DS89C420-QCL Datasheet, PDF (124/139 Pages) Maxim Integrated Products – Ultra-High-Speed Flash Microcontroller User’s Guide

Ultra-High-Speed Flash
Microcontroller User’s Guide
VALID TIMED-ACCESS PROCEDURES
Three Memory Cycles
MOV 0C7h, #0AAh
Three Memory Cycles
MOV 0C7h, #55h
Three Memory Cycles
SETB EWT
Three Memory Cycles
MOV 0C7h, #0AAh
Three Memory Cycles
MOV 0C7h, #55h
One Memory Cycle
NOP
Two Memory Cycles
SETB EWT
Three Memory Cycles
MOV 0C7h, #0AAh
Three Memory Cycles
MOV 0C7h, #55h
Three Memory Cycles
MOV WDCON, #02h
INVALID TIMED-ACCESS PROCEDURES
Three Memory Cycles
One Memory Cycle
Three Memory Cycles Two Memory Cycles
MOV 0C7h, #0AAh
NOP
MOV 0C7h, #55H
SETB EWT
*Second write to TA register does not occur within 3 cycles of first write.
Three Memory Cycles
Three Memory Cycles
One Memory Cycle
Three Memory Cycles
MOV 0C7h, #0AAh
MOV 0C7h, #55H
NOP
MOV WDCON, #02h
*Modification of protected bit did not occur with 3 cycles of second write to TA register.
Three Memory Cycles
Three Memory Cycles
Two Memory Cycles
Two Memory Cycles
MOV 0C7h, #0AAh
MOV 0C7h, #55h
SETB EWT
SETB EWT
*Modification of second protected bit did not complete within 3 cycles of second write to
TA register.
Figure 13-1. Timed-Access Examples
TIMED-ACCESS PROTECTS WATCHDOG
Any microcontroller-based system can be faced with environmental conditions that are beyond its designed abilities. These include
external signal transients due to component failure, fluctuating power conditions, massive electrostatic discharge (ESD), and other
unexpected system events. When a microcontroller is exposed to such conditions, program execution can become corrupted. The
ultra-high-speed microcontroller incorporates a watchdog timer that can initiate a reset to recover from these conditions. The primary
function of the timed-access feature is to protect against accidental disabling of the watchdog timer by an “out-of-control” device. This
allows the watchdog timer to reset the system in the event of program execution failure.
The following hypothetical example demonstrates how a single bit change can corrupt program execution. The timed-access proce-
dure protects against an accidental write to the EWT bit by the errant code, allowing the watchdog timer reset function to reset the
device. While this is a purely fictitious example, it illustrates how the watchdog timer and timed-access feature allow the ultra-high-
speed microcontroller to minimize the effect of accidental code corruption. Note: Timed access is not optional and must be support-
ed if the protected bits are used. This example helps explain the category of problem that the timed access prevents.
EXAMPLE: A TRANSIENT CAUSES THE WATCHDOG TO BE DISABLED:
TABLE_READ:
C2D2 90 0A 00
C2D5 79 FF
C2D7 78 90
C2D9
C2DA
C2DB
C2DC
C2DD
E0
F6
06
A3
D9 C2 D9
MOV
MOV
MOV
LOOP:
MOVX
MOV
INC
INC
DJNZ
DPTR, 0A00H
R1, #0FFH
R0, #90H
A, @DPTR
@R0, A
R0
DPTR
R1, LOOP
;LOAD TABLE POINTER
;LOAD COUNTER
;DESTINATION POINTER
;READ DATA BYTE
;STORE IT IN RAM
;NEXT TABLE LOCATION
;NEXT DATA VALUE
;NEXT BYTE OR DONE ?
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