imx6开机启动就进入download模式，有的板子进入文件系统之后会进入download模式。查看datasheet，Chapter 8 System Boot查找原因,记录于此。
freescale论坛有关于这个问题的讨论，有硬件也有软件方面的原因。
参考链接
https://community.nxp.com/thread/316232
https://community.nxp.com/thread/338433
boot mode pin settings
8.2.1 Boot mode pin settings
BOOT_MODE is initialized by sampling the BOOT_MODE0 and BOOT_MODE1
inputs on the rising edge of POR_B. After these inputs are sampled, their subsequent
state does not affect the contents of the BOOT_MODE internal register. The state of the
internal BOOT_MODE register may be read from the BMOD[1:0] field of the SRC Boot
Mode Register (SRC_SBMR2). The available boot modes are: Boot From Fuses, serial
boot via USB, and Internal Boot. See the table below for settings
根据BOOT_MODE[1:0]引脚的值选择启动类型
BOOT_MODE[1:0] Boot Type
00 Boot From Fuses
01 Serial Downloader
10 Internal Boot
11 Reserved
在POR_B引脚的上升沿读取BOOT_MODE0和BOOT_MODE1的值作为启动的模式。
如果boot一开始就进入download模式，应该查看POR_B上升沿时，BOOT_MODE, BOOT_CFG引脚配置的值是否正确。
Boot devices(Internal Boot)
当选择为Internal Boot时，根据BOOT_CFG1[7:4]选择启动设备的类型，如果没有选择的设备，则进入download模式。
NOR Flash with External Interface Module (EIM), located on CS0, 16-bit bus width
• OneNAND Flash with EIM interface, located on CS0, 16-bits bus width
• Raw NAND (MLC and SLC), and Toggle-mode NAND flash through GPMI-2
interface. Page sizes of 2 Kbyte, 4 Kbyte and 8 Kbyte. Bus widths of 8-bit with 2
through 40-bit BCH Hardware ECC (Error Correction) are supported.
• SD/MMC/eSD/SDXC/eMMC4.4 via USDHC interface, supporting high capacity
cards
• EEPROM boot via SPI (serial flash) and I2C(via ECSPI and I2C blocks respectively)
The selection of external boot device type is controlled by BOOT_CFG1[7:4] eFUSEs.
See the table below for more details
BOOT_CFG1[7:4] Boot Device
0000 NOR/OneNAND (EIM)
0001 Reserved
0011 Serial ROM (I2C/SPI)
010x SD/eSD/SDXC
011x MMC/eMMC
1xxx Raw NAND
BOOT_CFG其他引脚
BOOT_CFG1,BOOT_CFG2,BOOT_CFG3,BOOT_CFG4引脚进一步确定其他的参数。
例如sd/emmc,如下：
8.5.3 Expansion Device
The ROM supports booting from MMC/eMMC and SD/eSD compliant devices.
8.5.3.1 Expansion Device eFUSE Configuration
SD/MMC/eSD/eMMC/SDXC boot can be performed using either USDHC ports, based
on setting of the BOOT_CFG2[4:3] (Port Select) fuse or it's associated GPIO input value
at boot. All USDHC ports support eMMC4.3 and eMMC4.4 fast boot.
BOOT_CFG2[7:5] 选择SD/EMMC的参数
SD/eSD/SDXC (BOOT_CFG1[5]=0)
Bus Width
xx0 - 1-bit
xx1 - 4-bit
SD Calibration Step
00x - 1 delay cells
01x - 1 delay cells
10x - 2 delay cells
11x - 3 delay cells
MMC/eMMC (BOOT_CFG1[5]=1)
000 - 1-bit
001 - 4-bit
010 - 8-bit
101 - 4-bit DDR (MMC 4.4)
110 - 8-bit DDR (MMC 4.4)
Else - reserved.
BOOT_CFG2[4:3] 选择从哪个设备启动
01 - USDHC-2
10 - USDHC-3
11 - USDHC-4
Author
Tony Liu
2016-10-31, Shenzhen