at91_sdcard源码分析(3)

SD_MCI_Init函数做的工作是初始化SD卡。有可能是SD卡或是MMC卡。

//------------------------------------------------------------------------------
/// Run the SDcard SD Mode initialization sequence. This function runs the     根据SD协议初始化SD卡（或MMC卡），此函数完成后进入数据传输模式
/// initialisation procedure and the identification process, then it sets the
/// SD card in transfer state to set the block length and the bus width.
/// Returns 0 if successful; otherwise returns an SD_ERROR code.
/// \param pSd  Pointer to a SD card driver instance.
/// \param pSdDriver  Pointer to SD driver already initialized
//------------------------------------------------------------------------------
unsigned char SD_MCI_Init(SdCard *pSd, SdDriver *pSdDriver)
{
    unsigned int sdCid[4];
    unsigned char isCCSet;
    unsigned char error;
    unsigned int status;
    unsigned char cmd8Retries = 2;
    unsigned char cmd1Retries = 100;

    // The command GO_IDLE_STATE (CMD0) is the software reset command and sets card into Idle State
    // regardless of the current card state.
    error = Cmd0(pSd);　　//软件复位
    if (error) {
        TRACE_ERROR("Error during initialization (%d)\n\r", error);
        return error;
    }

    // CMD8 is newly added in the Physical Layer Specification Version 2.00 to support multiple voltage
    // ranges and used to check whether the card supports supplied voltage. The version 2.00 host shall
    // issue CMD8 and verify voltage before card initialization.
    // The host that does not support CMD8 shall supply high voltage range...  检查是否支持CMD8命令
    TRACE_DEBUG("Cmd8()\n\r");
    do {
        error = Cmd8(pSd, 1);
    }
    while ((error == SD_ERROR_NORESPONSE) && (cmd8Retries-- > 0));

    if (error == SD_ERROR_NORESPONSE) {  //不支持CMD8命令，所以SD卡的版本是2.00以下，或者它不是SD卡或MMC卡。
        // No response : Ver2.00 or later SD Memory Card(voltage mismatch)
        // or Ver1.X SD Memory Card
        // or not SD Memory Card

        TRACE_DEBUG("No response to Cmd8\n\r");

        // ACMD41 is a synchronization command used to negotiate the operation voltage range and to poll the
        // cards until they are out of their power-up sequence.
        error = Acmd41(pSd, 0, &isCCSet);  //发送ACMD41命令
        if (error) {
            // Acmd41 failed : MMC card or unknown card  不支持ACMD41，它可能是MMC卡，下面先复位再发送CMD1命令偿试初始化MMC卡
            error = Cmd0(pSd);
            if (error) {
                TRACE_ERROR("Error during initialization (%d)\n\r", error);
                return error;
            }
            do {
                error = Cmd1(pSd);
            }
            while ((error) && (cmd1Retries-- > 0));
            if (error) {
                TRACE_ERROR("Error during initialization (%d)\n\r", error);
                return error;
            }
            else {
                TRACE_DEBUG("CARD MMC\n\r");
                pSd->cardType = CARD_MMC;
            }
        }
        else {
            if(isCCSet == 0) {
                TRACE_DEBUG("CARD SD\n\r");
                pSd->cardType = CARD_SD;
            }
        }
    }
    else if (!error) {   //发送CMD8命令成功，那么它是版本2.00以上的SD卡。 

        // Valid response : Ver2.00 or later SD Memory Card
        error = Acmd41(pSd, 1, &isCCSet);
        if (error) {
            TRACE_ERROR("Error during initialization (%d)\n\r", error);
            return error;
        }
        if (isCCSet) {
            TRACE_DEBUG("CARD SDHC\n\r");
            pSd->cardType = CARD_SDHC;
        }
        else {
            TRACE_DEBUG("CARD SD\n\r");
            pSd->cardType = CARD_SD;
        }
    }
    else {
        TRACE_ERROR("Error during initialization (%d)\n\r", error);
        return error;
    }

    // The host then issues the command ALL_SEND_CID (CMD2) to the card to get its unique card identification (CID) number.
    // Card that is unidentified (i.e. which is in Ready State) sends its CID number as the response (on the CMD line).
    error = Cmd2(pSd, sdCid);    //获取CID寄存器的内容。
    if (error) {
        TRACE_ERROR("Error during initialization (%d)\n\r", error);
        return error;
    }

    // Thereafter, the host issues CMD3 (SEND_RELATIVE_ADDR) asks the
    // card to publish a new relative card address (RCA), which is shorter than CID and which is used to
    // address the card in the future data transfer mode. Once the RCA is received the card state changes to
    // the Stand-by State. At this point, if the host wants to assign another RCA number, it can ask the card to
    // publish a new number by sending another CMD3 command to the card. The last published RCA is the
    // actual RCA number of the card.
    error = Cmd3(pSd); 
    if (error) {
        TRACE_ERROR("Error during initialization (%d)\n\r", error);
        return error;
    }

    // The host issues SEND_CSD (CMD9) to obtain the Card Specific Data (CSD register),
    // e.g. block length, card storage capacity, etc...
    error = Cmd9(pSd);
    if (error) {
        TRACE_ERROR("Error during initialization (%d)\n\r", error);
        return error;
    }

    // At this stage the Initialization and identification process is achieved
    // The SD card is supposed to be in Stand-by State
    do {
        error = Cmd13(pSd, &status);
        if (error) {
            TRACE_ERROR("Error during initialization (%d)\n\r", error);
            return error;
        }
    }
    while ((status & STATUS_READY_FOR_DATA) == 0);

    // If the 4 bit bus transfer is supported switch to this mode  选择当前的SD卡或MMC卡，使它进入数据传输模式。
    // Select the current SD, goto transfer state 
    error = Cmd7(pSd, pSd->cardAddress);
    if (error) {
        TRACE_ERROR("Error during initialization (%d)\n\r", error);
        return error;
    }

    if (pSd->cardType != CARD_MMC) {     //如果是SD卡，那么设置成4位数据线模式。
        // Switch to 4 bits bus width (All SD Card shall support 1-bit, 4 bitswidth
        error = Acmd6(pSd, 4);
        if (error) {
            TRACE_ERROR("Error during initialization (%d)\n\r", error);
            return error;
        }
    }
    else {
        MCI_SetBusWidth((Mci *)pSdDriver, MCI_SDCBUS_1BIT);
    }
    return 0;
}