DS80C320 Tutorial: Timed Access
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.
Programming Tip:
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,#0AAh
MOV TA,#55h
SETB WDCON.1
You can also use the MOV command. So the above example could also be written
in the following manner:
MOV TA,#0AAh
MOV TA,#55h
MOV WDCON,#02h
It is very important to note that the following code would
not work:
MOV TA,#0AAh
MOV TA,#55h
SETB WDCON.0 ;This instruction will take effect
SETB WDCON.1 ;This instruction will NOT take effect
The 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:
MOV TA,#0AAh
MOV TA,#55h
MOV WDCON,#03h
The following bits are protected and, thus, you must execute the two MOV TA instructions before
attempting to modify any of them:
| Protected Bit |
Bit Name |
| EXIF.0 |
BGS |
| WDCON.6 |
POR |
| WDCON.3 |
WDIF |
| WDCON.1 |
EWT |
| WDCON.0 |
RWT |
