關於74HC595芯片的規格參數以及時序圖可參考德州儀器CD74HC595或其他博客,在此不多做分析。
現如今,小梅哥用兩片74HC595級聯,從而驅動八個八段8位的數碼管。輸入分別爲位選信號sel[7:0]和段選信號seg[7:0],經過設計的驅動電路,從而產生輸入到74HC595上的DS信號、移位信號SHCP以及存儲信號STCP。驅動原理圖如下:
源碼
RTL
對應小梅哥的verilog代碼如下:
/***************************************************
* Module Name : HC595_Driver
* Engineer : 小梅哥
* Target Device : EP4CE10F17C8
* Tool versions : Quartus II 13.0
* Create Date : 2017-3-31
* Revision : v1.0
* Description : 74HC595移位寄存器驅動設計
**************************************************/
module HC595_Driver(
Clk,
Rst_n,
Data,
S_EN,
SH_CP,
ST_CP,
DS
);
parameter DATA_WIDTH = 16;
input Clk;
input Rst_n;
input [DATA_WIDTH-1 : 0] Data; //data to send
input S_EN; //send en
output reg SH_CP; //shift clock
output reg ST_CP; //latch data clock
output reg DS; //shift serial data
parameter CNT_MAX = 4; //4
reg [15:0] divider_cnt;//分頻計數器
wire sck_pluse;
reg [4:0]SHCP_EDGE_CNT;//SH_CP EDGE counter
reg [15:0]r_data;
always@(posedge Clk or negedge Rst_n)
if(!Rst_n)
r_data <= 16'd0;
else if(S_EN)
r_data <= Data;
else
r_data <= r_data;
//clock divide
always@(posedge Clk or negedge Rst_n)
if(!Rst_n)
divider_cnt <= 16'd0;
else if(divider_cnt == CNT_MAX)
divider_cnt <= 16'd0;
else
divider_cnt <= divider_cnt + 1'b1;
assign sck_pluse = (divider_cnt == CNT_MAX);
always@(posedge Clk or negedge Rst_n)
if(!Rst_n)
SHCP_EDGE_CNT <= 5'd0;
else if(sck_pluse)begin
if(SHCP_EDGE_CNT == 5'd31)
SHCP_EDGE_CNT <= 5'd0;
else
SHCP_EDGE_CNT <= SHCP_EDGE_CNT + 1'd1;
end
else
SHCP_EDGE_CNT <= SHCP_EDGE_CNT;
always@(posedge Clk or negedge Rst_n)
if(!Rst_n)begin
SH_CP <= 1'b0;
ST_CP <= 1'b0;
DS <= 1'b0;
end
else begin
case(SHCP_EDGE_CNT)
5'd0:begin SH_CP <= 1'b0; ST_CP <= 1'b1; DS <= r_data[15]; end
5'd1:begin SH_CP <= 1'b1; ST_CP <= 1'b0;end
5'd2:begin SH_CP <= 1'b0; DS <= r_data[14];end
5'd3:begin SH_CP <= 1'b1; end
5'd4:begin SH_CP <= 1'b0; DS <= r_data[13];end
5'd5:begin SH_CP <= 1'b1; end
5'd6:begin SH_CP <= 1'b0; DS <= r_data[12];end
5'd7:begin SH_CP <= 1'b1; end
5'd8:begin SH_CP <= 1'b0; DS <= r_data[11];end
5'd9:begin SH_CP <= 1'b1; end
5'd10:begin SH_CP <= 1'b0; DS <= r_data[10];end
5'd11:begin SH_CP <= 1'b1; end
5'd12:begin SH_CP <= 1'b0; DS <= r_data[9];end
5'd13:begin SH_CP <= 1'b1; end
5'd14:begin SH_CP <= 1'b0; DS <= r_data[8];end
5'd15:begin SH_CP <= 1'b1; end
5'd16:begin SH_CP <= 1'b0; DS <= r_data[7];end
5'd17:begin SH_CP <= 1'b1; end
5'd18:begin SH_CP <= 1'b0; DS <= r_data[6];end
5'd19:begin SH_CP <= 1'b1; end
5'd20:begin SH_CP <= 1'b0; DS <= r_data[5];end
5'd21:begin SH_CP <= 1'b1; end
5'd22:begin SH_CP <= 1'b0; DS <= r_data[4];end
5'd23:begin SH_CP <= 1'b1; end
5'd24:begin SH_CP <= 1'b0; DS <= r_data[3];end
5'd25:begin SH_CP <= 1'b1; end
5'd26:begin SH_CP <= 1'b0; DS <= r_data[2];end
5'd27:begin SH_CP <= 1'b1; end
5'd28:begin SH_CP <= 1'b0; DS <= r_data[1];end
5'd29:begin SH_CP <= 1'b1; end
5'd30:begin SH_CP <= 1'b0; DS <= r_data[0];end
5'd31:begin SH_CP <= 1'b1; end
endcase
end
endmodule
testbench
testbench代碼如下:
`timescale 1ns/1ns
`define clk_period 20
module HC595_Driver_tb;
reg Clk;
reg Rst_n;
reg [15 : 0] Data; //data to send
reg S_EN; //send en
wire SH_CP; //shift clock
wire ST_CP; //latch data clock
wire DS; //shift serial data
HC595_Driver HC595_Driver(
.Clk(Clk),
.Rst_n(Rst_n),
.Data(Data),
.S_EN(S_EN),
.SH_CP(SH_CP),
.ST_CP(ST_CP),
.DS(DS)
);
initial Clk = 1;
always#(`clk_period/2) Clk = ~Clk;
initial begin
Rst_n = 1'b0;
S_EN = 1;
Data = 16'b1010_1111_0110_0101;
#(`clk_period*20);
Rst_n = 1;
#(`clk_period*20);
#5000;
Data = 16'b0101_0101_1010_0101;
#5000;
$stop;
end
endmodule
仿真結果
爲了只測試74HC595_driver.v,在quartus軟件中將74HC595_driver.v設置爲工程頂層,將74HC595_driver_tb.v設置爲testbench文件,仿真結果如下:
分析
- 所有的代碼生成的電路都運行在全局時鐘clk之下,沒有用到門控時鐘。
- 當
CNT_MAX = 4
時,在全局時鐘下,每當divide_cnt == CNT_MAX
時,divide_cnt被清零,同時sck_pluse產生一個時鐘週期的脈衝信號,該脈衝信號時隔五個時鐘週期,如圖: - 在全局時鐘的大前提下,每產生一個sck_pluse的脈衝,
SHCP_EDGE_CNT
會進行自加1操作(除非記滿清零),代碼爲
always@(posedge Clk or negedge Rst_n)
if(!Rst_n)
SHCP_EDGE_CNT <= 5'd0;
else if(sck_pluse)begin
if(SHCP_EDGE_CNT == 5'd31)
SHCP_EDGE_CNT <= 5'd0;
else
SHCP_EDGE_CNT <= SHCP_EDGE_CNT + 1'd1;
end
else
SHCP_EDGE_CNT <= SHCP_EDGE_CNT;
由此可知,在全局時鐘的條件下,每五個clk週期產生一個sck_pluse的脈衝信號,相應地,每隔五個clk週期SHCP_EDGE_CNT信號加1。
- 而對於SH_CP信號來說,由於SHCP信號在case語句中產生,代碼爲:
always@(posedge Clk or negedge Rst_n)
...
...
case(SHCP_EDGE_CNT)
5'd0:begin SH_CP <= 1'b0; ST_CP <= 1'b1; DS <= r_data[15]; end
5'd1:begin SH_CP <= 1'b1; ST_CP <= 1'b0;end
5'd2:begin SH_CP <= 1'b0; DS <= r_data[14];end
5'd3:begin SH_CP <= 1'b1; end
5'd4:begin SH_CP <= 1'b0; DS <= r_data[13];end
......
endcase
end
end
當SHCP_EDGE_CNT == 5'd0;
,SH_CP信號爲0;
當SHCP_EDGE_CNT == 5'd1;
,SH_CP信號爲1;
當SHCP_EDGE_CNT == 5'd2;
,SH_CP信號爲0;
當SHCP_EDGE_CNT == 5'd3;
,SH_CP信號爲1;
…
即信號SHCP_EDGE_CNT
每五個時鐘週期變化一次,
當其爲偶數時,SH_CP爲0;
當其爲奇數是,SH_CP爲1;
故SH_CP則以每10個clk週期爲一個SH_CP的時鐘週期,巧妙地實現了clk信號的十分頻。。
- 對於ST_CP信號,由於在case語句中只有當
SHCP_EDGE_CNT == 5'd0;
時纔會爲高電平,其餘時候都爲低電平,同時高電平也正好在SH_CP信號的低電平時產生。而且此case語句中,正好是傳輸完16個數據之後,ST_CP對這16個數據進行了一次採集,實在巧妙。
改進
由於SH_CP信號是clk信號的十分頻,究其原因是CNT_MAX = 4
,此種方式欠妥,應該改爲CNT_MAX = 10
,且divider_cnt清零的條件爲(divider_cnt == CNT_MAX/2 - 1)
,此方式爲最佳。同時sck_pluse的賦值條件也應爲sck_pluse = (divider_cnt == CNT_MAX/2 - 1)
。