Question about SPI communications between MCF51MM256 and external RTC

[aydsae]

Hi,

I have problems trying to communicate a MCF51MM256 with PCF2127A RTC through SPI. I am using a TWR_MCF51MM-Kit. I set MCF51MM256 as master and it seems that the transfer of commands and data go out but i am not getting the answer I hope from the RTC, i.e, when I try to do a sequential red i just got a byte with an illogical value. I'll put my code below:

//*************************************
//   MCF51MM256 Init
//***************************************/
void MCU_Init(void)
{ 
    SOPT1 = 0x23;          /* disable COP, enable stop mode, enable BKGD and RESET pin */
    SCGC1 = 0x00;          /* Disable Bus clock to unused peripherals */
    SCGC2 = 0x01;          /* Bus Clock to the SPI1 module is enable */
    SCGC3 = 0x00;     /* Disable Bus clock to unused peripherals */
}

//*************************************
//   SPI_Init
//**********************************/
void SPI_Init(void)
{ 
    SPI1C1 = 0x54;// Enable SPI1, sets SPI as master Device,
                  // transfers start with most significant bit
    SPI1C2 = 0x00;// SPI1 in 8-bit mode, separate pins for input/ output  

    SPI1BR = 0x04;  // prescale divisor 1, baudrate divisor 32
                  // getting aprox 131 kHz

    SPI2C1 = 0x00;  //0 0 0 0 0 0 0 0
                    // Disable SPI2
                  
    SPI1S = 0x00;
                 
    // Wait until ready to send
    while ( !(SPI1S & SPI1S_SPTEF_MASK) );
    SPI1ML = 0xFF;
}

//*********************************
// Init_RTC
//***************************************/
void Init_RTC( void)
{
    byte i, buffer[ 2 ];

    buffer[0] = 0x20 | 0x03;  // command for writng Seconds register on RTC
    buffer[1] = i2bcd( 15 ); // value to write on Seconds register

    CS_ON;     // set chip enable sign for RTC through a GPIO
    while ( !(SPI1S & SPI1S_SPTEF_MASK) );   
    for ( i = 0; i < 2; i++ )
    {
        SPI1DL = buffer[i];   // Send byte value
        while( !(SPI1S & SPI1S_SPTEF_MASK) );
    }
    CS_OFF;  // disable connection sign to RTC
}

//********************************
// Read_RTC
//**************************************/
byte Read_RTC( void)
{
    byte i, buffer[ 2];

    buffer[0] = 0xA0 | 0x03; // command for reading Seconds register on RTC

    CS_ON; // set chip enable sign for RTC through a GPIO
    while ( !(SPI1S & SPI1S_SPTEF_MASK) );
    SPI1D = buffer[0];   // Send command byte
    while ( !(SPI1S & SPI1S_SPTEF_MASK) );      

    while ( !(SPI1S & SPI1S_SPRF_MASK) );   
    buffer[1]= SPI1DL;
    CS_OFF;
   
    return bcd2i(buffer[1]);
}

//***************************************
// MainLoop
//*************************************/
void main(void)
{
    EnableInterrupts; /* enable interrupts */ 
    /* include your code here */
    MCU_Init();
    // ... Use here some code that initializes the Ports of the MCU
    SPI_Init();

    Init_RTC();
    // ... Use here some dummy code to introduce a time delay
    byte s = Read_RTC();

    for(;;)
    {
        __RESET_WATCHDOG();
       
    } //
    // please make sure that you never leave main
}

Some suggestions about causes for not getting the "logical" values ?

While I am trying to debug this code with CodeWarrior Real Time Debugger  I can not see changes in the contents of the Memory at the address of SPI1DL register after 

SPI1DL = buffer[i];

instructions is executed.

 i2bcd() and bcd2i() are just funstions for coding numbers.

Thanks for your attention, in advance

[kef]

1. Double check CPOL and CPHA settings.

2. You are setting CS off prematurely in Init_RTC(). SPTEF flag is set long before transfer is complete. I hope you have scope to check it.

3. You can't read SPID output buffer. Reads from data register return data from SPI input buffer.

[aydsae]

Thank you very much Kef for your answer.

I have archived a lot of progress... However, I can not found the way to read data from the slave. As I understand, master have to generate the clock signal to the slave but I have not found in the documentation how set it. I have tested using the GPIO ports and I get data from the RTC properly. But when I use the SPI module, I see in the scope that clock is stoped when I want to get data. I guess I am missing something but not idea about what.

Thanks in advance,

[kef · Accepted solution]

Do you know what FLS module is? You disabled clock to them, but maybe it is somehow coupled to SPI?

S08 have similar SPI modules. Try searching 8-bitters forum for code examples.

I think your code should produce some SPI clocks, though I don't have MCF51MM to try what's wrong. I can only advice that routine for master I/O, both read from spi and write to spi  should look like this

char spiio(char tosend)

{

   while ( !(SPI1S & SPI1S_SPTEF_MASK) );    
   SPIDL = tosend;

   while ( !(SPI1S & SPI1S_SPRF_MASK) );    
   return  SPIDL;.

}

Doing master SPI you can't receive without sending something. Also when sending, you receive some data, like you that or not. Routine is for both send and receive.

[aydsae]

Thanks kef, yesterday I made some tests and everything went right.

I consider that the sentence

"Doing master SPI you can't receive without sending something"

is the principal point that I was not applying.

Best regards,