There are a number of bits that the 80C320 considers vital to correct operation. If any of these bits were to be inadvertently changed by a runaway (crashed) CPU, system reliability could suffer. In order to insure that these bits are not accidentally modified, a "timed access protection system" was created to protect them.
The "Timed Accessed" procedure explained in this page also applies to other Dallas Semiconductor high-speed microcontrollers.
The timed access protection system requires that the following instructions be executed immediately prior to modifying a protected bit (note that the Timed Access
TA SFR is at C7h):
Executing these two instructions (in the specified order) opens a 3-cycle window during which a protected bit can be modified. If an attempt is made to modify a protected bit without these two instructions being executed immediately beforehand, the modification will not take effect.
For example, if you wish to enable the Watchdog by setting EWT (WDCON.1), you would need to execute the following code:
MOV TA,#0AAhYou can also use the MOV command. So the above example could also be written in the following manner:
It is very important to note that the following code would
MOV TA,#0AAhThe above code segment will not work because the timed access window opened by the first two instructions is only open for 3 instruction cycles. The first
SETB instruction requires 2 cycles to execute, so the timed access window will be closed while the second SETB instruction is still executing-- thus it will not take effect. If you wish to set both
WDCON.0 and WDCON.1, the most efficient way to do it is, again, with a MOV command. For example:
SETB WDCON.0 ;This instruction will take effect
SETB WDCON.1 ;This instruction will NOT take effect
The following bits are protected and, thus, you must execute the two
MOV TA instructions before attempting to modify any of them: