我是买的开发板，用直接下载的BOOT.bin能正常启动，我自己在xilinx官网下载的u-boot-xlnx-master.zip，生成BOOT.bin启动的时候出现下面的问题，MMC超时，请问是什么原因呀，我读mmc的信息也是显示超时
U-Boot 2014.01 (Dec 10 2014 - 21:42:04)


I2C:   ready
Memory: ECC disabled
DRAM:  1 GiB
MMC:   zynq_sdhci: 0
SF: Detected S25FL128S_64K with page size 512 Bytes, erase size 128 KiB, total 32 MiB
*** Warning - bad CRC, using default environment


In:    serial
Out:   serial
Err:   serial
Net:   Gem.e000b000
Hit any key to stop autoboot:  3  2  1  0 
sdhci_send_command: MMC: 0 busy timeout increasing to: 200 ms.
sdhci_send_command: MMC: 0 busy timeout increasing to: 400 ms.
sdhci_send_command: MMC: 0 busy timeout increasing to: 800 ms.
sdhci_send_command: MMC: 0 busy timeout increasing to: 1600 ms.
sdhci_send_command: MMC: 0 busy timeout increasing to: 3200 ms.
sdhci_send_command: MMC: 0 busy timeout.
sdhci_send_command: MMC: 0 busy timeout.
sdhci_send_command: MMC: 0 busy timeout.
sdhci_send_command: MMC: 0 busy timeout.
sdhci_send_command: MMC: 0 busy timeout.
sdhci_send_command: MMC: 0 busy timeout.
zynq-uboot>