Interfacing a MAX6900 RTC with an 8051-Type Microcontroller
Abstract: This app note provides example hardware and software for interfacing the MAX6900 with an 8051-type microcontroller.
Description
This app note shows how to interface a MAX6900 I²C-Compatible RTC (real-time clock) to an 8051-type microcontroller (µC) and provides example code showing basic interface routines. The microcontroller used in this example is the DS2250, and the software is written in C.
Operation
The program uses two general-purpose port pins on the micro to operate as a master on the I²C bus. The MAX6900 operates as a slave device on the same bus.
A schematic of the circuit is shown in Figure 1. The software is shown in Figure 2.
/***************************************************************************/
/* DEMO6900.c */
/***************************************************************************/
/* This program is for example only and is not supported by Dallas Maxim */
#include <stdio.h> /* Prototypes for I/O functions */
#include <DS5000.h> /* 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 Address\nADDRESS (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 %s\n", __DATE__);
printf("CI Clock Init\n");
printf("CR Clock Read BW Byte Write\n");
printf("RR RAM Read RW RAM Write\n");
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;
}
}
}
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