W79E632A Datasheet, PDF (59/79 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E632A/W79L632A
12. TIMED ACCESS PROTECTION
The W79E(L)632 has several new features, like the Watchdog timer, on-chip ROM size adjustment, wait
state control signal and Power on/fail reset flag, which are crucial to proper operation of the system. If left
unprotected, errant code may write to the Watchdog control bits resulting in incorrect operation and loss
of control. In order to prevent this, the W79E(L)632 has a protection scheme which controls the write
access to critical bits. This protection scheme is done using a timed access.
In this method, the bits which are to be protected have a timed write enable window. A write is successful
only if this window is active, otherwise the write will be discarded. This write enable window is open for 3
machine cycles if certain conditions are met. After 3 machine cycles, this window automatically closes.
The window is opened by writing AAh and immediately 55h to the Timed Access(TA) SFR. This SFR is
located at address C7h. The suggested code for opening the timed access window is
TA REG 0C7h
;define new register TA, located at 0C7h
MOV TA, #0AAh
MOV TA, #055h
When the software writes AAh to the TA SFR, a counter is started. This counter waits for 3 machine
cycles looking for a write of 55h to TA. If the second write (55h) occurs within 3 machine cycles of the
first write (AAh), then the timed access window is opened. It remains open for 3 machine cycles, during
which the user may write to the protected bits. Once the window closes the procedure must be repeated
to access the other protected bits.
Examples of Timed Assessing are shown below.
Example 1: Valid access
MOV TA, #0AAh 3 M/C
MOV TA, #055h
3 M/C
MOV WDCON, #00h 3 M/C
Note: M/C = Machine Cycles
Example 2: Valid access
MOV
MOV
NOP
SETB
TA, #0AAh
TA, #055h
EWT
3 M/C
3 M/C
1 M/C
2 M/C
Example 3: Valid access
MOV TA, #0Aah 3 M/C
MOV TA, #055h
3 M/C
ORL WDCON, #00000010B 3M/C
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Publication Release Date: Nov. 11, 2009
Revision A10