Interfacing a MAX6902 RTC With an 8051-Type Microcontroller
Abstract: This app note provides example hardware and software for interfacing the MAX6902 with an 8051-type microcontroller.
MAX6902 Pin Assignment
Description
This app note demonstrates how to interface a MAX6902 real-time clock to an 8051-type microcontroller 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 four general-purpose port pins on the microcontroller to control the SPI bus. The microcontroller initiates a data transfer by sending a command/address byte to the MAX6902. The microcontroller then sends additional data and/or SCLKs to the MAX6902, which transmits or receives data based upon the command byte.
A schematic of the circuit is shown in Figure 1. The software is shown in Figure 2.
/***************************************************************************/
/* DEMO6902.c This program is for example only and is not supported by */
/* Dallas Semiconductor Maxim */
/***************************************************************************/
#include /* Prototypes for I/O functions */
#include /* Register declarations for DS5000 */
/***************************** Defines ********************************/
sbit CS = P0^0;
sbit SCLK = P0^1;
sbit DIN = P0^2;
sbit DOUT = P0^3;
/*********************** Function Prototypes **************************/
void writebyte();
void initialize();
void disp_clk_regs();
void burstramread();
void burstramwrt();
/************************* Global Variables ***************************/
uchar cy, yr, mn, dt, dy, hr, min, sec, msec, CPOL = 1;
void set_spi() /* ----- enable DUT with SCLK preset based upon CPOL ----- */
{
if(CPOL)
{
CS = 1; /* make sure CS is high */
SCLK = 1; /* set SCLK for CPOL=1 */
CS = 0; /* enable DUT */
}
else
{
CS = 1; /* make sure CS is high */
SCLK = 0; /* set SCLK for CPOL=0 */
CS = 0; /* enable DUT */
}
}
void reset_spi() /* ----- reset DUT using SPI protocol ----- */
{
if(CPOL)
{
CS = 1;
SCLK = 1;
}
else
{
CS = 1;
SCLK = 0;
}
}
void wbyte_spi(uchar W_Byte) /* ----- write one byte to DUT ----- */
{
uchar i;
CS = 0;
if(CPOL)
{
for(i = 0; i < 8; ++i)
{
DIN = 0;
if(W_Byte & 0x80)
{
DIN = 1;
}
SCLK = 0;
SCLK = 1;
W_Byte <<= 1;
}
}
else
{
for(i = 0; i < 8; ++i)
{
DIN = 0;
if(W_Byte & 0x80)
{
DIN = 1;
}
SCLK = 1;
SCLK = 0;
W_Byte <<= 1;
}
}
}
uchar rbyte_spi(void) /* ----- read one byte from DUT ----- */
{
uchar i;
uchar R_Byte;
uchar TmpByte;
R_Byte = 0x00;
DOUT = 1; /* set up port for read */
CS = 0;
if(CPOL)
{
for(i=0; i<8; ++i)
{
SCLK = 0;
TmpByte = (uchar)DOUT;
SCLK = 1;
R_Byte <<= 1;
R_Byte |= TmpByte;
}
}
else
{
for(i=0; i<8; ++i)
{
SCLK = 1;
TmpByte = (uchar)DOUT;
SCLK = 0;
R_Byte <<= 1;
R_Byte |= TmpByte;
}
}
return R_Byte;
}
void writebyte() /* ----- write one byte, prompt for address and data ------ */
{
uchar add;
uchar dat;
/* Get Address & Data */
printf("
Enter the Read Address
ADDRESS (1,2,3...):");
scanf("%bx", &add);
printf("
DATA (0-ff):");
scanf("%bx", &dat);
set_spi();
wbyte_spi(add);
wbyte_spi(dat);
reset_spi();
}
void initialize() /* ----- init clock data using user entries ----- */
/* Note: NO error checking is done on the user entries! */
{
set_spi();
wbyte_spi(0x0f); /* control register write address */
wbyte_spi(0x00); /* clear write protect */
reset_spi();
printf("
Enter 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);
set_spi();
wbyte_spi(0x3f); /* clock burst write */
wbyte_spi(sec);
wbyte_spi(min);
wbyte_spi(hr);
wbyte_spi(dt);
wbyte_spi(mn);
wbyte_spi(dy);
wbyte_spi(yr);
wbyte_spi(0); /* control */
reset_spi();
set_spi();
wbyte_spi(0x13);
wbyte_spi(0x20); /* century data */
reset_spi();
}
void disp_clk_regs() /* --- loop reading clock, display when secs change --- */
{
uchar mil, pm, prv_sec = 99;
while(!RI) /* Read & Display Clock Registers */
{
set_spi();
wbyte_spi(0xbf); /* clock burst read */
sec = rbyte_spi();
min = rbyte_spi();
hr = rbyte_spi();
dt = rbyte_spi();
mn = rbyte_spi();
dy = rbyte_spi();
yr = rbyte_spi();
cy = rbyte_spi(); /* dummy read of control register */
reset_spi();
set_spi();
wbyte_spi(0x93); /* century byte read address */
cy = rbyte_spi();
reset_spi();
if(hr & 0x80)
mil = 0;
else
mil = 1;
if(sec != prv_sec) /* display every time seconds change */
{
if(mil)
{
printf("
%02bX%02bX/%02bX/%02bX %01bX", cy, yr, mn, dt, dy);
printf(" %02bX:%02bX:%02bX", hr, min, sec);
}
else
{
if(hr & 0x20)
pm = 'P';
else
pm = 'A';
hr &= 0x1f; /* strip mode and am/pm bits */
printf("
%02bx%02bx/%02bx/%02bx %02bx", cy, yr, (mn & 0x1f), dt, dy);
printf(" %02bx:%02bx:%02bx %cM", hr, min, sec, pm);
}
}
prv_sec = sec;
}
RI = 0; /* Swallow keypress to exit loop */
}
void burstramread() /* ------ read RAM using burst mode ----- */
{
uchar k;
printf("
MAX6901 RAM contents:
");
set_spi();
wbyte_spi(0xff); /* ram burst read */
for (k = 0; k < 31; k++)
{
if(!(k % 8) ) printf("
");
printf("%02.bX ", rbyte_spi() );
}
reset_spi();
}
void burstramwrt(uchar Data) /* ------ write RAM using burst mode ------- */
{
uchar k;
set_spi();
wbyte_spi(0x7f); /* ram burst write */
for (k=0; k < 31; k++)
{
wbyte_spi(Data);
}
reset_spi();
}
main (void) /* ----------------------------------------------------- */
{
uchar i, M, M1;
while (1)
{
printf("
MAX6902 build %s
", __DATE__);
printf("CI. Initialize MAX6902
");
printf("CW. Write Byte
");
printf("CR. Read Time
");
printf("RW. Write RAM
");
printf("RR. Read RAM
");
printf("Enter Menu Selection: ");
M = _getkey();
switch(M)
{
case 'C':
case 'c':
printf("\rEnter Clock Routine to run:C");
M1 = _getkey();
switch(M1)
{
case 'I':
case 'i': initialize();
break;
case 'R':
case 'r': disp_clk_regs();
break;
case 'W':
case 'w': writebyte();
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("
Enter the data to write: ");
scanf("%bx", &i);
burstramwrt(i); break;
}
break;
}
}
}
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