14.2.15. Why do I get TFTP timeouts?

Question:

When trying to download a file from the TFTP server I always get timeouts like these:

...
Loading: #######T ##################################T###################T ####T ##T #
       ###T #T #########T ########T #############T ##T #############T ########T #############T
       #####T ###T ######T #######T #######T #############T ##T ##############T ###########
       ###########
done

If the target is connected directly to the host PC (i. e. without a switch inbetween) the problem goes away or is at least less incisive.

What's wrong?

Answer:

Most probably you have a full duplex/half duplex problem. Verify that U-Boot is setting the ethernet interface on your board to the proper duplex mode (full/half). I'm guessing your board is half duplex but your switch is full (typical of a switch ;-).

The switch sends traffic to your board while your board is transmitting... that is a collision (late collision at that) to your board but is OK to the switch. This doesn't happen nearly as much with a direct link to your PC since then you have a dedicated link without much asynchronous traffic.

The software (U-Boot/Linux) needs to poll the PHY chip for duplex mode and then (re)configure the MAC chip (separate or built into the CPU) to match. If the poll isn't happening or has a bug, you have problems like described above.