[Soekris] hardware watchdog again...

Soren Kristensen soren at soekris.com
Fri Jul 19 03:29:35 UTC 2002

Hi Jasper & Everybody.

Jasper Wallace wrote:
> I've been pokeing at the watchdog registers using ew/dw in the bios, and i
> wrote enough of a driver for netbsd to be able to test it[1], but no matter
> what i try i can't get it to do anything - so has anyone got the watchdog
> driver to react?
> I've tried on both a Net4501 and a Net4521...

I've been planning to make a document about it, but never got around to
it. So here is the quick version:

There is a bug in the AMD ElanSC520 processor rev A1 that indirectly
affect the Watchdog timer. If the GP bus echo mode is enabled (bit 0 in
MMCR:0C00, see page 10-2 in the SC520 register set manual), writing to
the watchdog timer control register (MMCR:0CB0) is blocked.

The net4501 BIOS set the GP echo mode bit to fix another bug in the rev
A1 chips regarding to a pll noise problem. The recommended fix is to
read and save the GP echo mode register, clear the echo mode bit (write
0), then access the watchdog register, and when done, write back the
saved GP echo mode value.

Boards using the rev B0 do not have that problem, but due to long
leadtimes on the ElanSC520 I've been using A1's that I could get from
different stocks.

Best Regards,

Soren Kristensen

Soekris Engineering
19185 Taylor Avenue
Morgan Hill, CA 95037-2714

Phone (408)782-2307, Fax (408)782-2317

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