I finally have a DCM476 of my own to play with, and I've been playing.
(well, I had a DCM475 of my own way way back, but it sold quickly)
Turns out it is possible to upgrade the DCM476 with just a TTL serial cable and a TFTP server. The method is similar to the DCM425 method.. Apply a short to pin 2 of the SPI flash at exactly the right moment and you get an open console.
(edit to add some instruction)Serial console settings
Data bits: 8
Stop bits: 1
Flow control: Xon/XoffThe moment for the short
First power up with the serial console connected. After about 3 seconds of being powered on, the console should display a bunch of information in plain text. Get used to how long it takes from applying power to seeing the output. Then apply power again, but this time apply a short between ground and pin 2 of the SPI flash (on the 476, or pin 8 on the 475) just at the moment that the information would be displayed.
If you see the normal output, you were either way too early, or too late.
If you see the Broadcom wavy logo, but the output is stuck, you've done it very slightly too early.
If the output is much longer, and results in a prompt that you can type instructions at, you are in.Command to flash
dload -i2 192.168.100.10 02.50.bin
(or, you know, 02.16.bin if it is a 475)
A thing of beauty :D
I don't mind making up detailed instructions if the demand is there, but for now just pm me if you'd like to give it a try.
And, I'm almost certain that this will work just as well on a DCM475, but we won't know until somebody tries.
So I just did the upgrade on a DCM475 using the short method, and it works just as well. Of course the DCM475 has a 16 pin SPI flash, so you have to apply the short to pin 8 instead of pin 2 on the 8 pin SPI flash. Everything else is exactly the same.--