我的设计方式，如有更好的设计方式，那更好，欢迎分享。
2. 代码实现
状态机的代码如下所示，清晰的描述了各状态之间的跳变及其跳变条件。其中信号ctrl_valid即为上图中命令有效期的时间区间。在各状态描述的时序逻辑模块中，只是产生了读、写或刷新执行模块的使能信号，即在“写”状态的时候，使能写模块，完成相信的写操作。
always @ (posedge clk or negedge rst_n)
begin
if(rst_n == 1'b0)
begin
current_status <= IDLE;
end
else if(init_ing == 1'b0)
begin
current_status <= next_status;
end
else
begin
current_status <= IDLE;
end
end
always @ (rst_n or current_status or sdram_wrreq or sdram_rdreq or ref_req_auto or wr_done or rd_done or ref_done or ctrl_valid)
begin
next_status = 5'dx;
case(current_status)
IDLE:
begin
if(ref_req_auto == 1'b1) //收到自动刷新请求
begin
next_status = AUTO_REF;
end
else if(ctrl_valid == 1'b1 && sdram_wrreq == 1'b1)//在读写控制有效区内收到写请求
begin
next_status = WRITE;
end
else if(ctrl_valid == 1'b1 && sdram_rdreq == 1'b1) //在读写控制有效区内收到读请求
begin
next_status = READ;
end
else
begin
next_status = IDLE;
end
end
WRITE:
begin
if(wr_done == 1'b1)
begin
next_status = AUTO_REF;
end
else
begin
next_status = WRITE;
end
end
READ:
begin
if(rd_done == 1'b1)
begin
next_status = AUTO_REF;
end
else
begin
next_status = READ;
end
end
AUTO_REF:
begin
if(ref_done == 1'b1)
begin
next_status = IDLE;
end
else
begin
next_status = AUTO_REF;
end
end
default:
begin
next_status = IDLE;
end
endcase
end
//各个状态下的使能信号，以控制相应的模块执行相应的操作
always @ (posedge clk or negedge rst_n)
begin
if(rst_n == 1'b0)
begin
wr_start <= 1'b0;
rd_start <= 1'b0;
ref_start <= 1'b0;
end
else
begin
case(next_status)
IDLE:
begin
wr_start <= 1'b0;
rd_start <= 1'b0;
ref_start <= 1'b0;
end
WRITE:
begin
wr_start <= 1'b1;
rd_start <= 1'b0;
ref_start <= 1'b0;
end
READ:
begin
wr_start <= 1'b0;
rd_start <= 1'b1;
ref_start <= 1'b0;
end
AUTO_REF:
begin
wr_start <= 1'b0;
rd_start <= 1'b0;
ref_start <= 1'b1;
end
default:
begin
wr_start <= 1'b0;
rd_start <= 1'b0;
ref_start <= 1'b0;
end
endcase
end
end
以下给出写操作模块的部分代码，读操作和刷新同理。中间有些信号是我工程需要，参考一下思路即可。
always @(posedge clk or negedge rst_n)
begin
if(rst_n == 1'b0)
begin
cke_wr <= 1'b0;
cmd_wr <= NOP;
dqm_wr <= DQM_DIS;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= 4'd0;
end
else if(wr_start == 1'b1)
begin
case(status_wr)
4'd0:
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr + 4'd1;
end
4'd1:
begin
cke_wr <= 1'b1;
cmd_wr <= ACT;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= row_addr; //行地址
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr + 4'd1;
end
4'd2: //4'd2和4'd3是为了延时T_RCD，即两个时钟
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr + 4'd1;
end
4'd3:
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr + 4'd1;
end
4'd4:
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b1; //用于写入第一个数据的时序标记
status_wr <= status_wr + 4'd1;
end
4'd5:
begin
cke_wr <= 1'b1;
cmd_wr <= WR;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= column_addr; //{A12A11，A10,column_address}
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr + 4'd1;
end
4'd6:
begin
if(sdram_wr_done == 1'b1) //用于增加NOP持续周期
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_DIS;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b1;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr + 4'd1;
end
else
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= status_wr;
end
end
4'd7:
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_DIS;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= 4'd0;
end
default:
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= 4'd0;
end
endcase
end
else
begin
cke_wr <= 1'b1;
cmd_wr <= NOP;
dqm_wr <= DQM_EN;
bank_addr_wr <= BANK0;
addr_wr <= DONT_CARE_ADDR;
wr_done <= 1'b0;
wr_first_flag_r <= 1'b0;
status_wr <= 4'd0;
end
end