pi i2c 读取24c256

树霉pi ioctl i2c读取 24c256，其它pi也行

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#define ADDRESS 0x50
#define WRITECYCLE 5  //ms
#define PORT 1
int fd;

int iicwrite(char* buf, int addr, int start, int len)
{
        struct i2c_msg msgs ;
        char* buf0= malloc(len+2);
        msgs.addr = addr;
        msgs.flags = 0;
        msgs.len = len+2;
        buf0[0] = (start>>8)&0xff;
        buf0[1] = start&0xff;
        memcpy(buf0+2,buf,len);
        msgs.buf = buf0;
        struct i2c_rdwr_ioctl_data rdwr;
        rdwr.msgs = &msgs;
        rdwr.nmsgs = 1;
        int res = ioctl(fd, I2C_RDWR, &rdwr);
        usleep(WRITECYCLE*1000);
        free(buf0);
        return res;
}
int iicread(char* buf,int addr, int start, int len)
{
        struct i2c_msg msgs[2] ;
        char buf0[2];
        msgs[0].addr = addr;
        msgs[0].flags = 0;
        msgs[0].len = 2;
        buf0[0] = (start>>8)&0xff;
        buf0[1] = start&0xff;
        msgs[0].buf = buf0;

        msgs[1].addr = addr;
        msgs[1].flags = 1; // read
        msgs[1].len = len;
        msgs[1].buf = buf;

        struct i2c_rdwr_ioctl_data rdwr;
        rdwr.msgs = msgs;
        rdwr.nmsgs = 2;
        int res = ioctl(fd, I2C_RDWR, &rdwr);
        return res;
}

int setup(int port, int addr) {
        char fname[16];

        snprintf(fname,sizeof(fname),"/dev/i2c-%d",port);
        if((fd=open(fname, O_RDWR))<0)
                exit(1);
        int res = ioctl(fd,I2C_SLAVE_FORCE,addr);
        return res;
}
int main(){
        int plen = 94; //printable ascii
        char astr[plen+1];
        unsigned char  i;
        for( i = 0; i < 95; i++)
                astr[i] = i+' ';
        int addr = ADDRESS;
        setup(PORT,addr);
        iicwrite( astr, addr, 0, plen);
        memset(astr,0,plen);
        iicread( astr, addr, 0, plen);
        astr[plen+1] = 0;
        printf("%s\n",astr+1);
        fclose(fd);
}

PORT 端口号 /dev/i2c-1 默认1
24c256 write cycle time 5 ms
a0 a1 a2 接地 地址 0x50
两个问题
一 i2c 不通优先检查杜邦线，浪费很长时间，才发现是线的事。
二 at24c256 write cycle time max 5毫秒， 写后延时5ms 再读。