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Datasheet: AN574 (Maxim Integrated Products)

App Note 101: Using The Secure Microcontroller Watchdog Timer

 

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Maxim Integrated Products
MICROCONTROLLERS
Aug 10, 2001
App Note 101: Using the Secure Microcontroller
Watchdog Timer
The Secure Microcontroller Family (DS5000FP, DS5001FP, DS5002FP, and
associated modules) integrate an internal watchdog timer to prevent code
execution errors. The watchdog timer uses the high precision crystal oscillator that
is also used by the microcontroller. This eliminates the need for a RC oscillator
while providing greater accuracy.
Overview
Microcontrollers are often used in harsh environments where power supply transients, electromagnetic
interference (EMI), and electrostatic discharge (ESD) are abundant. Program corruption caused by bus
corruption and electromagnetic discharges can cause a microprocessor to execute erroneous
instructions. In these environments, a watchdog timer is a useful peripheral that can help catch and
reset a microcontroller that has gone "out of control."
A watchdog timer is a simple countdown timer which is used to reset a microprocessor after a specific
interval of time. In a properly operating system, software will periodically "pet" or restart the watchdog
timer. After being restarted, the watchdog will begin timing another predetermined interval. When
software or the device is not functioning correctly, software will not restart the watchdog timer before it
times out. When the watchdog timer times out, it will cause a reset of the microcontroller. If the system
software has been designed correctly and there has been no hardware failure, the reset will cause the
system to operate properly again. The reset condition must be a "safe" state. For instance, it would not
be wise to have the reset state of a magnetic stripe card reader enabling the write head.
Many systems have been designed using an external watchdog timer. The Secure Microcontroller family
eliminates the need for external components by incorporating an internal watchdog timer. By moving the
watchdog timer inside the microcontroller, the number of devices in the system is reduced, increasing
the overall system reliability. The watchdog timer can take advantage of the high-precision crystal
oscillator used by the microcontroller, rather than the imprecise RC oscillator used by most independent
watchdog timers. The operation of the watchdog timer is independent of the microcontroller, unless
specifically addressed via the Timed Access procedure. The possibility of an out-of-control
microcontroller accidentally disabling the watchdog timer is less than 1 in 7.2 X 10
16
. This application
note describes the features and use of the Secure Microcontroller's watchdog timer.
General Use of a Watchdog Timer
The primary application of a watchdog timer is as a system monitor to detect and reset an "out of
control" microprocessor. When program execution goes awry it will not properly execute the code that
restarts the watchdog. In such a case the watchdog timer will timeout and cause a microcontroller reset.
In a properly designed system, the reset will correct the error.
Regardless of how capable a watchdog timer might be, there are certain failures that cannot be
corrected by a reset. For instance, a watchdog timer cannot prevent or detect the corruption of data
memory. Unless corruption of data affects program flow, or some extra measures are taken, data
corruption will not cause a watchdog timeout. Of course, self-diagnostic software can be written in such
a way as to make restarting the watchdog contingent on verification of data memory. While many
applications implement such a data verification scheme, it is beyond the scope of this document.
It should be remembered that a watchdog timer cannot detect a fault instantaneously. By definition, the
watchdog timer must reach the end of its timeout interval before it resets the processor. The system
designer should be aware of the maximum time interval that can occur between the execution of a bad
instruction and the watchdog timer reset.
Placing the Restart Instructions
In the Secure Microcontroller family the watchdog timer is driven by the main system clock. The timeout
interval is fixed at 122,800 machine cycles (1,473,600 external clock cycles). When the timeout is
reached a reset will occur. Table 1 shows the reset time intervals associated with different crystal
frequencies.
Table 1. Watchdog Timeout Intervals
Clock Frequency
Timeout Interval
16.0000 MHz
92 ms
14.7456 MHz
100 ms
11.0592 MHz
133 ms
7.73280 MHz
191 ms
5.52960 MHz
266 ms
1.84320 MHz
800 ms
A primary concern is the location of the watchdog timer reset command (setting the RWT bit) in the
software. The most desirable approach is to have a single location within the main loop of the system
software that restarts the watchdog timer periodically. The time required to pass through the main
program loop must be less than the timeout interval or the device will reset itself during normal
operation. In some systems, however, the program flow is not linear enough to allow the placement of a
single watchdog timer reset function. Multiple reset functions should be placed in the code,
corresponding to the longest software paths.
Often a system will need to know if a watchdog timer reset has occurred. The WTR bit (PCON.4) will be
set whenever this occurs, and software can test for this early in the reset sequence if a system fault has
occurred. If so, the system may decide to go into a "safe" mode and alert the user to an error condition.
Watchdog Reset Example
A short program illustrating the initialization and basic function of the watchdog timer is shown below. It
illustrates the Timed Access feature, which prevents the accidental modification of the watchdog control
bits. A Timed Access operation is a sequence of steps that must be executed together, in sequence;
otherwise, the access fails. The example program shows the timed access being used for restarting the
watchdog and enabling its reset. Further details on Timed Access operation may be found in the Secure
Microcontroller User's Guide. The watchdog timer bits that are protected by the Timed Access
procedure are the Enable Watchdog Timer Reset (EWT;PCON.2) and Restart Watchdog
Timer(RWT;IP.7) bits.
; WD_RST.ASM Program
;
; This program demonstrates the use of the watchdog timer.
; When running, the program counts on port 1 to indicate the device is
; running and periodically resetting the watchdog timer. After counting
; to 16, it stops resetting the watchdog timer, simulating a system fault.
;
; The program begins by checking to see if the WTR bit is set. If so, the
; reset was caused by the watchdog timer, and the program will execute
; the FAULT subroutine. Port 1 is set to F0h to indicate this condition.
; If the WTR bit is not set, the reset was caused by another source and
; execution should continue normally.
;************************************
RWT EQU 0BFh ;Reset Watchdog Timer bit
TA EQU 0C7h ;Timed Access Register
PCON EQU 87h ;Power Control Register
ACC EQU 0E0h ;Accumulator
P1 EQU 090h ;Port 1
ORG 00h ;Reset Vector
SJMP START

;************************************
ORG 080h ;Program starts at 80h in this example.
START: MOV A, PCON ;If reset was caused by watchdog timeout,
JB ACC.4, FAULT ; (WTR bit =1) execute fault subroutine.

;**********************************
;A normal power-on reset has occurred. Start initialization sequence.
MOV P1, #00h ;Clear P1 to signal start of program.

;Watchdog timer initialization sequence
MOV TA, #0AAh ;First restart the Watchdog timer
MOV TA, #055h ; using timed
SETB RWT ; access.

MOV TA, #0AAh ;Next enable the Watchdog timer reset
MOV TA, #055h ; function using timed
ORL PCON, #04h ; access.

;**********************************
;Main program loop. This simulates a program that is operating
; correctly and then goes awry. After the program has counted to 16
; on Port 1 it will skip over the watchdog timer reset function. This
; will simulate a fault and allow the watchdog timer reset to be asserted.
;**********************************
MAIN: MOV R1, #0FFh ;Create a delay loop. This simulates
LOOP1: MOV R2, #0FFh ; a device actually "doing something."
LOOP2: JB P1.4, SKIP_WD_RST ;Have we been through loop 16 times?
MOV TA, #0AAh ;Watchdog timer reset. In a user application it
MOV TA, #055h ; should be placed at strategic locations
SETB RWT ; where it will be executed periodically.

SKIP_WD_RST:
DJNZ R2, LOOP2
JNZ R1, LOOP1

INC P1 ;Increment counter.
SJMP MAIN ;Go back to main program loop.
;************************************
;Watchdog timeout fault. This subroutine would normally have special
; routines to be executed in the event of a system fault. In this example,
; it disables the watchdog reset and sets Port 1 to F0h to indicate a fault.
; In a real application, this routine could either clear the fault and
; restart the software, or signal a fault and halt further operation.
;************************************
FAULT: MOV P1, #0F0h ;Signal a fault
MOV TA, #0AAh ;Disable watchdog timer reset
MOV TA, #55h ; using timed
ANL PCON, #0FBh ; access.
SJMP $ ;Halt further operation.
Summary
A number of considerations must go into any design that uses a watchdog as a monitor. Once the
timeout period is determined, the system software must be analyzed to determine where to locate the
watchdog restart instructions. For an effective design, the number of watchdog restarts should be kept to
a minimum, and some consideration should be given to the likelihood of incorrectly executing a restart.
As mentioned previously, some system software is too convoluted or data-dependent to ensure that all
software flow paths are covered by a watchdog restart. This may dictate that a self-diagnostic software
approach might be required. If there is an expected failure mechanism such as a periodic EMI burst or
power supply glitch, the watchdog timeout should consider this period.
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