interfacing a SD/MMC card to microcontroller

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/*  PIC MMC Interface Test
    [url]http://www.captain.at/electronics/[/url]
    
    Writes ASCII characters to the MMC, reads them back and sends the characters
    to the linux machine via the serial port. Use "serterm" for testing:
 
    # ./serterm
    baud=9600
    baud=9600
    listening...
    PIC online MMC online 0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcde
    fghijklmno0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmno01
    23456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmno0123456789:;<=
    >?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnoCCCCCCCCCCCCCCCCCCCCCCCCCC
    CCCCCCCCCCC [more C's follow]
    exiting...
    
    Furthermore the PIC returns any character sent to it via the interrupt
    service routine. Use "ser" for testing:
    
    # ./ser
    baud=9600
    baud=9600
    written:ABC
    readport=ABC
    #
    
    Compile with CC5x: [url]http://www.bknd.com/cc5x/[/url]
    c:\picc\cc5x mmc.c -Ic:\picc\ -ammc.ASM -u
    
    CC5x runs nicely on Linux with "dosemu" [url]http://www.dosemu.org/[/url]
    
    Credit:
    SPI functions made by Michael Dworkin
    [I]**broken link removed**[/I]
*/
 
#include <16F876.h>
#include "int16CXX.H"   // interrupt stuff
 
#define CP_off  |= 0x3F30
#define LVP |= 128
#pragma config CP_off,PWRTE=on,WDTE=off,FOSC=HS,BODEN=on,LVP
 
#pragma bit CS @ PORTC.2    // output pin for chip select (MMC)
 
#pragma rambank 0
char sector1[64];
int currentrambank;
int counter;
 
#pragma rambank 1
char sector2[64];
 
#pragma rambank 2
char sector3[64];
 
#pragma rambank 3
char sector4[64];
 
#pragma origin 4        // force interrupt service routine at address 4
 
interrupt myInterrupt(void)     // ISR
{
    int_save_registers
    char rec;
    if (RCIF)               // USART interrupt ?
    {
        while(!TXIF) {      // wait for serial register to be sent, if there is still something in there
            nop();
        }
        rec = RCREG;        // Get the received character
        TXREG = rec;        // write to serial register -> start transmission
 
        RCIF = 0;           // clear USART int flag
    }
    int_restore_registers
}
 
void InitUSART()
{
    PORTA = 0;
    PORTB = 0;
    PORTC = 0;
    
    BRGH = 1;       // high speed serial mode
    SPBRG = 25;     // Set 9600 baud for 4 MHz oscillator
    SYNC = 0;       // Clear SYNC bit -> Set ASYNC Mode
    SPEN = 1;       // Set serial port enable
    TX9 = 0;            // 8-bit transmissions
    TXEN = 1;       // Enable transmission
    RCIE = 1;       // Rx interrupts are desired
    RX9 = 0;            // 8-bit receptions
    CREN = 1;       // Enable reception
}
 
void initint()      // init interrupts
{
    GIE = 1;        // Set Global Interrupt Enable
    PEIE = 1;       // Set Peripheral Interrupt Enable
}
 
void fillram()      // fill RAM (sector1..4) with ASCII characters
{
    int i;
    int x;
    for (i=0;i<=63;i++) {
        x = i + 48;
        sector1[i] = x;
        sector2[i] = x;
        sector3[i] = x;
        sector4[i] = x;
    }
}
 
char SPI(char d)        // send character over SPI
{
    SSPBUF = d;         // send character
    while (!BF);        // wait until sent
    return SSPBUF;      // and return the received character
}
 
char Command(char befF,uns16 AdrH,uns16 AdrL,char befH )
{   // sends a command to the MMC
    char a;
    SPI(0xFF);
    SPI(befF);
    SPI(AdrH.high8);
    SPI(AdrH.low8);
    SPI(AdrL.high8);
    SPI(AdrL.low8);
    SPI(befH);
    SPI(0xFF);
    return SPI(0xFF);   // return the last received character
}
 
bit MMC_Init()  // init SPI
{
    SMP = 0;        // input is valid in the middle of clock
    CKE = 0;        // rising edge is data capture
    CKP = 1;        // high value is passive state
    SSPM1 = 1;  // speed f/64(312kHz), Master
    SSPEN = 1;  // enable SPI
    CS = 1;         // disable MMC
 
    char i;
    // start MMC in SPI mode
    for(i=0; i < 10; i++)SPI(0xFF);     // send 10*8=80 clock pulses
    CS=0;           // MMC-Enabled
 
    if (Command(0x40,0,0,0x95) !=1) goto mmcerror;  // Reset MMC
 
st: // if there is no MMC, prg. loops here
    if (Command(0x41,0,0,0xFF) !=0) goto st;
 
    return 1;
mmcerror:
    return 0;
}
 
void serialterminate() { // terminate sent string!!!
    while(!TXIF);
    TXREG = 0x0d;
    while(!TXIF);
    TXREG = 0x0a;
}
 
void SerString(const char *str) // send string via RS232
{
    char ps;
    ps = *str;                  // pointer of start of string into ps
    while(ps>0)                 // test if end of string is reached
    {
        str++;                  // set pointer to next char
        if (ps== 0) break;  // test if end of string is reached
        while(!TXIF);           // test if TXREG empty
        TXREG = ps ;            // send character
        ps = *str;              // content of pointer into ps
    }
    serialterminate();
}
 
bit writeramtommc()     // write RAM (sector1..4) to MMC
{
    // 512 byte-write-mode
    if (Command(0x58,0,512,0xFF) !=0) {
        SerString("MMC: write error 1 ");
        return 1;   
    }
    SPI(0xFF);
    SPI(0xFF);
    SPI(0xFE);
    uns16 i;
    // write ram sectors to MMC
    for (i=0;i<=63;i++) {
        SPI(sector1[i]);
    }
    for (i=0;i<=63;i++) {
        SPI(sector2[i]);
    }
    for (i=0;i<=63;i++) {
        SPI(sector3[i]);
    }
    for (i=0;i<=63;i++) {
        SPI(sector4[i]);
    }
    for (i=0;i<256;i++) {   // fill rest with 'C'
        SPI('C');
    }
    SPI(255);                   // at the end, send 2 dummy bytes
    SPI(255);
 
    i = SPI(0xFF);
    i &= 0b.0001.1111;
    if (i != 0b.0000.0101) {
        SerString("MMC: write error 2 ");
        return 1;
    }
    while(SPI(0xFF) != 0xFF);   // wait until MMC is not busy anymore
    return 0;
}
 
bit sendmmc()       // send 512 bytes from the MMC via the serial port
{
    uns16 i;
    // 512 byte-read-mode 
    if (Command(0x51,0,512,0xFF) !=0) {
        SerString("MMC: read error 1 ");
        return 1;
    }
    while(SPI(0xFF) != 0xFE);   // wait for 0xFE - start of any transmission
    for(i=0; i < 512; i++)
    {
        while(!TXIF);               // wait until TXREG is empty
        TXREG = SPI(0xFF);      // get MMC byte and send via RS232
    }
    serialterminate();
    SPI(0xFF);                      // at the end, send 2 dummy bytes
    SPI(0xFF);
    return 0;
}
 
void main(void)
{
    InitUSART();                    // initialize serial port
    initint();                      // initialize interrupts
 
    ADCON1=0x6;                     // PortA Digital
    // set ports for I/O
    TRISC = 0b.1101.0011;       // sck rc3-0, sdo rc5-0, CS rc2-0.
    TRISB = 0b.0000.0010;       // RB2>TX, RB1>RX
 
    SerString("PIC online ");   // start message
//  SerString(0x00);    // start message
    
    // init MMC and send message if ok
    if (MMC_Init()) SerString("MMC online ");
    
    fillram();                      // fill the RAM with ASCII characters
    writeramtommc();                // write RAM to MMC
    sendmmc();                      // send 512 bytes of the MMC via serial port
            
    while(1) {
        nop();
        nop();
    }
}