Need to undestand SD_Card.h

[Altera_Forum]

Hi, so I'm just a newbie and I plan to use the SD card of my Altera DE2 board for a project. I managed to find the example that comes with the CD named as DE2_SD_Card_Audio. So I saw that there's a header file named "SD_Card.h". I understand this is the one responsible for reading files that's inside the SD Card. Can someone help me understand the codes in this header file?  

#ifndef __SD_Card_H__ # define __SD_Card_H__ //------------------------------------------------------------------------- // SD Card Set I/O Direction # define SD_CMD_IN IOWR(SD_CMD_BASE, 1, 0) # define SD_CMD_OUT IOWR(SD_CMD_BASE, 1, 1) # define SD_DAT_IN IOWR(SD_DAT_BASE, 1, 0) # define SD_DAT_OUT IOWR(SD_DAT_BASE, 1, 1) // SD Card Output High/Low # define SD_CMD_LOW IOWR(SD_CMD_BASE, 0, 0) # define SD_CMD_HIGH IOWR(SD_CMD_BASE, 0, 1) # define SD_DAT_LOW IOWR(SD_DAT_BASE, 0, 0) # define SD_DAT_HIGH IOWR(SD_DAT_BASE, 0, 1) # define SD_CLK_LOW IOWR(SD_CLK_BASE, 0, 0) # define SD_CLK_HIGH IOWR(SD_CLK_BASE, 0, 1) // SD Card Input Read # define SD_TEST_CMD IORD(SD_CMD_BASE, 0) # define SD_TEST_DAT IORD(SD_DAT_BASE, 0) //------------------------------------------------------------------------- # define BYTE unsigned char # define UINT16 unsigned int # define UINT32 unsigned long //------------------------------------------------------------------------- void Ncr(void); void Ncc(void); BYTE response_R(BYTE); BYTE send_cmd(BYTE *); BYTE SD_read_lba(BYTE *,UINT32,UINT32); BYTE SD_card_init(void); //------------------------------------------------------------------------- BYTE read_status; BYTE response_buffer; BYTE RCA; BYTE cmd_buffer; const BYTE cmd0 = {0x40,0x00,0x00,0x00,0x00}; const BYTE cmd55 = {0x77,0x00,0x00,0x00,0x00}; const BYTE cmd2 = {0x42,0x00,0x00,0x00,0x00}; const BYTE cmd3 = {0x43,0x00,0x00,0x00,0x00}; const BYTE cmd7 = {0x47,0x00,0x00,0x00,0x00}; const BYTE cmd9 = {0x49,0x00,0x00,0x00,0x00}; const BYTE cmd16 = {0x50,0x00,0x00,0x02,0x00}; const BYTE cmd17 = {0x51,0x00,0x00,0x00,0x00}; const BYTE acmd6 = {0x46,0x00,0x00,0x00,0x02}; const BYTE acmd41 = {0x69,0x0f,0xf0,0x00,0x00}; const BYTE acmd51 = {0x73,0x00,0x00,0x00,0x00}; //------------------------------------------------------------------------- void Ncr(void) { SD_CMD_IN; SD_CLK_LOW; SD_CLK_HIGH; SD_CLK_LOW; SD_CLK_HIGH; } //------------------------------------------------------------------------- void Ncc(void) { int i; for(i=0;i<8;i++) { SD_CLK_LOW; SD_CLK_HIGH; } } //------------------------------------------------------------------------- BYTE SD_card_init(void) { BYTE x,y; SD_CMD_OUT; SD_DAT_IN; SD_CLK_HIGH; SD_CMD_HIGH; SD_DAT_LOW; read_status=0; for(x=0;x<40;x++) Ncr(); for(x=0;x<5;x++) cmd_buffer=cmd0; y = send_cmd(cmd_buffer); do { for(x=0;x<40;x++); Ncc(); for(x=0;x<5;x++) cmd_buffer=cmd55; y = send_cmd(cmd_buffer); Ncr(); if(response_R(1)>1) //response too long or crc error return 1; Ncc(); for(x=0;x<5;x++) cmd_buffer=acmd41; y = send_cmd(cmd_buffer); Ncr(); } while(response_R(3)==1); Ncc(); for(x=0;x<5;x++) cmd_buffer=cmd2; y = send_cmd(cmd_buffer); Ncr(); if(response_R(2)>1) return 1; Ncc(); for(x=0;x<5;x++) cmd_buffer=cmd3; y = send_cmd(cmd_buffer); Ncr(); if(response_R(6)>1) return 1; RCA=response_buffer; RCA=response_buffer; Ncc(); for(x=0;x<5;x++) cmd_buffer=cmd9; cmd_buffer = RCA; cmd_buffer = RCA; y = send_cmd(cmd_buffer); Ncr(); if(response_R(2)>1) return 1; Ncc(); for(x=0;x<5;x++) cmd_buffer=cmd7; cmd_buffer = RCA; cmd_buffer = RCA; y = send_cmd(cmd_buffer); Ncr(); if(response_R(1)>1) return 1; Ncc(); for(x=0;x<5;x++) cmd_buffer=cmd16; y = send_cmd(cmd_buffer); Ncr(); if(response_R(1)>1) return 1; read_status =1; //sd card ready return 0; } //------------------------------------------------------------------------- BYTE SD_read_lba(BYTE *buff,UINT32 lba,UINT32 seccnt) { BYTE c=0; UINT32 i,j; lba+=101; for(j=0;j<seccnt;j++) { { Ncc(); cmd_buffer = cmd17; cmd_buffer = (lba>>15)&0xff; cmd_buffer = (lba>>7)&0xff; cmd_buffer = (lba<<1)&0xff; cmd_buffer = 0; lba++; send_cmd(cmd_buffer); Ncr(); } while(1) { SD_CLK_LOW; SD_CLK_HIGH; if(!(SD_TEST_DAT)) break; } for(i=0;i<512;i++) { BYTE j; for(j=0;j<8;j++) { SD_CLK_LOW; SD_CLK_HIGH; c <<= 1; if(SD_TEST_DAT) c |= 0x01; } *buff=c; buff++; } for(i=0; i<16; i++) { SD_CLK_LOW; SD_CLK_HIGH; } } read_status = 1; //SD data next in return 0; } //------------------------------------------------------------------------- BYTE response_R(BYTE s) { BYTE a=0,b=0,c=0,r=0,crc=0; BYTE i,j=6,k; while(1) { SD_CLK_LOW; SD_CLK_HIGH; if(!(SD_TEST_CMD)) break; if(crc++ >100) return 2; } crc =0; if(s == 2) j = 17; for(k=0; k<j; k++) { c = 0; if(k > 0) //for crc culcar b = response_buffer; for(i=0; i<8; i++) { SD_CLK_LOW; if(a > 0) c <<= 1; else i++; a++; SD_CLK_HIGH; if(SD_TEST_CMD) c |= 0x01; if(k > 0) { crc <<= 1; if((crc ^ b) & 0x80) crc ^= 0x09; b <<= 1; crc &= 0x7f; } } if(s==3) { if( k==1 &&(!(c&0x80))) r=1; } response_buffer = c; } if(s==1 || s==6) { if(c != ((crc<<1)+1)) r=2; } return r; } //------------------------------------------------------------------------- BYTE send_cmd(BYTE *in) { int i,j; BYTE b,crc=0; SD_CMD_OUT; for(i=0; i < 5; i++) { b = in; for(j=0; j<8; j++) { SD_CLK_LOW; if(b&0x80) SD_CMD_HIGH; else SD_CMD_LOW; crc <<= 1; SD_CLK_HIGH; if((crc ^ b) & 0x80) crc ^= 0x09; b<<=1; } crc &= 0x7f; } crc =((crc<<1)|0x01); b = crc; for(j=0; j<8; j++) { SD_CLK_LOW; if(crc&0x80) SD_CMD_HIGH; else SD_CMD_LOW; SD_CLK_HIGH; crc<<=1; } return b; } //------------------------------------------------------------------------- # endif