MPC83xx watchdog reset board dead lock

[Leon Woestenberg <leon.woestenberg@gmail.com>]

Hello,

this is a hardware, even board issue, but I hope to find the right
target audience here.


In our MPC83xx design I would like to prevent dead lock in case where
a field upgrade is performed, i.e. NOR Flash is erased or written, and
the MPC83xx built-in hardware watchdog triggers.

In u-boot the scenario can be easily reproduced by running this
command (WARNING, erases some sectors!) on an MPC8313E-RDB:

erase_wdg=mw.l 0xe0000204 0x10000007 1;mw.w 0xe000020e 0x556c 1;mw.w
0xe000020e 0xaa39 1;erase 1:10-30

This sets up the watchdog to reset soonish, then starts erasing NOR
sectors. Watchdog triggers and resets -> Dead lock.


Most MPC8xxx board designs I have seen suffer from this possible dead lock:
- NOR Flash is put in erase mode or write mode
- Hardware watchdog triggers
- HRESET# is asserted by the processor, during which the configuration
words are read from NOR Flash.

Either
HRESET# is not attached to NOR, NOR stays in erase/write mode and
invalid words will be read -> dead lock

or either:
HRESET# is attached to NOR reset, NOR is reset, but stays in reset as
HRESET# stays asserted.



We have been looking at several solutions hardware wise that reset the
NOR flash on HRESET# going low, but the processors are stubborn,
read the config words only once, than dead lock.

I wonder if there are known-working designs for this.

Regards,
-- 
Leon