编写fpga的串口代码时遇到的一个大坑！
【编写fpga的串口代码时遇到的一个大坑！】
编写fpga的串口代码时遇到的一个大坑！
- 1 warning 10240！！！
- 2 代码
- 2.1 uart_tx.v
- 2.2 send_freq.v
- 2.3 send_freq_top.v
1 warning 10240！！！ ??在调试串口时遇到一个很诡异的问题。我打算用串口循环发送一个频率值，间隔为1s，然而下载到板子后只能发一次。我反复地检查我的时序设计图，然后用testbench仿真观察波形，都没有错！第一次遇到仿真正确然而下板验证时却出错的情况，值得纪念一下。不过难受的是我为这件事卡了一个星期，这一个星期里我反复地修改串口模块、发送频率模块和顶层模块，然而都无济于事，甚至还出现很多莫名其妙的情况，比如发送间隔为1s时只发了一次，改为10ms时就发送了两次，再改为1us时就什么也收不到了，完全不知道里面发生了什么……
??转机出现在今晚，我想起了以前看过的fpga开发分享贴，里面提到调试fpga时如果实在找不出bug所在，就看看warning。这一看不得了了！当我把下面这条10240警告改掉之后，程序就跑通了！！！
10240 Verilog HDL Always Construct warning at send_freq.v(38): inferring latch(es) for variable "r_next_state", which holds its previous value in one or more paths through the always construct
??这一条警告出现的原因是写了if没写else，或者写了case没写default，产生了锁存器。我在send_freq.v里面是这样写的：
always @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_cur_state <= S_IDLE;
else
r_cur_state <= r_next_state;
endalways @(*) begin
case(r_cur_state)
S_IDLE:
if(i_freq_send_req)
r_next_state <= S_SEND_TENTH;
S_SEND_TENTH:
...
default: r_next_state <= S_IDLE;
endcase
end
??我没有给r_next_state赋初值，复位后r_cur_state为S_IDLE，如果在紧接着的时钟沿到来前i_freq_send_req没有出现，那么r_cur_state就被赋值为r_next_state，一个未初始化的值。然后再次进入case判断时，因为有default的存在，r_next_state第一次被赋值为S_IDLE。接着r_cur_state又被赋值为S_IDLE，这个时候程序才走上正轨，该赋值的变量都赋值了。按理说不应该出现我遇到的问题，所以这个玄学我暂时没法解释。先记录在此，以后回过头来看也许就能一目了然。如果各位看官知道其中道理的，还望不吝赐教。
??以后要是要重视编程规范呀。
2 代码 ??这部分是可以运行的代码，分别是串口发送模块，频率发送控制模块和顶层模块。
2.1 uart_tx.v
module uart_tx(
inputi_clk,
inputi_rst_n,
inputi_uart_send_req,
input[7:0] i_uart_send_byte,
outputo_uart_send_ack,
outputo_uart_tx_pin
);
localparam C_CYCLE = 50 * 1000000 / 115200;
localparam S_IDLE= 5'b00001;
localparam S_START= 5'b00010;
localparam S_DATA= https://www.it610.com/article/5'b00100;
localparam S_STOP= 5'b01000;
localparam S_ACK = 5'b10000;
reg[4:0] r_cur_state;
reg[4:0] r_next_state;
reg[31:0]r_cycle_cnt;
reg[2:0] r_bit_cnt;
reg[7:0] r_uart_send_byte;
regr_uart_send_ack;
regr_uart_tx_pin;
assign o_uart_tx_pin = r_uart_tx_pin;
assign o_uart_send_ack = r_uart_send_ack;
always @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_cur_state <= S_IDLE;
else
r_cur_state <= r_next_state;
endalways @(*) begin
case(r_cur_state)
S_IDLE:
if(i_uart_send_req)
r_next_state <= S_START;
else
r_next_state <= S_IDLE;
S_START:
if(r_cycle_cnt == C_CYCLE - 1)
r_next_state <= S_DATA;
else
r_next_state <= S_START;
S_DATA:
if((r_cycle_cnt == C_CYCLE - 1) && r_bit_cnt == 3'd7)
r_next_state <= S_STOP;
else
r_next_state <= S_DATA;
S_STOP:
if(r_cycle_cnt == C_CYCLE - 1)
r_next_state <= S_ACK;
else
r_next_state <= S_STOP;
S_ACK: r_next_state <= S_IDLE;
default: r_next_state <= S_IDLE;
endcase
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_uart_tx_pin <= 1'b1;
else
case(r_next_state)
S_IDLE: r_uart_tx_pin <= 1'b1;
S_START: r_uart_tx_pin <= 1'b0;
S_DATA: r_uart_tx_pin <= r_uart_send_byte[r_bit_cnt];
S_STOP, S_ACK: r_uart_tx_pin <= 1'b1;
default: r_uart_tx_pin <= 1'b1;
endcase
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_cycle_cnt <= 32'd0;
else if(r_cur_state != r_next_state || (r_next_state == S_DATA && r_cycle_cnt == C_CYCLE - 1))
r_cycle_cnt <= 32'd0;
else
r_cycle_cnt <= r_cycle_cnt + 32'd1;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_bit_cnt <= 3'd0;
else if(r_cur_state == S_DATA)
if(r_cycle_cnt == C_CYCLE - 1)
r_bit_cnt <= r_bit_cnt + 3'd1;
else
r_bit_cnt <= r_bit_cnt;
else
r_bit_cnt <= 3'd0;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_uart_send_byte <= 8'd0;
else if(i_uart_send_req)
r_uart_send_byte <= i_uart_send_byte;
else
r_uart_send_byte <= r_uart_send_byte;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_uart_send_ack <= 1'b0;
else if(r_next_state == S_ACK)
r_uart_send_ack <= 1'b1;
else
r_uart_send_ack <= 1'b0;
endendmodule
2.2 send_freq.v
module send_freq(
inputi_clk,
inputi_rst_n,
inputi_freq_send_req,
input[9:0] i_freq,
outputo_freq_send_ack,
outputo_uart_tx_pin
);
localparam S_IDLE= 6'b000001;
localparam S_SEND_TENTH = 6'b000010;
localparam S_SEND_DOT= 6'b000100;
localparam S_SEND_INT= 6'b001000;
localparam S_WAIT= 6'b010000;
localparam S_ACK= 6'b100000;
reg[5:0] r_cur_state;
reg[5:0] r_next_state;
reg[5:0] r_last_state;
reg[9:0] r_freq;
reg[7:0] r_uart_send_byte;
regr_freq_send_ack;
regr_uart_send_req;
wirer_uart_send_ack;
assign o_freq_send_ack = r_freq_send_ack;
always @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_cur_state <= S_IDLE;
else
r_cur_state <= r_next_state;
endalways @(*) begin
case(r_cur_state)
S_IDLE:
if(i_freq_send_req)
r_next_state <= S_SEND_TENTH;
else // important!!
r_next_state <= S_IDLE;
S_SEND_TENTH,S_SEND_DOT,S_SEND_INT: r_next_state <= S_WAIT;
S_WAIT:
if(r_uart_send_ack) begin
if(r_last_state == S_SEND_TENTH)
r_next_state <= S_SEND_DOT;
else if(r_last_state == S_SEND_DOT)
r_next_state <= S_SEND_INT;
else if(r_last_state == S_SEND_INT && (|r_freq))
r_next_state <= S_SEND_INT;
else
r_next_state <= S_ACK;
end
else
r_next_state <= S_WAIT;
S_ACK: r_next_state <= S_IDLE;
default: r_next_state <= S_IDLE;
endcase
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_freq_send_ack <= 1'b0;
else if(r_next_state == S_ACK)
r_freq_send_ack <= 1'b1;
else
r_freq_send_ack <= 1'b0;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_last_state <= S_IDLE;
else if(r_next_state == S_SEND_TENTH || r_next_state == S_SEND_DOT || r_next_state == S_SEND_INT)
r_last_state <= r_next_state;
else
r_last_state <= r_last_state;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_uart_send_req <= 1'b0;
else if(r_next_state == S_SEND_TENTH)
r_uart_send_req <= 1'b1;
else if(r_next_state == S_ACK)
r_uart_send_req <= 1'b0;
else
r_uart_send_req <= r_uart_send_req;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_freq <= 10'd0;
else if(r_next_state == S_SEND_TENTH)
r_freq <= i_freq / 10'd10;
else if(r_next_state == S_SEND_INT && (|r_freq))
r_freq <= r_freq / 10'd10;
else
r_freq <= r_freq;
endalways @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n)
r_uart_send_byte <= 8'hff;
else if(r_next_state == S_SEND_TENTH)
r_uart_send_byte <= i_freq % 10'd10 + "0";
else if(r_next_state == S_SEND_DOT)
r_uart_send_byte <= ".";
else if(r_next_state == S_SEND_INT && (|r_freq))
r_uart_send_byte <= r_freq % 10'd10 + "0";
else
r_uart_send_byte <= r_uart_send_byte;
enduart_tx uart_tx_inst(
.i_clk(i_clk),
.i_rst_n(i_rst_n),
.i_uart_send_req(r_uart_send_req),
.i_uart_send_byte(r_uart_send_byte),
.o_uart_send_ack(r_uart_send_ack),
.o_uart_tx_pin(o_uart_tx_pin)
);
endmodule
2.3 send_freq_top.v
module send_freq_top(
inputi_clk,
inputi_rst_n,
output o_uart_tx_pin
);
localparam S_IDLE = 4'b0001;
localparam S_SEND = 4'b0010;
localparam S_WAIT = 4'b0100;
localparam S_SEND2= 4'b1000;
reg[3:0]r_next_state;
reg[31:0] r_clk_cnt;
reg[9:0]r_freq;
regr_freq_send_req;
wirer_freq_send_ack;
always @(posedge i_clk or negedge i_rst_n) begin
if(!i_rst_n) begin
r_next_state <= S_IDLE;
r_freq <= 10'd1021;
r_freq_send_req <= 1'b0;
r_clk_cnt <= 32'd0;
end
else
case(r_next_state)
S_IDLE: r_next_state <= S_SEND;
S_SEND: begin
r_clk_cnt <= 32'd0;
if(r_freq_send_req && r_freq_send_ack) begin
r_freq_send_req <= 1'b0;
r_next_state <= S_WAIT;
end
else if(!r_freq_send_req)
r_freq_send_req <= 1'b1;
end
S_WAIT: begin
r_clk_cnt <= r_clk_cnt + 32'd1;
if(r_clk_cnt >= 32'd50000000)
r_next_state <= S_SEND;
end
default: r_next_state <= S_IDLE;
endcase
endsend_freq send_freq_inst(
.i_clk(i_clk),
.i_rst_n(i_rst_n),
.i_freq_send_req(r_freq_send_req),
.i_freq(r_freq),
.o_freq_send_ack(r_freq_send_ack),
.o_uart_tx_pin(o_uart_tx_pin)
);
endmodule
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