watchdog: add sl28cpld watchdog driver

The watchdog timer is part of the sl28cpld management controller. The
watchdog timer usually supervises the bootloader boot-up and if it bites
the failsafe bootloader will be activated. Apart from that it supports
the usual board level reset and one SMARC speciality: driving the
WDT_TIMEOUT# signal.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>

1 files changed, 36 insertions, 17 deletions

diff --git a/doc/board/kontron/sl28.rst b/doc/board/kontron/sl28.rst
index c7b18bed10..c2cdc5e424 100644
--- a/doc/board/kontron/sl28.rst
+++ b/doc/board/kontron/sl28.rst
@@ -35,23 +35,6 @@ The board is fully failsafe, you can't break anything. But because you've
 disabled the builtin watchdog you might have to manually enter failsafe
 mode by asserting the ``FORCE_RECOV#`` line during board reset.
 
-Disable the builtin watchdog
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-- boot into the failsafe bootloader, either by asserting the
-  ``FORCE_RECOV#`` line or if you still have the original bootloader
-  installed you can use the command::
-
-  > wdt dev cpld_watchdog@4a; wdt expire 1
-
-- in the failsafe bootloader use the "sl28 nvm" command to disable
-  the automatic start of the builtin watchdog::
-
-  > sl28 nvm 0008
-
-- power-cycle the board
-
-
 Update image
 ------------
 
@@ -82,6 +65,42 @@ u-boot (yet). But you can use the i2c command to access it.
   > i2c md 4a 3.1 1
 
 
+Builtin watchdog
+----------------
+
+The builtin watchdog will supervise the bootloader startup. If anything
+goes wrong it will reset the board and boot into the failsafe bootloader.
+
+Once the bootloader is started successfully, it will disable the watchdog
+timer.
+
+wdt command flags
+^^^^^^^^^^^^^^^^^
+
+The `wdt start` as well as the `wdt expire` command take a flags argument.
+The supported bitmask is as follows.
+
+| Bit | Description                   |
+| --- | ----------------------------- |
+|   0 | Enable failsafe mode          |
+|   1 | Lock the control register     |
+|   2 | Disable board reset           |
+|   3 | Enable WDT_TIME_OUT# line     |
+
+For example, you can use `wdt expire 1` to issue a reset and boot into the
+failsafe bootloader.
+
+Disable the builtin watchdog
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If for some reason, this isn't a desired behavior, the watchdog can also
+be configured to not be enabled on board reset. It's configuration is saved
+in the non-volatile board configuration bits. To change these you can use
+the `sl28 nvm` command.
+
+For more information on the non-volatile board configuration bits, see the
+following section.
+
 Non-volatile Board Configuration Bits
 -------------------------------------