Console and outputs

Coreboot has various possible consoles:

Output name	direction (from the coreboot target machine point of view)	Requirements	Compatibility with software loaded after coreboot, like OS and payloads
Serial console (sends coreboot logs over the serial port)	input output	An internal(not USB) serial port: it can be on the mainboard it can also be a PCIe OXPCIe952 card with the 0x1415 and 0xc158 vendor and device ID (respectively). USB serial ports are not supported by coreboot yet.	seabios grub as a payload grub running after seabios libpayload linux kernel (loaded after the payload) most GNU/Linux init systems(sysvinit, systemd, upstart etc...)
Console Over EHCI debug port (sends coreboot logs over the usb debug port)	input? output	An USB debug port supported by coreboot. To find on which usb connector it goes.	grub linux kernel in its early initialisation (I'm not sure that it can be kept afterward, because, after the early initialisation, by default it tries to initialise the USB port normally)
Console trough spkmodem (sends coreboot logs over the sound card)	output only	A working "beep" sound card at boot. On some laptops it's necessary to tell the embedded controller to enable sound output to get it working.	For writting to it from the coreboot target computer: coreboot's console grub's console For reading it from a remote computer: coreboot's/grub's spkmodem_recv uttilty (same uttility, but it's available in both git repositories)
Network console (sends coreboot logs over the network)	output probably no input	A ne2k compatible ethernet card on the coreboot target system. A network between the coreboot computer and the computer receiving the logs.	For sending the logs: the linux kernel For receiving the logs: nc and similar networking uttilities.
Cbmem console (Ram buffer, like dmesg)	output only for coreboot. output for grub. Can also print cbmem console content. It requires the computer to be booted to retrive the logs.	A coreboot device that supports cbmem: not all devices supports it in coreboot.	For writting to the buffer which is in the coreboot target computer: coreboot console output grub console output seabios For reading the buffer on the coreboot target computer, after coreboot booted: grub with the cbmemc command coreboot's userspace cbmem -c uttility
POST card	output only.

If none of the above works for your case, you might consider:

• Trying Flash emulators to trace the code being accessed.
• Osciloscopes for hardware related lower level debuging.
• SerialICE if you can add support for your board serial or EHCI Debug Port in SerialICE