[Android Head-Units] teletype:
I've tried several different SD card images and thus far haven't been able to get any to boot. I got sick of flying blind with no way to know what the problem was. So, before installing it in my car or even really using the thing, I sacrificed my warranty in the name of science.

After an evening of hacking and exploration, I have found the TX/RX pins on the SoM and successfully added a serial console port. In case anyone is curious, here are the pins. Note this is 3.3V, DO NOT connect it to an RS232 serial port. I'm using the UART pins on a Raspberry Pi GPIO connector to talk to it. I ran this out to an external connector and managed to get the thing back together without breaking anything but my warranty.

Here's the stock ROM boot sequence:

DDR Version 1.04 20130830
In
DDR3
300MHz
Bus Width=32 Col=10 Bank=8 Row=15 CS=2 Die Bus-Width=8 Size=2048MB
Memory OK
OUT
Boot1 Release Time: 2016-09-13, version: 2.32
ChipType = 4 1318
SdmmcInit=2 0
FwPartOffset=2000 , 400
StorageInit ok = 132539
chipEID = 33504b52
theLoader 60200000 168604

U-Boot 2014.01-gb92ab43-dirty (Aug 09 2017 - 16:44:27)

CPU: RK3188
Board: RK30xx platform Board
config base at ddr end
config base at ddr end,config_base = 67bf0000, size = 00400000
logo base at ddr end, logo_base = 671f0000, size = 00a00000
backcar base at ddr end, backcar_base = 667f0000, size = 00a00000
total reserving memory(except stack) is :68m
DRAM: 128 MiB
arch_interrupt_init
DMAInit: OK
storage init OK!
Using default environment

In: serial
Out: serial
Err: serial
board_late_init
unsigned!
SecureBootEn = 0 0
##################################################
uboot version: U-Boot 2014.01-gb92ab43-dirty (Aug 09 2017 - 16:44:27)

#Boot ver: 2017-06-06#0.02

##################################################
setup gpio int
GetParam
LBA:0, 2000, 80check parameter success
partition(uboot): offset=0x00002000, size=0x00002000
partition(misc): offset=0x00004000, size=0x00002000
partition(resource): offset=0x00006000, size=0x00004000
partition(kernel): offset=0x0000A000, size=0x0000C000
partition(boot): offset=0x00016000, size=0x00010000
partition(recovery): offset=0x00026000, size=0x00010000
partition(backup): offset=0x00036000, size=0x00014000
partition(config): offset=0x0004A000, size=0x00002000
partition(logo): offset=0x0004C000, size=0x00005000
partition(backcar): offset=0x00051000, size=0x00005000
partition(cache): offset=0x00056000, size=0x00040000
partition(metadata): offset=0x00096000, size=0x00008000
partition(kpanic): offset=0x0009E000, size=0x00002000
partition(system): offset=0x000A0000, size=0x00300000
partition(userdata): offset=0x003A0000, size=0xFFFFFFFF
empty serial no.
checkKey
vbus = 1
LBA:4020, 2000, 3LBA:16000, 2000, 4LBA:16000, 2000, 1LBA:16001, 2000, c70LBA:a000, 2000, 1LBA:a001, 2000, 4000LBA:e001, 2000, 2a0log0
LBA:51000, 2000, 5000loadDataImage:0x0,0x0logo start = 0x4c000,size = 10240
LBA:4c000, 2000, 5000loadDataImage:0x42,0x4dlogo start = 0x4a000,size = 4096
LBA:4a000, 2000, 2000loadDataImage:0x5b,0x7aSecureBootCheckOK:0
booti: do_bootm_linux...

Starting kernel ...

I'm pretty confident I can get this to boot from SD now that I can actually see what is going on. First goal is to create a bootable Linux image that can be used for block-level backups/restores of the mmc device.

I think it might be possible to boot from USB. Once I get SD going, I'll give it a shot and let you know.