Toshiba TLCS-90 Series Data Book page 328

TOSHIBA TMP90C840
(i)
WDT enable/ disable control (WDTE)
WDTE is initialized to "I" by resetting, which enables the watchdog
timer function.
To disab Ie the func t ion, the bi t shou ld be cleared to
"a"
and the
disable code "BlH" should be written into the WDCR register. By
using this dual procedure, it becomes hard to disable the WDT even
if the malfunction occurs.
The disable state can be returned to the enable state by setting the
WOTE bit to "I".
MPU90-l30