AN589| Application Note
Maxim/Dallas > App Notes > MICROPROCESSOR SUPERVISOR CIRCUITS Keywords: cpu supervisors, watchdog timer, chip enable, sense input, cpu supervisor, chip enable gating, early warning signal, watchdog, CPU
Aug 10, 2001
APPLICATION NOTE 589
CPU Supervisors: Frequently Asked Questions
Addresses the function of a CPU supervisor and how they can be applied to monitor a system. Discusses the function of a watchdog timer, chip enable gating pin, as well as a sense input for an early warning signal. The document also focuses on the difference between a reset circuit and a voltage detector and how to choose the proper voltage threshold.
What Is A Watchdog Used For?
The watchdog timer feature found on many CPU Supervisors allows the supervisor to monitor and restart a processor based on its operation. In simplest terms, all the watchdog timer does is restart a clock each time a transition on its input occurs. If the clock times out, then a reset or watchdog status output goes active. The input for the watchdog can be derived from many processor or system signals. Some of the most commonly used include data/address I/Os and Interrupt outputs. In many cases a processor stalls because of a power transient, data, or software error. Monitoring processor operation can allow the supervisor to restart a stalled processor when no actual system problem exists. This provides for cost savings in system service personnel and can reduce down time.
What Is The Difference Between A Reset And A Voltage Detector?
Voltage detectors simply indicate that a voltage is above or below a specific value. They do not provide any timing delays and generally have very limited noise immunity. Resets provide a digital signal to the processor that not only indicates the voltage level but also provides that the reset signal is delayed to allow the voltage to arrive at its nominal value. That delay also allows the power supply and board to fully stabilize prior to starting operation.
What CPU Supervisor Device Tolerance Should Be Used For Best Results?
This is a matter of personal preference as much as engineering necessity. Generally speaking, the reset tolerance should be set below the worst-case operation of the power supply. This means a 5-volt power supply rated at 10% would typically be used with a 5-volt 10% tolerance supervisor. Although a 5% supervisor could be used in this application, the possibility exists that spurious resets could be generated under normal conditions. Also, a 5-volt 15% supervisor might be used to provide for greater noise or actual power conditions at the supervisor location, which are generally going to be worse than the conditions at the power supply output. Consideration of operating voltages for system components is also important. However, because supervisors provide a delay on power-up, in many cases they are less critical in this selection process.
I need A Dual 5V Reset. What Do You Have?
The 1834 can have 5V applied to the 3.3V side in order to have two resets with different trip points. If a resistor divider is used, at least 10mA of current should be available to the input.
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