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FAQs: Microprocessor supervisors

Posted: 05 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FAQ? supervisor microprocessor?

What is a Supervisor?
A Supervisor is a protection device. It monitors certain system parameters and reacts to protect the system when those parameters go out-of-tolerance. The most common supervisor function is the Reset circuit or Voltage Monitor which monitors the main system supply voltage and resets the system in case the voltage is too high or too low. Other supervisor functions include protection against runaway software, high temperature, and intrusion, to name just a few.

What is a Microprocessor (?P) Supervisor?
The key Microprocessor Supervisory functions are the Reset circuit and the Watchdog. These two functions are contained in most Microprocessor Supervisors.

Reset Circuit:Also known as a Voltage Monitor, it protects the microprocessor (and the system) by monitoring the system power (Vcc) and generating a RESET signal to the ?P (micro) if the system power is out-of-tolerance. If the voltage is too low, the micro can behave unpredictably, and the Reset circuit prevents that.

When Vcc falls below Vref, RESET is asserted. This is called, "Low-Voltage Detect (LVD)." At power-up, the voltage monitor also asserts RESET until Vcc has risen above Vref, then holds it for a short timetypically 100msto ensure the system has stabilized. This is called, "Power-on Reset (POR)." Together, these two functions are called, "POR-LVD," providing yet another synonym for the basic Reset chip.

Watchdog: The Watchdog is the other most common Microprocessor Supervisory function. It is a special timer which will reset the ?P when it times out. Software periodically resets the timer to prevent that. If the software gets hung up or in a runaway condition, it will not reset the timer, so the timer will then reset the micro to restore the system to proper operation.

What is an NVRAM Supervisor?
NVRAM Supervisors turn standard, Low-Power SRAMs into NVRAMs with control and battery back-up functions at significant cost and space savings. The key NVRAM Supervisory functions are the Battery Switchover circuit and the Write Protect circuit.

Battery Switchover: The battery switchover circuit monitors Vcc, and when it falls below the switchover threshold, Vso, switches power to the battery, thereby providing a continuous, uninterrupted supply to the LPSRAM to preserve the data in it.

Write Protect: The Write Protect circuit monitors Vcc, and if it falls below the Power-fail Detect threshold voltage, Vpfd, it gates off access to the LPSRAM.

Since a microprocessor can behave erratically at low voltages, the Write Protect circuit protects the LPSRAM contents from possible corruption as power is failing.

Write Protect and Reset:By bringing the internal Write Protect signal to an output pin, the Write Protect circuit can be used to provide Power-on Reset/Low Voltage Detect, as well as Write Protection.

Many of ST's NVRAM Supervisors and Serial RTCs include a Reset or POR-LVD output.

Push-Button Reset: Many of ST's Reset circuits include reset inputs which can trigger the same Reset output timing as provided by the POR-LVD function. These inputs are suitable for use with external switches to provide a Push-button Reset function, often necessary in systems.

Battery Monitor:Many of ST's Reset, Supervisor, and Serial RTC devices include a battery monitor to inform the system and the user when it's time to replace the battery.

What is a Securitizor?
A Securitizor is a Supervisor that includes ST's new Tamper Detect/RAM Clear function. The M41ST87 is one such device. More are planned.

Tamper Detect and RAM Clear: Applications requiring data protection are perfect candidates for this new supervisory function. Whether Vcc is active, or the system is in battery-backed mode, the Tamper Detect circuit is constantly monitoring its two independent Tamper Detect inputs, looking for any condition which would indicate someone was tampering with the system.

When a Tamper Event occurs, the event time is automatically stored, plus the user can configure up to three other responses:

  1. the circuit can interrupt the processor;

  2. the device's internal RAM can be cleared; and

  3. an external signal can be asserted to clear external RAM.

Tamper Detect options include:

  1. detect high-to-low or low-to-high transition, and

  2. detect the making or breaking of a connection.

The Tamper Detect/RAM Clear function is currently available on the M41ST87.

Early Power-fail Warning: Also known as PFI-PFO (Power-fail In/Power-fail Out), this circuit combines a comparator with a precision reference. It is used to monitor the power supply upstream of the regulator and provide advanced warning that the power is about to fail. By providing a few milliseconds of advanced warning, the system can save any critical parameters to NVRAM and gracefully shut down.

When the unregulated supply, Vunreg, first begins to fall, Vcc will stay steady. Eventually, as Vunreg continues to fall, Vcc will begin to fall. The time between these two events can be used to gracefully shut down the system.

The resistors R1 and R2 determine the threshold voltage, Vpfi-th. When Vunreg drops below Vpfi-th, PFO is asserted to the microprocessor. After Vunreg has continued falling for some time, Vcc will begin to fall. a short time later, as Vcc falls out of tolerance usually about 10 percent below nominal - RESET will be asserted, and the system will shut down abruptly.

"tsave," the time between the assertion of PFO and RESET, can be used to save important status information and parameters. The amount of time available is a function of the system load current and the bulk store capacitance in the regulator. This will vary widely, but can often be 10msecs or more.

Real-Time Clock (RTC): Since Real-Time Clocks require Battery Switchover and Write Protect circuits, it is a natural partnership to include RTCs on Supervisor ICs. Several ST devices include this combination, examples of which are the M41ST85 and the M48T212. Both include Microprocessor Supervisory features, such as Power-on Reset/Low-Voltage Detect, and Watchdog. NVRAM Supervisors that incorporate the RTC are called TIMEKEEPER Supervisors.

Other ST Serial Real-Time Clocks also include Microprocessor Supervisor functions, but without the NVRAM Switchover. The M41ST84 and M41T94 are two such devices which offer POR-LVD and Watchdog.

Alarm: The Alarm function works just like an ordinary alarm clock. The user programs in an alarm time, and when the current time is equal to the alarm time, an interrupt is generated to the micro.

All TIMEKEEPER Supervisors and many Serial RTCs include the Alarm function.

Source: STMicroelectronics

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