Entering usbboot through U+POWERON

Bas Wijnen wijnen at debian.org
Thu Dec 17 09:06:49 EST 2009

On Wed, Dec 16, 2009 at 03:16:12PM +0100, Kristoffer Ericson wrote:
> On lars suggestion I increased the timeout to first 200, then 1000 and still getting
> same issue. Could it be that my bootloader is too old for some reason? I mean
> it obviously enters usbboot mode with U+POWERON but did you have earlier versions
> of the bootloader that had that but still didnt work?

No, that is not possible.  The boot loader that is used during usbboot
is the boot rom inside the jz4740 (not in the flash rom, but really
inside the cpu chip).  This is a very small program which lets the chip
work as a usb device.  It accepts a few commands, which make it possible
to boot the device over usb, even if the flash does not have a bootable
image on it.

If things go wrong after running stage 1, as seems to be the case,
several things can be wrong.  Most likely, the stage 1 code crashes for
some reason, and the chip no longer responds to usb commands.  In some
cases, it is possible to get meaningful output from the serial port.  If
not, then I would add lots of output-producing commands to stage 1
(starting after serial port setup), to see where it breaks.

No delay at all should be required between running stage 1 and loading
stage 2, because the usb device module is not touched yet.  Stage 2 will
set up its own idea of the usb device, which will be different from the
previous situation, so there you do need a delay.

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