DE29623692U1 - Micro-control device with brown-out detection - Google Patents

Description

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Reference number: M80160EPGM

MICROCONTROL DEVICE WITH BROWN-OUT DETECTION

Applicant:

Microchip Technology Incorporated 2355 W Chandler Boulevard Chandler, AZ 85027 USA

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MICROCONTROL DEVICE WITH BROWN-OUT DETECTION

Background of the invention

The present invention relates generally to semiconductor devices and, more particularly, to semiconductor microprocessors or microcontrollers having low power protection.

Low supply voltage on a semiconductor microcontroller chip (device), i.e. a supply voltage that falls below the minimum acceptable value or level for proper operation of the chip or end system (i.e., necessary to avoid malfunction), can be particularly severe for sensitive applications such as an automotive anti-lock braking system. For this reason, microcontroller chips have typically been provided with "brown-out protection", i.e. protection against not a complete loss of power, but a reduction in the power supply level that is large enough to cause malfunction of the circuit or system. Applications requiring brown-out protection are generally systems with a controlled power supply and without battery operation.

In theory, brown-out protection should reset the microcontroller chip or system any time Vdd falls below the minimum acceptable value. However, this minimum value V _D d depends on the operating frequency of the device or other system constraints. For example, the critical value may be 4.0 volts (V) at 20 megahertz (MHz) operation and only 1.8 V at 4 MHz.

For this and other reasons, brown-out requirements may be qualified by market considerations, i.e., by the specific application in which the device is to be used. Applications requiring brown-out protection primarily fall into many application categories, such as instrumentation, industrial control, and automotive. Operational (plugged-in) instruments encounter brown-out situations during line voltage dips. Peripheral voltage-sensitive elements will malfunction at this time, even though the instrument is not even being used for its primary function.

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must have been switched on. Automotive electronics encounter brown-out situations when the ignition is operated. For example, the car radio may be switched on by the ignition key only because the starter is engaged, causing a dip in the power supply. The affected devices must be reset or left in an unstable state.

Brown-out does not occur in low voltage battery applications where power consumption, operating frequency and noise are at a relative minimum.

One can then define brown-out as the situation where Vdd falls to or below a predetermined voltage threshold level. Since microcontrollers operate at high speeds, it is important to distinguish a true brown-out from normal I/O switching noise, which may last for, say, 200 nanoseconds (ns). Under the above circumstances, for example, for a 5-volt supply, the brown-out detection should respond to reset the chip if Vdd falls below a level of 4.0 V for more than 200 ns.

Problems arise, however, in situations of relatively shallow dips in the supply voltage below the specified voltage threshold level, whether as an isolated slip or a sequence of slight slips of V _D d below and then back above the threshold. In such cases, whether due to noise, I/O switching, or other line disturbances, the brown-out detection circuit typically initiates an immediate reset of the microcontroller chip at the first opportunity and, if no further recurrence occurs within a set interval, restores operation of the chip immediately after the set interval has elapsed. This, of course, causes an interruption in the operation of the microcontroller if there is no need for a reset. Also, if Vdd falls to the trigger voltage and remains there for an extended period of time, the circuit may oscillate with accompanying unstable operation.

It is a primary object of the present invention to provide a brown-out detection circuit capable of analyzing the nature of supply voltage dips and providing predetermined criteria and hysteresis for determining whether or not

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Reset should be initiated each time the supply voltage drops below a threshold level.

Ideally, in microcontroller applications, brown-out protection should reset the microcontroller whenever the supply voltage falls below a predetermined value. Brown-out typically occurs in applications where the microcontroller is used with a controlled power supply but not a battery, so the voltage width is the supply system voltage +/- a delta value.

The present invention uses a brown-out detection and correction circuit fabricated in three sections in CMOS as one section of the microcontroller chip. One section of the brown-out circuit is a voltage reference circuit that establishes a precise voltage reference. A second section of the circuit is a dual comparator circuit that has the reference voltage as one input and a filtered power supply sample level as another input. The dual comparator is responsible for circuit hysteresis and keeps the circuit from oscillating. The third section has a signal filter and supply tracking mechanism that are used to sample the supply level and determine if the brown-out is of the type that causes a reset.

According to the invention, this" final section of the brown-out circuit has a built-in sensor that serves to filter out the depth of the V _DD spikes with frequency. If a spike is below the specified threshold level and very deep, the brown-out detection circuit will cause the chip to reset immediately, which is subject to the time-out interval of the start-up timer. On the other hand, if the spike is relatively shallow or at a very high frequency, the brown-out detection circuit will respond more slowly and consequently may not reset the chip. This is highly desirable in an application because it eliminates device reset as a consequence of noise or other minor disturbances alone. Essentially, the circuit filters out normal noise, but responds quickly to trigger the reset if an actual brown-out condition is detected.

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Once reset, the circuit uses hysteresis to eliminate "oscillations" and maintains the device in a reset state until the power supply returns to a normal operating voltage level, at which time a power ramp-up timer is invoked to maintain the reset state for a set time interval. Hysteresis greatly assists the dual comparator in distinguishing a true brown-out event from normal noise or other mild disturbances and is accomplished by dual sampling the power supply and establishing a pair of trip points for comparison with portions of the supply voltage level.

In a preferred embodiment, the microcontroller device is processed in an integrated circuit (IC) of a semiconductor chip for controlling an external system with which the device is to be installed in circuit. The microcontroller comprises a central processing unit (CPU), a program memory for storing program instructions to be implemented by the CPU, a data memory for storing data comprising data pertaining to parameters of the external system to be controlled by operations of the CPU in accordance with the instructions, and various peripheral devices. A brown-out protection circuit monitors the supply voltage level of the chip relative to a ground reference level and is operative to reset the device to protect it against malfunction in the event of a reduction in the difference between the supply voltage and the ground reference level to a value less than a predetermined threshold operating voltage level. The reset causes the operation of the device to be halted while maintaining the status quo of implementation of program instructions by the CPU and data stored in the data memory as they were at the time the reset occurred.

A discriminating means of the brown-out protection circuit distinguishes between a reduction of the aforementioned voltage difference, which is representative of a brown-out event, justifying calling a reset of the device, and a reduction which is representative of a simple transient voltage oscillation, which generally occurs during device operation, which does not justify calling a reset of the device. Consequently, malfunctions and unnecessary resets of the device during the performance of its control function are

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the external system. The discrimination means includes filter means for delaying the response of the brown-out protection circuit for each reduction representative of the transient voltage fluctuations generally occurring, so that a transient voltage fluctuation may occur before a normal device operating voltage level is restored.

The filter means includes sampling filters that tap a pair of samples from the chip's supply voltage for establishing when a reset should be invoked in response to fluctuations in the supply voltage relative to the lower of the sampling voltages and when the invoked reset should be terminated in response to fluctuations in the supply voltage relative to the higher of the sampling voltages. The dual sampling also provides hysteresis means coupled to the sampling filters and adapted to be set to a value related to the pair of samples to remove oscillations in the circuit response. A power-up timer is responsive to a fluctuation in the supply voltage at a second time at which the other sample establishes when a reset should be terminated to maintain the device in the reset state for a predetermined time interval and thereafter restores device operation to a point corresponding to the status quo at which the reset was initially invoked.

The above and other objects, features, aspects and attendant advantages of the invention will become more apparent when the following detailed description of the best mode presently contemplated for carrying out the invention is considered in conjunction with the accompanying drawings, in which:

Fig. 1 is a simplified block diagram of a microcontroller chip with certain peripheral components including brown-out detection;

Fig. 2 is a simplified block diagram of a brown-out detection circuit for use in the microcontroller of Fig. 1, according to the present invention;

3A and 3B are graphs illustrating the internal operation of the current monitoring circuit in the microcontroller of FIG. 1;

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Fig. 4 is a curve illustrating a deep V _D d drop with immediate reset;

5A and 5B are simplified block diagrams and a somewhat more detailed schematic circuit diagram, respectively, of a signal filter and a sampling circuit used in the brown-out detection of the invention;

Fig. 6 is a curve representing normal ground noise without reset; and

Fig. 7 is a curve illustrating an increase in the ground reference level causing a reset.

Referring to Figure 1, an exemplary microcontroller device embodying the principles of brown-out protection according to the present invention includes a central processing unit (CPU) 10, a program memory 12, a data memory 13, and various peripherals shown as block 15. Power is supplied to the system from a source designated Vdd. A power monitor 16 or brown-out detector is connected to the power supply to estimate the instantaneous level of Vdd and, if it falls below a predetermined minimum acceptable value, to detect this situation and subsequently reset the device. As pointed out above, the level of Vdd is dependent in part on the frequency of the I/O switching characteristics of the device.

The current monitor 16 is therefore adapted to respond to a situation where Vdd falls below the designated minimum acceptable level for a predetermined time interval dependent upon normally acceptable noise frequencies. Although brown-out detection should reset the chip in these circumstances, it is important to avoid a reset where there are disturbances in _Vdd associated with noise or 1/0 switching that temporarily drive the voltage level below and then back to or above this threshold. The current monitor is particularly designed to delay any reset of the microcontroller that may cause unnecessary disruption or instability of system operations.

As shown in Fig. 2, the current monitoring device is a brown-out detection and correction circuit comprising three sections, one of which is a precise

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reference voltage generator 20. The precise reference voltage is set to a "stable" output voltage which is applied to a comparator circuit 21. The comparator and associated logic circuits are designed to compare the precise reference output voltage of generator 20 with a balanced sample of the instantaneous level of V _D d at any given time. However, the comparison must be undertaken and accomplished in such a way as to avoid the aforementioned unnecessary and potentially error-causing interruptions or oscillations in system operation. This is particularly important in applications where personal injury or property damage may be a consequence of such interruption or instability, such as in anti-lock braking systems (ABS) in automobiles.

A current sampling and filtering circuit 23 is employed to measure and periodically sample the VoD level, but also to filter out spikes that bring Vdd below the reference level and to perform the filtering function according to the I/O switching frequency characteristics of the microcontroller operation. In operation, the comparator logic of the circuit 21 responds to the deviation from the reference voltage to reset the microcontroller with an output Rl based on a pair of sampling voltages Vl and V2 of the circuit 23 in the event of a sharp drop (negative spike) in Vdd occurring during a voltage sampling. A circuit 24 to which the output Rl is applied comprises a current ramp-up timer with a time-out interval selected as necessary for system stabilization. In cases where only a flat spike or a series of spikes occurring at a high frequency (relative to the microcontroller I/O characteristics) is detected, the filter function provided by circuit 23 produces a variable slower response to voltages Vl and V2 so that triggering a reset is much less likely than in the sudden deep spike circumstance.

Some examples of this operation are shown in Figs. 3A and 3B. For oscillation protection, a detection range is selected with upper and lower voltage threshold limits Vl and V2, each based on a delta difference from a threshold level shifted by a certain voltage from the normal voltage level of Vdd. A reference voltage V _re f is used for comparison. For example, for a 5 V supply, an allowable drop may be 1.0 V with a delta of +/- 0.2 V. Vl is then selected to cross V _re f when Vdd ⁼ 4.2 V, and V2 is selected to cross V _re f

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cross when V _D d = 3.8 V. Based on normal high frequency noise characteristics for a microcontroller, the predetermined delay time interval of 200 ns to trigger a chip reset is imposed by the filter starting from the time when V _DD falls below V _ref until the time when Vl falls below V _ref .

The chip is then driven into reset and is held there until V _D d returns to a level indicative of current restoration. At this time, the current run-up timer of the brown-out detection circuit is called to hold the chip in the reset state for a time interval of 72 milliseconds (ms). In the example of Fig. 3A, this event is measured from the time at which Vdd reaches a level such that V2 again exceeds V _ref .

On the other hand, the example of Fig. 3B shows that the Vl /V2 delay introduced by the filter causes fast disturbances to be ignored. In this case, the chip is not reset.

Under the circumstances shown in Fig. 4, a relatively deep spike in Vdd results in an immediate reset of the microcontroller chip. The reset condition is then maintained for a time-out interval of 72 ms after which the current is restored to a level corresponding to that of Fig. 3A where V2 exceeds _Vref .

The filter network of the sampling and filtering circuit 23 is shown in block diagram form in Fig. 5A. A pair of supply filters 30 and 31 provide the levels Vl and V2 to the comparator circuit 21 (Fig. 2), respectively. The V _D d supply voltage is used relative to the ground reference level by the supply filter 30 to tap the voltage level Vl, and another voltage level V2 is tapped from the combination of the supply filter 31 and the hysteresis filter 33. In the circuit diagram of Fig. 5B, the voltage Vl is tapped between the electrical resistances 36 and 37 of the three resistors connected in series between V _D d and ground (the third resistor is 38) and a circuit node of the hysteresis filter at a capacitive ground resistor 39. The voltage level V2 is taken from a node between the resistors 37 and 38, from another capacitive grounding resistor 40 of the hysteresis filter. The circuits in which the two capacitive resistors are arranged comprise a pair of matching RC filters. It

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it is then obvious that the voltages Vl and V2 vary with each fluctuation of VdB*.

Because the brown-out detector (current monitor) 16 monitors the power supply Vdd, it also automatically monitors the ground reference level, since voltages are always measured between two nodes. This may be undesirable, since normal ground noise can cause the microcontroller to reset, but the sampling and filtering circuit 23 prevents this, since the appropriate RC filters connected to ground serve, among other things, to filter out normal ground noise. In this way, the same filtering mechanism as for the supply side is employed on the ground side (i.e., the circuit of Fig. 5A - or 5B - affects both sides in the same way), so that the ground level is also sampled and can only reset the chip if the difference in level between Vdd and ground is less than the trigger voltage that would cause a reset (i.e., if Vdd - GND < Vb ₀ , where Vb ₀ is the brown-out voltage).

Fig. 6 illustrates the situation where normal ground noise occurs, but since the difference between Vdd and ground is not less than Vb ₀ , no reset occurs.

Fig. 7, on the other hand, is a point where their difference is equal to (and ultimately less than) the trip voltage, and at this point the reset condition occurs.

When the microcontroller chip is in the reset state, the circuit of Fig. 5A is operated to damp out oscillations through the hysteresis filter 33 and hold the chip in the reset state until Vdd is restored to its normal operating level (i.e., V _d d - GND > V _bo )--at which point the power run-up timer 24 (Fig. 2) in the brown-out detection circuit is called to initiate its fixed time-out interval to hold the microcontroller chip in the reset state. When this interval expires (i.e., the power run-up timer counts out), the chip is brought out of the reset state. In this way, the application of hysteresis serves to distinguish the occurrence of an actual brown-out event from the presence of normal noise or switching transient on the power supply (or ground reference).

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The hysteresis is derived by tapping Vdd through the two supply filters, which creates trip points for initiating an internal reset (when _Vdd falls such that Vl < _Vref ) and for initiating the current ramp-up timer interval (when Vdd rises such that V2 > Vref). Consequently, the trip points can be written as: Vl > Vb ₀ > V2. In the illustrated embodiment of the invention, the hysteresis level (which is not greater than V2-V1) may have a value of about 0.1 V when the brown-out voltage V _b0 is about 4.0 V.

Although the best mode presently contemplated for practicing the invention has been described in terms of the preferred embodiment, it will be apparent that variations and modifications may readily be made thereto by those skilled in the art without departing from the scope of the invention. Therefore, the invention should be defined only by the scope of the claims.

Claims (11)

1. A microcontroller incorporated in an integrated circuit (IC) of a semiconductor chip for controlling an external system with which the microcontroller is to be installed in a circuit, and comprising a central processing unit (CPU), a program memory for storing program instructions to be implemented by the CPU, a data memory for storing data including data pertaining to parameters of the external system to be controlled by the operation of the CPU in accordance with its instructions, means for providing a supply voltage to operate the IC chip, the supply voltage being defined by a voltage level relative to a ground reference level, and an implemented brown-out protection device comprising monitoring means for monitoring the supply voltage level relative to the ground reference level to reset the microcontroller as protection against malfunction in response to reduction of an arithmetic difference between the supply voltage level and the ground reference level to a value less than a predetermined threshold operating voltage level, discrimination means for distinguishing between a reduction representative of a Brown-out event that is representative of a simple transient voltage variation that usually occurs during operation of the microcontroller and does not warrant a reset of the microcontroller, and suppression means for suppressing a reset of the microcontroller when the reduction of the difference between the supply voltage level and the ground reference level represents a transient voltage variation that does not warrant a reset, thereby avoiding malfunctions and unnecessary resets of the microcontroller controlling the external system. 2. A microcontroller according to claim 1, wherein the discriminating means comprises filtering means for delaying the response of the brown-out protection means for each reduction representative of the commonly occurring transient voltage fluctuations, thereby allowing a transient voltage fluctuation to decay before restoration of a normal device operating voltage level. 3. A microcontroller according to claim 2, wherein the filtering means comprises sampling filters coupled to the supply voltage means for taking a pair of samples from the supply voltage for justifying when a reset should be invoked in response to supply voltage fluctuations relative to one of the samples and when the invoked reset should be terminated in response to supply voltage fluctuations relative to that of the other samples. 4. The microcontroller of claim 3, wherein the filter means further comprises hysteresis means coupled to the sample filters and adapted to be set to a value related to the pair of samples to add a corresponding delay interval to a first time at which the first sample justifies when a reset should be invoked. 5. The microcontroller of claim 4, wherein the brown-out protection means further comprises timer means responsive to a supply voltage fluctuation at a second time, the other sample establishing when a reset is to be terminated, to maintain the device in a reset state for a predetermined time interval and thereafter to restore device operation to a point corresponding to the status quo at which the reset was initially invoked. 6. The microcontroller of claim 1, wherein the brown-out protection means further comprises timer means responsive to restoration of the normal device operating voltage level following a reset of the device for maintaining the device in the reset state for a predetermined time interval and thereafter for restoring device operation from a point corresponding to the status quo at which the reset was initially invoked. 7. A microcontroller comprising a microprocessor, memory and peripherals for controlling a controlled system, and a supply voltage source for operating the microprocessor, memory and peripherals, the microcontroller further comprising a brown-out protection device for monitoring the supply voltage to invoke a reset of the microcontroller to avoid malfunction when the supply voltage level drops below a predetermined threshold operating voltage level, the reset defining a halt in operation of the microcontroller while freezing the status quo of the microprocessor, memory and peripherals at the point of halting operation, the brown-out protection device comprising means for selectively disabling the response of the brown-out protection device to supply voltage level fluctuations that would otherwise result in invoking a reset of the microcontroller attributable only to noise or switching transients that normally occur in operation of the microcontroller, thereby preventing malfunctions and unnecessary resets. of the microcontroller during control of the controlled system. 8. The microcontroller of claim 7, wherein the selective disabling means includes delay means for delaying response to fluctuations attributable to noise or switching transients, thereby enhancing recovery of the normal device operating voltage level without suspending operation of the microprocessor, memory and peripherals. 9. The microcontroller of claim 8, wherein the delay means comprises sampling means responsive to the supply voltage for taking a plurality of samples therefrom for establishing a differential level of the supply voltage fluctuations for invoking a reset and terminating a reset. 10. The microcontroller of claim 9, wherein the sampling means further comprises hysteresis means electrically connected to the sampling means for shifting both samples by a variable amount to delay the invocation of a reset accordingly. 11. The microcontroller of claim 10, wherein the brown-out protection means further comprises timer means for maintaining the reset state of the microcontroller for a predetermined time interval and thereafter restoring device operation to a point corresponding to the status quo at which operation was halted, responsive to one of the differential levels of supply voltage fluctuation to terminate a reset.