Mitsubishi DS907x SIP, DS5000TK manual Timed Access Protected Control Bits ±1

USER'S GUIDE

This code allows the reset of the Watchdog Timer:

MOV	0C7H,#0AAH	; 1st TA Value
MOV	0C7H,#055H	; 2nd TA Value	2 Cycles
SETB	IP.7	; Reset Watchdog Timer	1 Cycle

The Watchdog Timer bit may have been set using ORL IP, #80H which takes two cycles.

This code allows the reset of the Watchdog Timer using a different ap- proach:

MOV	A, #55H	; Setup Acc for fast write
MOV	0C7H, #0AAH	; 1st TA Value
MOV	0C7H, A	; 2nd TA Value	1 Cycle
MOV	A, IP	; Get Current IP	1 Cycle
ORL	A, #80H	; Prepare for fast write	1 Cycle
MOV	IP, A	; Reset Watchdog Timer	1 Cycle

Note that a new value for IP could have been retrieved from any direct register instead of the current IP.

The bits which are write access±protected by the Timed Access function are listed in Table 8±1.

TIMED ACCESS PROTECTED CONTROL BITS Table 8±1

BIT NAME	MICRO VERSION	LOCATION	DESCRIPTION
EWT	All Secure Micro	PCON.2	Enables the Watchdog Timer Reset function
RWT	All Secure Micro	IP.7	Resets the Watchdog Timer count
STOP	All Secure Micro	PCON.1	Stop Mode Enable
POR	All Secure Micro	PCON.6	Power On Reset
PAA	DS5000 series	MCON.1	Partition Address Access bit (protects PA3±0)
PA3±0	DS5001, DS5002	MCON.7±4	Partition Address bits
	series
AE	DS5001, DS5002	RPCTL.4	Access Enable
	series

The Secure Microcontroller family has a variety of con- trol bits that are critical to the correct operation of the processor. Several of these are nonvolatile and will not be altered by a reset. Thus they must be protected from an accidental write by software that has gone out of con- trol. This is a possibility in all microprocessor based sys- tems, especially those in an industrial environment. While the Watchdog Timer will recover from this condi- tion, the critical bits must be protected during the interval before the time±out of the Watchdog Timer.

The Secure Microcontroller family actually has two lev- els of protection for these critical bits. The most critical SFR bits can only be altered using the Bootstrap Load- er. An example is the Range function that determines the total memory. There is no need for an application to modify this bit during normal operation. For those critical bits that might need to be modified during normal opera- tion, the Timed Access procedure protects against an inadvertent write operation.

050396 65/173

66