Maxim Integrated High-Speed Microcontroller - Manual (Page 155)

High-Speed Microcontroller User’s Guide
Rev: 062210 155 of 176
EXAMPLE: A TRANSIENT CAUSES THE WATCHDOG TO BE DISABLED
TABLE_READ:
C2D2 90 0A 00
MOV DPTR, 0A00H ;LOAD TABLE POINTER
C2D5 79 FF MOV R1, #0FFH ;LOAD COUNTER
C2D7 78 90 MOV R0, #90H ;DESTINATION POINTER
LOOP:
C2D9 E0 MOVX A, @DPTR ;READ DATA BYTE
C2DA F6 MOV @R0, A ;STORE IT IN RAM
C2DB 06 INC R0 ;NEXT TABLE LOCATION
C2DC A3 INC DPTR ;NEXT DATA VALUE
C2DD D9 C2 D9 DJNZ R1, LOOP ;NEXT BYTE OR DONE ?
A transient occurs while the op code is being fetched for the first instruction. The transient causes one bit
of the op code in the first instruction to be read as a 0 instead of 1. The resulting program is what the
microcontroller would actually execute:
TABLE_READ:
C2D2 80 0A 00 SJMP 0BH ;RELATIVE JUMP BY 10 LOCATIONS
C2D5 79 FF MOV R1, #0FFH ;LOAD COUNTER
C2D7 78 90 MOV R0, #90H ;DESTINATION POINTER
LOOP:
C2D9 E0 MOVX A, @DPTR ;READ DATA BYTE
C2DA F6 MOV @R0, A ;STORE IT IN RAM
C2DB 06 INC R0 ;NEXT TABLE LOCATION
C2DC A3 INC DPTR ;NEXT DATA VALUE
C2DD D9 C2 D9 DJNZ R1, LOOP ;NEXT BYTE OR DONE ?
The resulting jump is to address C2DE. This is not even a real op code, but would be treated as such. The
resulting fetch is the value C2 D9. This is the op code for CLR D9h. The bit addressable location D9h
corresponds to the EWT. If the timed-access procedure did not prevent it, this errant instruction would
disable the watchdog. Note that now, the program execution is completely lost. Real op codes are being
replaced by operands, data, and garbage. In the high-speed microcontroller, the watchdog will recover
from this state as soon as it times out since it could not have been disabled in this way.
In the high-speed microcontroller it is very hard to contrive a situation that will accidentally disable the
watchdog. Note that the timed access prevents accidentally writing a bit. It cannot prevent accidentally
calling the correct code that writes a bit. This is much more unlikely however.