51 MCU Introduction - SPI Bus

UART, I2C and SPI are the three most commonly used communication protocols in microcontroller systems.

1. Introduction

SPI is a high-speed, full-duplex, synchronous communication bus. The standard SPI uses only 4 pins and is commonly used for communication between microcontrollers and EEPROM, FLASH, real-time clocks, digital signal processors and other devices. The SPI communication principle is simpler than I2C. It is mainly a master-slave communication mode. This mode usually has only one host and one or more slaves. The standard SPI has 4 wires, namely SSEL (chip select, also written as SCS), SCLK (clock, also written as SCK), MOSI (master output slave input Master

Output/Slave Input) and MISO (Master Input/Slave Output).

SSEL: Slave chip select enable signal. If the slave device is low level enabled, when this pin is pulled low, the slave device will be selected, and the host will communicate with the selected slave.

SCLK: Clock signal, generated by the host, somewhat similar to the SCL of I2C communication.

MOSI: The channel through which the host sends instructions or data to the slave. MISO: The channel through which the host reads the status or data of the slave.

2. Working mode

The host of SPI communication is also our microcontroller. There are four modes in the process of reading and writing data timing;

CPOL: Clock Polarity, which is the polarity of the clock. The entire communication process is divided into idle time and communication time. If the idle state of SCLK before and after data transmission is high, then CPOL=1, if the idle state SCLK is low, then CPOL=0.

CPHA: Clock Phase, which is the phase of the clock.

#include

typedef unsigned char uchar;

sbit DS1302_CE = P1^7;

sbit DS1302_CK = P3^5;

sbit DS1302_IO = P3^4;

struct sTime //Date and time structure definition

{

  unsigned int year; //year

  unsigned char mon; //month

  unsigned char day; //day

  unsigned char hour; //hour

  unsigned char min; // points

  unsigned char sec; //seconds

  unsigned char week; //week

};

/* Send a byte to the DS1302 communication bus */

void DS1302ByteWrite(uchar dat)

{

  uchar mask;

  for (mask = 0x01; mask != 0; mask <<= 1) // Low bit first, shift out one by one

  {

    if ((mask & dat) != 0) // first output the data bit

    {

      DS1302_IO = 1;

    }

    else

    {

      DS1302_IO = 0;

    }

    DS1302_CK = 1; //Then pull the clock high

    DS1302_CK = 0; //Pull the clock down again to complete a bit operation

  }

  DS1302_IO = 1; //Finally make sure to release the IO pin

}

/* Read a byte from the DS1302 communication bus */

uchar DS1302ByteRead()

{

  uchar mask;

  uchar dat = 0;

  for (mask = 0x01; mask != 0; mask <<= 1) //low bit first, read bit by bit

  {

    if (DS1302_IO != 0) //First read the current IO pin and set the corresponding bit in dat

    {

      dat |= mask;

    }

    DS1302_CK = 1; //Then pull the clock high

    DS1302_CK = 0; //Pull the clock down again to complete a bit operation

  }

  return dat; //Finally return the read byte data

}

/* Use a single write operation to write a byte to a register, reg-register address, dat-byte to be written*/

void DS1302SingleWrite(uchar reg, uchar dat)

{

  DS1302_CE = 1; // Enable chip select signal

  DS1302ByteWrite((reg << 1) | 0x80); //Send write register instruction

  DS1302ByteWrite(dat); //Write byte data

  DS1302_CE = 0; //Disable chip select signal

}

/* Use a single read operation to read a byte from a register, reg-register address, return value-read byte*/

uchar DS1302SingleRead(uchar reg)

{

  uchar dat;

  DS1302_CE = 1; // Enable chip select signal

  DS1302ByteWrite((reg << 1) | 0x81); //Send the read register instruction

  dat = DS1302ByteRead(); //Read byte data

  DS1302_CE = 0; //Disable chip select signal

  return dat;

}

/* Use burst mode to continuously write 8 register data, dat-data pointer to be written*/

void DS1302BurstWrite(uchar *dat)

{

  uchar i;

  DS1302_CE = 1;

  DS1302ByteWrite(0xBE); //Send burst write register instruction

  for (i = 0; i < 8; i++) //Continuously write 8 bytes of data

  {

    DS1302ByteWrite(dat[i]);

  }

  DS1302_CE = 0;

}

/* Use burst mode to continuously read the data of 8 registers, dat-read data receiving pointer*/

void DS1302BurstRead(uchar *dat)

{

  uchar i;

  DS1302_CE = 1;

  DS1302ByteWrite(0xBF); //Send burst read register instruction

  for (i = 0; i < 8; i++) //Read 8 bytes continuously

  {

    dat[i] = DS1302ByteRead();

  }

  DS1302_CE = 0;

}

/* Get real-time time, that is, read the current time of DS1302 and convert it into time structure format*/

void GetRealTime(struct sTime *time)

{

  uchar buf[8];

  DS1302BurstRead(buf);

  time->year = buf[6] + 0x2000;

  time->mon = buf[4];

  time->day = buf[3];

  time->hour = buf[2];

  time->min = buf[1];

  time->sec = buf[0];

  time->week = buf[5];

}

/* Set the real time, convert the set time in the time structure format into an array and write it into DS1302*/

void SetRealTime(struct sTime *time)

{

  uchar buf[8];

  buf[7] = 0;

  buf[6] = time->year;

  buf[5] = time->week;

  buf[4] = time->mon;

  buf[3] = time->day;

  buf[2] = time->hour;

  buf[1] = time->min;

  buf[0] = time->sec;

  DS1302BurstWrite(buf);

}

/* DS1302 initialization, if power failure occurs, reset the initial time*/

void InitDS1302()

{

  uchar dat;

  struct sTime code InitTime[] = //Tuesday, May 18, 2016, 9:00:00

  {

    0x2016, 0x05, 0x18, 0x09, 0x00, 0x00, 0x02

  };

  DS1302_CE = 0; //Initialize DS1302 communication pin

  DS1302_CK = 0;

  dat = DS1302SingleRead(0); //Read the seconds register

  if ((dat & 0x80) != 0) //Judge whether DS1302 has stopped by the value of the highest bit CH of the seconds register

  {

    DS1302SingleWrite(7, 0x00); //Remove write protection to allow data to be written

    SetRealTime(&InitTime); //Set DS1302 to the default initial time

  }

}