w79e532 Winbond Electronics Corp America, w79e532 Datasheet - Page 56

• Manufacturer Part Number: w79e532
• Description: 8-bit Microcontroller
• Manufacturer: Winbond Electronics Corp America

The SADDR and SADEN SFRs are located at address A9h and B9h respectively. On reset, these two
SFRs are initialized to 00h. This results in Given Address and Broadcast Address being set as XXXX
XXXX(i.e. all bits don't care). This effectively removes the multiprocessor communications feature,
since any selectivity is disabled.
13. TIMED ACCESS PROTECTION
The W79E(L)532 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)532 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
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
Example 2: Valid access
Example 3: Valid access
TA
MOV
MOV
MOV
MOV
MOV
MOV
MOV
NOP
SETB EWT
MOV
MOV
ORL
REG
TA, #0AAh
TA, #055h
TA, #0AAh
TA, #055h
WDCON, #00h 3 M/C
TA, #0AAh
TA, #055h
TA, #0Aah
TA, #055h
WDCON, #00000010B 3M/C
0C7h
3 M/C
3 M/C
3 M/C
1 M/C
2 M/C
3 M/C
3 M/C
3 M/C
;define new register TA, located at 0C7h
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Note: M/C = Machine Cycles
W79E532/W79L532