Future debug board and deprecating write-only

Werner Almesberger werner at openmoko.org
Tue Mar 11 14:57:25 CET 2008


Andy Green wrote:
> *every* phone would have a "debug board" built in

Aaah, good ideas never die :-) I actually suggested moving the debug
board inside the Neo a long time ago. The reasons why it was shut
down were the size of the FTDI chip, the cost of it, and
size+cost+issues of the USB hub, with issues being that the USB power
budget wouldn't be shared in the normal way.

The idea of putting a second USB connector didn't occur to me back
then. I like the simplicity of that. It would indeed eliminate the
need for the hub chip and simplify the power budget.

You'd then need two upstream ports, but even if you only have one
upstream port, you could just use a small unpowered hub.

A solution involving FTDI+USB connector+glue logic will take a lot
more PCB space than just the NOR. Also, the NOR isn't necessarily
optimal in the sense that, had our NAND better built-in protection
features (like most new NAND chips do), we wouldn't have needed an
extra chip just for that.

So I wouldn't expect us to need a separate memory chip just for
recovery in the future. Instead, the first sector of the memory
chip (maybe in and MCP), be it NOR or NAND, would just contain a
bit of code that locks down the pages needed for recovery and then
proceeds with normal execution.

Unlike our "old-style" NAND, more modern chips, ironically including
our NOR, have the option of write-protecting a page such that it
can't be unprotected by anything but a reset. We didn't use that
feature in our NOR on the grounds of it being simpler and safer to
just do everything in hardware, but perhaps that simplicity was a
bit overestimated.

A software-based protection scheme would also allow us more
flexibility in how we select the boot mode and how we disable the
write protection. E.g., instead of requiring a "hardware key", one
could just disable write protection by, say, holding down AUX and
tapping S-O-S in morse on the touch screen.

So even if we can't have the integrated debug board in future
products, we should be able to have a considerably less cumbersome
recovery option than in GTA02.

If we can pull off the integrated debug board, that would be great,
of course. Getting rid of the fumbly FPC would be a win already.

- Werner




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