I have a custom i.MX6-based design I'm bringing up with both MicroSD (on SDHC3) and eMMC (on SDHC1). I'm trying to boot from MicroSD right now. I've broken out the BT_CFG pins, and I'd like to set the boot config using GPIO pins.
Unfortunately, the i.MX6 always jumps to the USB serial download, no matter what settings I use on the BOOT_MODE[1:0] pins or the GPIO BT_CFG pins.
I've verified the correct voltages at the DIP switch used to set the boot mode, and I've tried it both from a cold boot, and by resetting the i.MX6 by pulling POR low -- either way, it jumps straight to USB serial download. I've verified the MicroSD card has a boot image that my RIoTBoard has no problems booting from.
Here's my schematic: