协议——SCCB与IIC的区别
SCCB（Serial Camera Control Bus，串行摄像头控制总线）是由OV（OmniVision的简称）公司定义和发展的三线式串行总线，该总线控制着摄像头大部分的功能，包括图像数据格式、分辨率以及图像处理参数等。结构框图如下所示：
OV公司为了减少传感器引脚的封装，现在SCCB总线大多采用两线式接口总线。OV7725使用的是两线式接口总线，该接口总线包括SIO_C串行时钟输入线和SIO_D串行双向数据线，分别相当于IIC协议的SCL信号线和SDA信号线。SIO_C的最小时间为10us，即最大频率为100K。一般来说，100K-400K之间都可以。
由此可见，SCCB就是改编版的IIC，完全可以按照IIC来理解，下面仔细讲解SCCB的时序以及和IIC的不同之处。
一、SCCB起始和结束（与IIC完全一致）
起始：SIO_C为高时，SIO_D由高拉低。
停止：SIO_C为高时，SIO_D由低拉高。
二、SCCB写（与IIC完全一致）
ID Address(W)里面就已经包括进了IIC中的“读写控制位”，所以没有额外写出。
即：start + phase_1 + phase_2 + phase_3 + stop
“X”的意思是“don't care”，该位是由从机发出应答信号来响应主机表示当前ID Address、Sub-address和Write Data是否传输完成，但是从机有可能不发出应答信号，因此主机（此处指FPGA）可不用判断此处是否有应答，直接默认当前传输完成即可。“X”即IIC中的ACK应答位。
三、SCCB读
数据手册中的SCCB读只写了上图的Phase3和Phase4，实际上它是和Phase1和Phase2联系在一起的。SCCB不支持连续读，Phase4的主机应答位必须为NA(no ack)，即为1，所以SCCB读其实就专指单次读，和IIC单次读几乎一样。
区别就一点：在IIC读传输协议中，写完寄存器地址后会有restart即重复开始的操作；而SCCB读传输协议中没有重复开始的概念，在写完寄存器地址后，需发起总线停止信号。
即：start_1 + phase_1 + phase_2 + stop_1 + start_2 + phase_3 + phase_4 + stop_2
四、SCCB和IIC的区别
1.SCCB的应答位称为X，表示“don't care”，而IIC应答位称为ACK。
2.SCCB只能单次读，而IIC除了单次读还支持连续读。
3.SCCB读操作中间有stop，而IIC读操作中间可以有stop也可以不需要stop，具体表现如下
SCCB读：start_1 + phase_1 + phase_2 + stop_1 + start_2 + phase_3 + phase_4 + stop_2 IIC读：start_1 + phase_1 + phase_2 + + start_2 + phase_3 + phase_4 + stop_2
除去上面三点，SCCB和IIC再无区别，因此如果只需要配置寄存器（只用到写），可以直接拿IIC的时序来当做SCCB用，如果需要读，读操作中间必须有一个stop。
五、SCCB控制器Verilog代码
1 //************************************************************************** 2 // *** 名称 : sccb.v 3 // *** 作者 : xianyu_FPGA 4 // *** 博客 : https://www.cnblogs.com/xianyufpga/ 5 // *** 日期 : 2019-08-10 6 // *** 描述 : SCCB控制器，只支持写 7 //************************************************************************** 8 9 module sccb 10 //========================< 参数 >========================================== 11 #( 12 parameter DEVICE_ID = 8'b01010000 , //器件ID 13 parameter CLK = 26'd50_000_000 , //本模块的时钟频率 14 parameter SCL = 18'd250_000 //输出的SCL时钟频率 15 ) 16 //========================< 端口 >========================================== 17 ( 18 input clk , //时钟 19 input rst_n , //复位，低电平有效 20 //SCCB control -------------------------------------- 21 input sccb_en , //SCCB触发信号 22 input addr16_en , //16位地址使能 23 input addr8_en , //8位地址使能 24 //SCCB input ---------------------------------------- 25 input [15:0] sccb_addr , //SCCB器件内地址 26 input [ 7:0] sccb_data , //SCCB要写的数据 27 //SCCB output --------------------------------------- 28 output reg sccb_done , //SCCB一次操作完成 29 output reg sccb_scl , //SCCB的SCL时钟信号 30 inout sccb_sda , //SCCB的SDA数据信号 31 //dri_clk ------------------------------------------- 32 output reg sccb_dri_clk //驱动SCCB操作的驱动时钟，1Mhz 33 ); 34 //========================< 状态机参数 >==================================== 35 localparam IDLE = 6'b00_0001 ; //空闲状态 36 localparam DEVICE = 6'b00_0010 ; //写器件地址 37 localparam ADDR_16 = 6'b00_0100 ; //写字地址高8位 38 localparam ADDR_8 = 6'b00_1000 ; //写字地址低8位 39 localparam DATA = 6'b01_0000 ; //写数据 40 localparam STOP = 6'b10_0000 ; //结束 41 //========================< 信号 >========================================== 42 reg sda_dir ; //SCCB数据(SDA)方向控制 43 reg sda_out ; //SDA输出信号 44 reg state_done ; //状态结束 45 reg [ 6:0] cnt ; //计数 46 reg [ 7:0] state_c ; //状态机当前状态 47 reg [ 7:0] state_n ; //状态机下一状态 48 reg [15:0] sccb_addr_t ; //地址寄存 49 reg [ 7:0] sccb_data_t ; //数据寄存 50 reg [ 9:0] clk_cnt ; //分频时钟计数 51 wire [ 8:0] clk_divide ; //模块驱动时钟的分频系数 52 53 //========================================================================== 54 //== sda控制 55 //========================================================================== 56 assign sccb_sda = sda_dir ? sda_out : 1'bz; //SDA数据输出或高阻 57 58 //========================================================================== 59 //== 生成SCL的4倍时钟来驱动后面SCCB的操作，生成1Mhz的sccb_dri_clk 60 //========================================================================== 61 assign clk_divide = (CLK/SCL) >> 3; // >>3即除以8 62 63 always @(posedge clk or negedge rst_n) begin 64 if(!rst_n) begin 65 sccb_dri_clk <= 1'b1; 66 clk_cnt <= 10'd0; 67 end 68 else if(clk_cnt == clk_divide - 1'd1) begin 69 clk_cnt <= 10'd0; 70 sccb_dri_clk <= ~sccb_dri_clk; 71 end 72 else 73 clk_cnt <= clk_cnt + 1'b1; 74 end 75 76 //========================================================================== 77 //== 状态机 78 //========================================================================== 79 always @(posedge sccb_dri_clk or negedge rst_n) begin 80 if(!rst_n) 81 state_c <= IDLE; 82 else 83 state_c <= state_n; 84 end 85 86 always @(*) begin 87 case(state_c) 88 IDLE: begin //空闲状态 89 if(sccb_en) 90 state_n = DEVICE; 91 else 92 state_n = IDLE; 93 end 94 DEVICE: begin //写器件ID 95 if(state_done) begin 96 if(addr16_en) 97 state_n = ADDR_16; 98 else if(addr8_en) 99 state_n = ADDR_8 ; 100 end 101 else 102 state_n = DEVICE; 103 end 104 ADDR_16: begin //写地址高8位 105 if(state_done) 106 state_n = ADDR_8; 107 else 108 state_n = ADDR_16; 109 end 110 ADDR_8: begin //写地址低8位 111 if(state_done) 112 state_n = DATA; 113 else 114 state_n = ADDR_8; 115 end 116 DATA: begin //写数据 117 if(state_done) 118 state_n = STOP; 119 else 120 state_n = DATA; 121 end 122 STOP: begin //结束 123 if(state_done) 124 state_n = IDLE; 125 else 126 state_n = STOP ; 127 end 128 default:state_n= IDLE; 129 endcase 130 end 131 132 //========================================================================== 133 //== 设计各路信号 134 //========================================================================== 135 always @(posedge sccb_dri_clk or negedge rst_n) begin 136 if(!rst_n) begin 137 sccb_scl <= 1'b1; 138 sda_out <= 1'b1; 139 sda_dir <= 1'b1; 140 sccb_done <= 1'b0; 141 cnt <= 1'b0; 142 state_done <= 1'b0; 143 sccb_addr_t <= 1'b0; 144 sccb_data_t <= 1'b0; 145 end 146 else begin 147 state_done <= 1'b0 ; 148 cnt <= cnt + 1'b1 ; 149 case(state_c) 150 //--------------------------------------------------- 空闲状态 151 IDLE: begin 152 sccb_scl <= 1'b1; 153 sda_out <= 1'b1; 154 sda_dir <= 1'b1; 155 sccb_done <= 1'b0; 156 cnt <= 7'b0; 157 if(sccb_en) begin 158 sccb_addr_t <= sccb_addr; 159 sccb_data_t <= sccb_data; 160 end 161 end 162 //--------------------------------------------------- 写器件ID 163 DEVICE: begin 164 case(cnt) 165 7'd1 : sda_out <= 1'b0; 166 7'd3 : sccb_scl <= 1'b0; 167 7'd4 : sda_out <= DEVICE_ID[7]; 168 7'd5 : sccb_scl <= 1'b1; 169 7'd7 : sccb_scl <= 1'b0; 170 7'd8 : sda_out <= DEVICE_ID[6]; 171 7'd9 : sccb_scl <= 1'b1; 172 7'd11: sccb_scl <= 1'b0; 173 7'd12: sda_out <= DEVICE_ID[5]; 174 7'd13: sccb_scl <= 1'b1; 175 7'd15: sccb_scl <= 1'b0; 176 7'd16: sda_out <= DEVICE_ID[4]; 177 7'd17: sccb_scl <= 1'b1; 178 7'd19: sccb_scl <= 1'b0; 179 7'd20: sda_out <= DEVICE_ID[3]; 180 7'd21: sccb_scl <= 1'b1; 181 7'd23: sccb_scl <= 1'b0; 182 7'd24: sda_out <= DEVICE_ID[2]; 183 7'd25: sccb_scl <= 1'b1; 184 7'd27: sccb_scl <= 1'b0; 185 7'd28: sda_out <= DEVICE_ID[1]; 186 7'd29: sccb_scl <= 1'b1; 187 7'd31: sccb_scl <= 1'b0; 188 7'd32: sda_out <= DEVICE_ID[0]; 189 7'd33: sccb_scl <= 1'b1; 190 7'd35: sccb_scl <= 1'b0; 191 7'd36: begin 192 sda_dir <= 1'b0; //从机应答 193 sda_out <= 1'b1; 194 end 195 7'd37: sccb_scl <= 1'b1; 196 7'd38: state_done <= 1'b1; //状态结束 197 7'd39: begin 198 sccb_scl <= 1'b0; 199 cnt <= 1'b0; 200 end 201 default : ; 202 endcase 203 end 204 //--------------------------------------------------- 写字地址高8位 205 ADDR_16: begin 206 case(cnt) 207 7'd0 : begin 208 sda_dir <= 1'b1 ; 209 sda_out <= sccb_addr_t[15]; 210 end 211 7'd1 : sccb_scl <= 1'b1; 212 7'd3 : sccb_scl <= 1'b0; 213 7'd4 : sda_out <= sccb_addr_t[14]; 214 7'd5 : sccb_scl <= 1'b1; 215 7'd7 : sccb_scl <= 1'b0; 216 7'd8 : sda_out <= sccb_addr_t[13]; 217 7'd9 : sccb_scl <= 1'b1; 218 7'd11: sccb_scl <= 1'b0; 219 7'd12: sda_out <= sccb_addr_t[12]; 220 7'd13: sccb_scl <= 1'b1; 221 7'd15: sccb_scl <= 1'b0; 222 7'd16: sda_out <= sccb_addr_t[11]; 223 7'd17: sccb_scl <= 1'b1; 224 7'd19: sccb_scl <= 1'b0; 225 7'd20: sda_out <= sccb_addr_t[10]; 226 7'd21: sccb_scl <= 1'b1; 227 7'd23: sccb_scl <= 1'b0; 228 7'd24: sda_out <= sccb_addr_t[9]; 229 7'd25: sccb_scl <= 1'b1; 230 7'd27: sccb_scl <= 1'b0; 231 7'd28: sda_out <= sccb_addr_t[8]; 232 7'd29: sccb_scl <= 1'b1; 233 7'd31: sccb_scl <= 1'b0; 234 7'd32: begin 235 sda_dir <= 1'b0; //从机应答 236 sda_out <= 1'b1; 237 end 238 7'd33: sccb_scl <= 1'b1; 239 7'd34: state_done <= 1'b1; //状态结束 240 7'd35: begin 241 sccb_scl <= 1'b0; 242 cnt <= 1'b0; 243 end 244 default : ; 245 endcase 246 end 247 //--------------------------------------------------- 写字地址低8位 248 ADDR_8: begin 249 case(cnt) 250 7'd0: begin 251 sda_dir <= 1'b1 ; 252 sda_out <= sccb_addr_t[7]; 253 end 254 7'd1 : sccb_scl <= 1'b1; 255 7'd3 : sccb_scl <= 1'b0; 256 7'd4 : sda_out <= sccb_addr_t[6]; 257 7'd5 : sccb_scl <= 1'b1; 258 7'd7 : sccb_scl <= 1'b0; 259 7'd8 : sda_out <= sccb_addr_t[5]; 260 7'd9 : sccb_scl <= 1'b1; 261 7'd11: sccb_scl <= 1'b0; 262 7'd12: sda_out <= sccb_addr_t[4]; 263 7'd13: sccb_scl <= 1'b1; 264 7'd15: sccb_scl <= 1'b0; 265 7'd16: sda_out <= sccb_addr_t[3]; 266 7'd17: sccb_scl <= 1'b1; 267 7'd19: sccb_scl <= 1'b0; 268 7'd20: sda_out <= sccb_addr_t[2]; 269 7'd21: sccb_scl <= 1'b1; 270 7'd23: sccb_scl <= 1'b0; 271 7'd24: sda_out <= sccb_addr_t[1]; 272 7'd25: sccb_scl <= 1'b1; 273 7'd27: sccb_scl <= 1'b0; 274 7'd28: sda_out <= sccb_addr_t[0]; 275 7'd29: sccb_scl <= 1'b1; 276 7'd31: sccb_scl <= 1'b0; 277 7'd32: begin 278 sda_dir <= 1'b0; //从机应答 279 sda_out <= 1'b1; 280 end 281 7'd33: sccb_scl <= 1'b1; 282 7'd34: state_done <= 1'b1; //状态结束 283 7'd35: begin 284 sccb_scl <= 1'b0; 285 cnt <= 1'b0; 286 end 287 default : ; 288 endcase 289 end 290 //--------------------------------------------------- 写数据 291 DATA: begin 292 case(cnt) 293 7'd0: begin 294 sda_out <= sccb_data_t[7]; 295 sda_dir <= 1'b1; 296 end 297 7'd1 : sccb_scl <= 1'b1; 298 7'd3 : sccb_scl <= 1'b0; 299 7'd4 : sda_out <= sccb_data_t[6]; 300 7'd5 : sccb_scl <= 1'b1; 301 7'd7 : sccb_scl <= 1'b0; 302 7'd8 : sda_out <= sccb_data_t[5]; 303 7'd9 : sccb_scl <= 1'b1; 304 7'd11: sccb_scl <= 1'b0; 305 7'd12: sda_out <= sccb_data_t[4]; 306 7'd13: sccb_scl <= 1'b1; 307 7'd15: sccb_scl <= 1'b0; 308 7'd16: sda_out <= sccb_data_t[3]; 309 7'd17: sccb_scl <= 1'b1; 310 7'd19: sccb_scl <= 1'b0; 311 7'd20: sda_out <= sccb_data_t[2]; 312 7'd21: sccb_scl <= 1'b1; 313 7'd23: sccb_scl <= 1'b0; 314 7'd24: sda_out <= sccb_data_t[1]; 315 7'd25: sccb_scl <= 1'b1; 316 7'd27: sccb_scl <= 1'b0; 317 7'd28: sda_out <= sccb_data_t[0]; 318 7'd29: sccb_scl <= 1'b1; 319 7'd31: sccb_scl <= 1'b0; 320 7'd32: begin 321 sda_dir <= 1'b0; //从机应答 322 sda_out <= 1'b1; 323 end 324 7'd33: sccb_scl <= 1'b1; 325 7'd34: state_done <= 1'b1; //状态结束 326 7'd35: begin 327 sccb_scl <= 1'b0; 328 cnt <= 1'b0; 329 end 330 default : ; 331 endcase 332 end 333 //--------------------------------------------------- 结束 334 STOP: begin 335 case(cnt) 336 7'd0: begin 337 sda_dir <= 1'b1; 338 sda_out <= 1'b0; 339 end 340 7'd1 : sccb_scl <= 1'b1; 341 7'd3 : sda_out <= 1'b1; 342 7'd15: state_done <= 1'b1; //状态结束 343 7'd16: begin 344 cnt <= 1'b0; 345 sccb_done <= 1'b1; //sccb配置完成 346 end 347 default : ; 348 endcase 349 end 350 endcase 351 end 352 end 353 354 355 356 endmodule
参考资料：
[1]OmniVision Serial Camera Control Bus (SCCB) Functional Specification
[2]正点原子FPGA教程
[3]开源骚客.SDRAM那些事儿
