Test mode programmable reset for a watchdog timer

Details Test mode programmable reset for a watchdog timer Test mode programmable reset for a watchdog timer

1998-10-31

2001-07-10

Ray, Gopal C. (Department: 2181)

Error detection/correction and fault detection/recovery

Data processing system error or fault handling

Reliability and availability

C714S025000

Reexamination Certificate

active

06260162

ABSTRACT:

SPECIFICATION
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to test support for processor-oriented devices, and more particularly to a test mode programmable reset for a watchdog timer.
2. Description of the Related Art
MICROCONTROLLERS
As technology advances, computer system components are providing specific services which previously were offered by a microprocessor or the computer system as a whole. A centerpiece of this advancing technology is known as a microcontroller, or embedded controller, which in effect is a microprocessor as used in a personal computer, but with a great deal of additional functionality combined onto the same monolithic semiconductor substrate (i.e., chip). In a typical personal computer, the microprocessor performs the basic computing functions, but other integrated circuits perform functions such as communicating over a network, controlling the computer memory, and providing input/output with the user.
In a typical microcontroller, many of these functions are embedded within the integrated circuit chip itself. A typical microcontroller, such as the Am186EM or Am186ES by Advanced Micro Devices, Inc., of Sunnyvale, Calif., not only includes a core microprocessor, but further includes a memory controller, a direct memory access (DMA) controller, an interrupt controller, and both asynchronous and synchronous serial interfaces. In computer systems, these devices are typically implemented as separate integrated circuits, requiring a larger area and increasing the size of the product. By embedding these functions within a single chip, size is dramatically reduced, often important in consumer products.
From a consumer products designer's viewpoint, often the particular combination of added features make a particular microcontroller attractive for a given application. Many microcontrollers are available that use the standard 80×86 microprocessor instructions, allowing for software to be easily developed for such microcontrollers. Because of the similar execution unit instruction sets, the added features often become principal differentiating criteria between particular microcontrollers.
In implementing microcontrollers in embedded systems, another common requirement or desirable feature is the reduction of the bandwidth needed by any particular portion of the microcontroller in negotiating with other portions. For example, the core of a microcontroller is the execution unit, which is essentially a microprocessor core. An execution unit should be free to perform the programmed task to which it is dedicated, rather than spending time waiting on other units within the microcontroller.
WATCHDOG TIMERS
An embedded system or other processor-oriented device such as a microcontroller-based device frequently provides a watchdog timer for permitting recovery or escape from software malfunctions. When a watchdog timer is enabled, the watchdog timer requires periodic refreshing; otherwise, after a programmed timeout period, the watchdog timer generates a watchdog timeout. Responsive to a watchdog timeout, a microcontroller is reset to restore control of a runaway software process.
In the past, a watchdog timer of the microcontroller has provided an internal reset signal for resetting the microcontroller. More recently, however, a microcontroller has also provided an external reset signal for resetting external devices (i.e., the entire system) after a watchdog timer timeout. The internal reset signal has been utilized in generating the external reset signal.
For a microcontroller providing an external reset signal, both an internal reset signal and an external reset signal may be asserted during a watchdog timer reset. In normal operation, the microcontroller is held in reset after a watchdog timer timeout for a period of time sufficiently long for all potential external devices to be reset. Since a watchdog timer acts as a fail-safe device, the duration of the watchdog timer reset has not been programmable during normal operation.
SUMMARY OF THE INVENTION
Briefly, a processor-oriented device (e.g., microcontroller) according to the present invention provides a watchdog timer having a test mode programmable reset. When the device is placed in a test mode by pulling a test mode hardware pin during a reset of the timer and then an appropriate write key is provided to the timer, a watchdog timer reset count is writeable, allowing for a programmable duration for a watchdog timer reset. In a disclosed embodiment, the watchdog timer reset count is a reset duration value maintained by a watchdog timer reset counter. The watchdog timer reset counter is provided in a watchdog timer reset control block of the watchdog timer and is programmed by a watchdog timer control register. Writes to the watchdog timer reset count are enabled in a test mode by watchdog timer reset write enable logic. In a disclosed embodiment, the watchdog timer reset write enable logic is a key detect logic block. The watchdog timer reset write enable logic receives a test mode signal from watchdog timer test mode enable logic of the device for indicating detection of the test mode. The watchdog timer reset write enable logic also receives a write key for granting write access to the watchdog timer reset count in a test mode. The watchdog timer test mode enable logic enables the test mode via pulling of the test mode hardware pin. The watchdog timer thus uniquely provides a hardware protection mechanism and software protection mechanism for maintaining the integrity of the watchdog timer during a test mode programmable reset. In the past, a long watchdog timer reset time has been expensive during production testing of a microcontroller-based device and prohibitive during simulation of a microcontroller-based device.


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Advanced Micro Devices, Inc. “Am186™ER and Am188™ER Microcontrollers User's Manual”, Copyright 1998, pp. 8-1 through 8-3.
Advanced Micro Devices, Inc. “Am186™ED/EDLV”, May 1997, pp. 1-2 and 45, Publication&pgr;21336, Rev: A Amendment/0.
Advanced Micro Devices, Inc. “Am186™ES and Am188™ES”, Jan. 1996, pp. 1-3,54, 61 and 62, Publication&pgr;20002, Rev. A Amendment/0.
Microchip Technology, Inc., “Microchip PIC16/17 Microcontroller Data Book”, May 1995, pp. DS00158A, DS00158A-p. ii, DS30412A-pp. 2-825, 2-921, 2-925, 2-926, 2-929 and 2-936.

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