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本应用笔迹介绍了MAX6900 I²C兼容RTC (实时时钟)与8051微控制器(µC)之间的连接方式，并提供了用于基本接口的程序代码。本范例中所使用的微控制器是DS2250, 软件用C语言编写。
操作过程
本程序利用作为I²C总线主机的微控制器的两个通用端口进行控制，MAX6900则作为同一总线上的从机器件。
电路原理图如图1所示，程序清单如图2所示。
图1. 子卡原理图
图2. 程序清单
/***************************************************************************/
/* DEMO6900.c */
/***************************************************************************/
/* This program is for example only and is not supported by Dallas Maxim */
#include /* Prototypes for I/O functions */
#include /* Register declarations for DS5000 */
#define ACK 0
#define NACK 1
#define ADD6900 0xa0 /* 2-wire addresses */
sbit scl = P0^0; /* 2-wire pin definitions */
sbit sda = P0^1;
void I2Cstart();
void I2Cstop();
uchar I2Cwrite(uchar);
unsigned char I2Cread(int);
void writebyte6900();
void Initialize_MAX6900();
void disp_clk_regs();
void burstramread();
void burstramwrt();
/* global variables */
void I2Cstart() /* ----------------------------------------------- */
{
sda = 1; /* Initiate start condition */
scl = 1;
sda = 0;
}
void I2Cstop() /* ----------------------------------------------- */
{
sda = 0; sda = 0; /* Initiate stop condition */
scl = 1; scl = 1;
sda = 1;
}
uchar I2Cwrite(uchar d) /* ----------------------------- */
{
int i;
scl = 0;
for (i = 0;i < 8; i++)
{
if (d & 0x80)
sda = 1; /* Send the msbits first */
else
sda = 0;
scl = 0;
scl = 1;
d = d < 1; /* add to scl high time */
scl = 0;
}
sda = 1; /* Release the sda line */
scl = 0;
scl = 1;
i = sda;
scl = 0;
if (i)
{
puts("Ack missing");
}
return(i);
}
uchar I2Cread(int b) /* ----------------------------------- */
{
uchar i, d;
d = 0;
sda = 1; /* Let go of sda line */
for (i=1; i<=8; i++) /* read the msb first */
{
scl = 0;
scl = 1;
d = d << 1;
d = d | (unsigned char)sda;
}
scl = 0;
sda = b; /* Hold sda low for ACK, high for NACK */
scl = 0; /* toggle clock */
scl = 1;
scl = 0;
sda = 1; /* Release the sda line */
return d;
}
void writebyte6900() /* ----- write a single byte; user enters read address ----- */
{
uchar add;
uchar dat;
/* Get Address & Data */
printf("nEnter the Read AddressnADDRESS (80,82,84...FC): ");
scanf("%bx", &add);
printf("DATA (0-ff):");
scanf("%bx", &dat);
I2Cstart();
I2Cwrite(ADD6900); /* slave address + write */
I2Cwrite(add);
I2Cwrite(dat);
I2Cstop();
}
void Initialize_MAX6900() /* ------- initialize from stdio entries ------- */
/* Note: NO error checking is done on the user entries! */
{
uchar yr, mn, dt, dy, hr, min, sec, day;
I2Cstart();
I2Cwrite(ADD6900); /* slave address + write */
I2Cwrite(0x8e); /* control register write address */
I2Cwrite(0x00); /* clear write protect */
I2Cstop();
printf("nEnter the year (0-99): ");
scanf("%bx", &yr);
printf("Enter the month (1-12): ");
scanf("%bx", &mn);
printf("Enter the date (1-31): ");
scanf("%bx", &dt);
printf("Enter the day (1-7): ");
scanf("%bx", &dy);
printf("Enter the hour (1-23): ");
scanf("%bx", &hr);
hr = hr & 0x3f; /* force clock to 24 hour mode */
printf("Enter the minute (0-59): ");
scanf("%bx", &min);
printf("Enter the second (0-59): ");
scanf("%bx", &sec);
I2Cstart();
I2Cwrite(ADD6900); /* slave address + write */
I2Cwrite(0xbe); /* clock burst write */
I2Cwrite(sec);
I2Cwrite(min);
I2Cwrite(hr);
I2Cwrite(dt);
I2Cwrite(mn);
I2Cwrite(dy);
I2Cwrite(yr);
I2Cwrite(0); /* control */
I2Cstart();
I2Cwrite(ADD6900); /* slave address + write */
I2Cwrite(0x92);
I2Cwrite(0x20); /* century data */
I2Cstop();
}
void disp_clk_regs() /* --------- display using burst mode --------- */
{
uchar Sec, prv_sec = 99, Min, Hrs, Dte, Mon, Day, Yr, cy;
while(!RI) /* Read & Display Clock Registers */
{
I2Cstart();
I2Cwrite(ADD6900); /* slave address + write */
I2Cwrite(0xbf); /* clock burst read */
I2Cstart();
I2Cwrite(ADD6900 + 1); /* slave address + read */
Sec = I2Cread(ACK); /* starts w/last address stored in register pointer */
Min = I2Cread(ACK);
Hrs = I2Cread(ACK);
Dte = I2Cread(ACK);
Mon = I2Cread(ACK);
Day = I2Cread(ACK);
Yr = I2Cread(ACK);
cy = I2Cread(NACK); /* dummy read of control register */
I2Cstart();
I2Cwrite(ADD6900); /* slave address + write */
I2Cwrite(0x93); /* century byte read address */
I2Cstart();
I2Cwrite(ADD6900 + 1); /* slave address + read */
cy = I2Cread(NACK);
I2Cstop();
if(Sec != prv_sec) /* display every time seconds change */
{
printf("n%02bX%02bX/%02bX/%02bX %01bX", cy, Yr, Mon, Dte, Day);
printf(" %02bX:%02bX:%02bX", Hrs, Min, Sec);
}
prv_sec = Sec;
}
RI = 0; /* Swallow keypress to exit loop */
}
void burstramread() /* ----------------------------------------- */
{
uchar j, k;
I2Cstart();
I2Cwrite(ADD6900); /* write slave address, write 6900 */
I2Cwrite(0xff); /* ram burst read */
I2Cstart();
I2Cwrite(ADD6900 + 1); /* slave address + read */
printf("nRAM contents");
for (j=0; j<30; ++j)
{
if(!(j % 8) ) printf("n");
printf("%2.bX ", I2Cread(ACK) );
}
printf("%2bX", I2Cread(NACK) ); /* last byte, NACK */
I2Cstop();
printf("n");
}
void burstramwrt(uchar Data) /* ----------------------------------------- */
{
uchar j, k;
I2Cstart();
I2Cwrite(ADD6900); /* write slave address, write 6900 */
I2Cwrite(0xfe); /* ram burst write */
for (k=0; k < 31; ++k)
{
I2Cwrite(Data);
}
I2Cstop();
}
main (void) /* ----------------------------------------------------- */
{
uchar i, M, M1;
while (1)
{
printf("nMAX6900 build %sn", __DATE__);
printf("CI Clock Initn");
printf("CR Clock Read BW Byte Writen");
printf("RR RAM Read RW RAM Writen");
printf("Enter Menu Selection: ");
M = _getkey();
switch(M)
{
case 'B':
case 'b':
printf("rByte: B");
M1 = _getkey();
switch(M1)
{
case 'W':
case 'w': writebyte6900();
break;
} break;
case 'C':
case 'c':
printf("rEnter Clock Routine to run:C");
M1 = _getkey();
switch(M1)
{
case 'I':
case 'i': Initialize_MAX6900();
break;
case 'R':
case 'r': disp_clk_regs();
break;
} break;
case 'R':
case 'r':
printf("rEnter Ram Routine to run:R");
M1 = _getkey();
switch(M1)
{
case 'R':
case 'r': burstramread();
break;
case 'W':
case 'w': printf("nEnter the data to write: ");
scanf("%bx", &i);
burstramwrt(i); break;
} break;
}
}
}