STM32F767のMbedでの時計,DS1307+で時刻表示(i2c)

#include "mbed.h"
#include <stdio.h>

//Serial pc(USBTX, USBRX); // tx, rx
Serial pc(SERIAL_TX, SERIAL_RX);

#define ADDR     (0x68<<1) //  address

I2C i2c(I2C_SDA, I2C_SCL);

int  ch_i;      //文字列のループカウンター
char ch_b[8+1]; //文字列のバッファー
int  ch_y;      //一文字分の一時バッファー

void ch_input()
{
    ch_i=0;
    while(1) {
        ch_y = pc.getc();
        if(ch_y == 13){ch_b[ch_i]=0;pc.printf("\n\r");break;}
        ch_b[ch_i]=ch_y;ch_i++;
        pc.putc(ch_y);
        if(ch_i >= 8){ch_b[ch_i]=0;pc.printf("\n\r");break;}     
    } //while
} //ch_input

char ch_hex_b[3];

char *ch_hex(int l_num)
{
    ch_hex_b[0] = '0' + (l_num >> 4);
    ch_hex_b[1] = '0' + (l_num & 0xf);
    ch_hex_b[2] = 0;
    return(ch_hex_b);
}

int h;
int m;
int a;
int b;

int p_val;
char data_read[16];
char cmd[2]={0,0};
int ii;     //ループカウンタ

int main() {

///*INIT start *********  

    pc.printf("\n\r");  //改行

    pc.printf("H 0-23>");
    ch_input();         //文字列の入力
    h=atoi(ch_b);


    pc.printf("M 0-59>");
    ch_input();         //文字列の入力
    m=atoi(ch_b);

    a=(m/10)*16+(m%10);
    b=(h/10)*16+(h%10);

    data_read[0]=0;
    data_read[1]=0;
    data_read[2]=a;
    data_read[3]=b;
    data_read[4]=3;    
    data_read[5]=1;
    data_read[6]=1;
    data_read[7]=0x20;

    p_val = i2c.write(ADDR, cmd, 1);
    p_val = i2c.write(ADDR, data_read, 8);

//*/  //INIT end *********

    while(1){

        p_val = i2c.write(ADDR, cmd, 1);
        p_val = i2c.read(ADDR | 1, data_read, 7);

        ii=2;
        while(1){
            pc.printf("%s",ch_hex(data_read[ii]));
            ii--;
            if(ii==-1){break;}else{pc.printf(":");}
        }
        pc.printf("\n\r");

        wait(1);

    } //while
}