Microcomputer power supply

Abstract: A low cost, less complex microcomputer power supply circuit with temperature stability and low current consumption in which a single voltage regulating element is used to provide the supply voltage, power-on reset (POR) and low voltage inhibitor (LVI) functions to the microcomputer (12). In an exemplary embodiment, the power supply circuit (10) includes a regulating circuit with a zener diode (30) that is conductive when the voltage level of the DC source (14) is above a threshold value and non-conductive when the voltage level of the DC source (14) is below a threshold value. The regulator circuit provides a regulated supply voltage to the microcomputer from the DC source (14). The regulator circuit also provides a regulation signal, whose state depends on the conductivity of the zener diode (30), to a reset circuit. The reset circuit of the power supply circuit is responsive to the regulation signal for providing a reset signal to the microcomputer (12). The state of the reset signal similarly depends upon the conductivity of the zener diode (30). Accordingly, when the zener diode (30) is non-conductive, operation of the microcomputer (12) is inhibited. Alternatively, when the zener diode (30) is conductive, operation of the microcomputer (12) is enabled. In one embodiment, the power supply (10) further includes a time delay circuit (28, 32 and 34) for delaying initialisation of the microcomputer to a known operating state until the DC source voltage rises above the threshold value for a period of time.

Description: The present invention relates generally to power supply circuitry for microcomputers and more particularly to power supply circuits that also provide power on reset (POR) and low voltage inhibit (LVI) functionality to the microcomputer. It is well known that microcomputers operating from regulated power supplies must be inhibited during both the power-up phase and in the event that the DC power to the regulated power supply is interrupted or drops below a predetermined level. Prior circuits such as that shown in U.S. Patent No. 5,313,112, which issued to the present inventor, sought to find a low cost solution to achieving the desired functions. Alternative and more costly solutions are embodied in custom integrated circuits such as the CS-8126 low dropout, high current linear regulator manufactured by the Cherry Semiconductor Corporation. A disadvantage of these prior approaches, however, is that separate voltage regulating elements are required. One to provide the regulated supply voltage, and at least one other for use as a comparison reference for the POR/LVI functions. These separate voltage regulating elements add cost and complexity to the microcomputer power supply circuitry. Accordingly, there is a need for a lower cost, less complex solution that combines the supply and reset functions so that only a single voltage regulating element is required to achieve the desired operation. US Patent 4,469,957 describes a resetting circuit for microprocessors whih includes a voltage regulator including a zener diode stabilization circuit and a flip flop. The flip flop is used to generate a reset signal to a microprocessor to reset the microprocessor when the supply voltage rises and falls relative to its minimum permissible value. The flip flop has a hysteresis between switching points for a rising and falling control voltage that controls the flip flop. The present invention now provides a circuit for providing regulated power to a microcomputer from a DC source, inhibiting operation of the microcomputer when the voltage supplied by the DC source drops below a predetermined threshold value, and resetting the microcomputer when the voltage supplied by the DC source rises above the predetermined threshold value, comprising; a regulator circuit including a zener diode having a conductive state when the voltage level of the DC source is above a threshold value and a non conductive state when the voltage level of the DC source is below a threshold value; said regulator circuit coupled to the DC source for providing a regulated supply voltage to an input power terminal of the microcomputer; a reset circuit having an input coupled to an electrode of the zener diode to receive a regulation signal having a first state when said zener diode is operating in said conductive state and a second state when said zener diode is operating in said non-conductive state; said reset circuit being responsive to said regulation signal for providing a reset signal to a reset terminal when said zener diode is operating in said conductive state and an inhibit signal to said reset terminal when said zener diode is operating in said non conductive state; characterised in that; said regulator circuit includes a capacitor and resistor delay circuit connected to the said electrode of the zener diode, said reset circuit includes a transistor circuit including a first transistor having a base electrode connected to be biased on and off by the delay circuit and a second transistor having a base electrode controlled by the first transistor, the delay time of the bias to the base electrode of the said first transistor is controlled by the time constant defined by the delay circuit thereby to delay the timing of the reset signal, and the second transistor is forced by the first transistor to supply the inhibit signal prior to the said supply voltage falling below the said predetermined threshold value. An advantage of the above aspect of the invention is that a single voltage regulating element provides the supply voltage, power-on reset (POR) and low voltage inhibitor (LVI) functions to the microcomputer with increased temperature stability and low current consumption. A further advantage of the present invention is its low cost and reduced complexity. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a schematic representation of a system embodying the present invention; and FIG. 2 graphically illustrates voltage as a function of time for defined nodes of the circuit shown in FIG. 1. Referring first to FIG. 1, power supply circuit 10 is showed coupled to power input terminal V CC of microcomputer 12. In this embodiment, as well as most applications in an automotive vehicle electronics environment, requires the voltage applied at V CC be 5.0 volts. Power supply circuit 10 receives an unregulated level of voltage V BATT from a conventional vehicle battery 14 that is ideally charged to a voltage between 12 and 14 volts DC by conventional charging device, such as an alternator/regulator system (not shown). Typically however, the voltage supplied by battery 14 fluctuates in a range of 8 volts to 18 volts. As shown in FIG. 1, conventional ignition switch 16 may be included so that power supply circuit 10 can be selectively energised by battery 14. In the present example, microcomputer 12 is a MC6805P9A microprocessor manufactured by Motorola Incorporated. However, power supply circuit 10 of the present invention is applicable to any number of microprocessors or microcomputers for which a reset signal is desirable to initialise the device to a known start up state or to inhibit operation when supply voltage drops below the manufacturer's recommended input voltage (typically about 4.5 volts). Continuing with FIG. 1, power supply circuit 10 includes signal conditioning circuitry 18 which includes diode 20 and capacitors 22 and 24. Diode 20 provides reverse battery protection if the battery terminals are reversed. Capacitor 22 bypasses electromagnetic noise and radio frequency interference. And, capacitor 24 allows the regulated output voltage to continue at the supply terminal V CC should battery power be momentarily interrupted. The output of signal conditioning circuitry 18 couples to the input (defined node A) of regulating circuitry that includes NPN transistor 26. Resistor 28 provides a current path so that a base voltage is established at transistor 26 to bias it as an emitter-follower. The cathode of zener diode 30 connects to the base of transistor 26. The anode of zener diode 30 (defined node B) couples to capacitor 32 and resistor 34. Together, resistors 28 and 34 along with capacitor 32 provide a time delay which will be described in greater detail later herein. The anode of zener diode 30 also couples to reset circuitry which includes transistor 36 and inverting transistor 38. The collector of transistor 36 (defined node C) couples to the output of the regulator circuitry at the emitter of transistor 26 (defined node D) through pull-up resistor 40. Similarly, the collector of transistor 38 couples to the emitter of transistor 26 through pull-up resistor 42. The collector of transistor 38 also couples to the reset terminal of microcomputer 12. Having described the various components of power supply circuit 10 and their various interconnections, the operation of power supply circuit 10 will now be described with concurrent reference to FIGS. 1 and 2. The curves associated with the respective defined nodes shown in FIG. 2 are intended to illustrate the operation of power supply circuit 10 in providing the power on reset (POR) and low voltage inhibit (LVI) functions. For these illustrative purposes, the conditioned battery voltage at node A is shown linearly ramping up to a level above a threshold value and then linearly ramping down to a level below the threshold value after a period of time (see curve A in FIG. 2). One skilled in the art will recognise that in practice, this voltage typically changes much more rapidly and most likely contains some transient noise. Generally, power supply circuit 10 provides a stable regulated output voltage signal (5 volts in this example) to power input terminal V CC when ignition switch 16 is closed and the voltage level V BATT of battery 14 is greater than 6.4 volts. In addition, power supply circuitry 10 provides a relatively low (ground level) reset signal to microcomputer 12 when the supply voltage present at input terminal V CC is below its specified voltage requirement. When the level of the DC source (battery 14) is greater than the specified 6.4 volt threshold, a regulation current flows through resistor 28, zener diode 30, and the base-emitter junction of transistor 36. In the current embodiment, zener diode 30 is a low test current 4.7 volt zener diode. Although zener diode 30 is nominally a 4.7 volt zener diode, at battery voltages between 8.5 volts and 16.5 volts, approximately 5 volts is nominally established across the device because of the increased current flow through it. Accordingly, for the purposes of this description, zener diode 30 is treated as having a 5 volt zener voltage. When the regulating current path is achieved, the base-emitter junction of transistor 36 is forward biased so that approximately 0.6 volts is present at the base of transistor 36. However, the base-emitter junction of transistor 36 cannot forward bias until capacitor 32 is charged to 0.6 volts. The delay time is controlled by the time constant defined by capacitor 32 and the parallel combination of resistors 28 and 34. Accordingly, until capacitor 32 is charged sufficiently, transistor 36 will remain off (see curve B in FIG. 2). Once zener diode 30 is conducting (t Z ON ), zener diode 30 regulates the voltage at the base of transistor 26 to a voltage level of approximately 5.6 volts. Accordingly, the base-emitter junction of transistor 26 is forward biased so that it operates as an emitter-follower to provide a regulated output voltage of approximately 5.0 volts at its emitter to the input power terminal V CC of microcomputer 12 (see node D of FIG. 2). Variations in the 5 volt regulated output delivered to power input terminal V CC due to temperature changes are negligible because each of the temperature coefficients of the devices in the circuit cancel each other out. For example, because zener diode 30 is a 4.7 zener diode it has a nominal temperature coefficient of 0. The base-emitter junctions of transistors 26 and 36 each have a -2 mv/°C temperature coefficient; however, they act to cancel each other out. For example, when transistor 36 has a higher base voltage due to cold temperature, the base of transistor 26 will have a correspondingly higher base-emitter voltage drop which cancels the out the effect of the other. Variations in the 5 volt supply due to variations in the voltage supplied by battery 14 are also negligible because the regulation current through zener diode 30 is well above its zener knee current, I ZK . Therefore, zener diode 30 operates in the region where its zener resistance is minimised. In this portion of its operating range, variations in current through the device have little effect on voltage variations across the device. An advantage is thereby obtained of providing a regulated output voltage less susceptible to variations in temperature or in the level of voltage supplied by battery 14. The operation of power supply circuit 10 in providing the power on reset (POR) function to microcomputer 12 will now be described. As noted above, transistor 36 cannot forward bias until capacitor 32 is sufficiently charged to 0.6 volts. With transistor 36 in its cut-off state (see curve C in FIG. 2), a current path is established through resistor 40 which forward biases the base-emitter junction of transistor 38. Sufficient current is then present through the base-emitter junction of transistor 38 to cause it to be in saturation. With transistor 38 in saturation, the collector of transistor 38 is pulled to a potential close to ground (see curve E in FIG. 2). Accordingly, a signal of a relatively low logic level is present on the reset terminal of microcomputer 12. The low level signal present at the reset pin inhibits the operation of microcomputer 12. Once capacitor 32 is charged, the base-emitter junction of transistor 36 forward biases so that transistor 36 turns on (see curve C in FIG. 2). With transistor 36 in saturation, the collector of transistor 36 is pulled close to ground. Transistor 38 then turns off since its base-emitter junction is no longer forward biased (see curve E in FIG. 2). With transistor 38 off, a signal of a relatively high logic level is applied to the reset terminal to enable initialisation of microcomputer 12 to a known operating state. Microcomputer 12 will then continue to operate according to its pre-programmed strategy so long as ignition switch 16 remains closed and battery voltage V BATT is greater than 6.5 volts such that power supply circuit 10 continues to provide the regulated 5 volts to power input terminal Vcc and the relatively high logic level signal to the reset terminal of microcomputer 12. Once ignition switch 16 is opened or the voltage supplied by battery 14 drops below 6.5 volts, capacitor 24 discharges to provide power to power supply circuitry 10. When the drop in battery voltage is only a momentary interruption, the discharging of capacitor 24 may prevent power supply circuit 10 from dropping out of regulation. However, once the voltage at node A drops below 5.6 volts, zener diode 30 no longer conducts the regulation current (at time t Z OFF) . In this state, capacitor 32 discharges through resistor 34 and the base of transistor 36 is pulled essentially to ground potential. Accordingly, transistor 36 turns off (see curve C in FIG. 2). With transistor 36 off, its collector-emitter junction becomes an open circuit and current flow is diverted through the base-emitter junction of transistor 38 thereby forcing transistor 38 into saturation (see curve E in FIG. 2). With transistor 38 in saturation, the collector of transistor 38 is pulled close to ground and a low level logic signal is applied to the reset terminal of microcomputer 12. Therefore, just as power supply circuit 10 falls out of regulation (approximately 4.8 volts at node D), a low level logic signal is applied at the reset terminal to inhibit the operation of microcomputer 12. An advantage is thereby obtained of insuring that microcomputer 12 is reset prior to the supply voltage V CC reaching 4.5 volts so that microcomputer 12 cannot operate in an unknown state. A further advantage of the above embodiment of the invention is that a single voltage regulating element is used by power supply circuit 10 to both provide a regulated output voltage to the power input terminal V CC of microcomputer 12 and to generate the reset/inhibit signal at the reset terminal. This concludes a description of an embodiment of the invention.

Claims: A circuit for providing regulated power to a microcomputer from a DC source, inhibiting operation of the microcomputer when the voltage supplied by the DC source drops below a predetermined threshold value, and resetting the microcomputer when the voltage supplied by the DC source rises above the predetermined threshold value, comprising; a regulator circuit including a zener diode (30) having a conductive state when the voltage level of the DC source (14) is above a threshold value and a non conductive state when the voltage level of the DC source (14) is below a threshold value; said regulator circuit coupled to the DC source (14) for providing a regulated supply voltage to an input power terminal (node D) of the microcomputer; a reset circuit having an input coupled to an electrode of the zener diode (node B) to receive a regulation signal having a first state when said zener diode (30) is operating in said conductive state and a second state when said zener diode is operating in said non-conductive state; said reset circuit being responsive to said regulation signal for providing a reset inhibit signal at a first logic level to a reset terminal when said zener diode (30) is operating in said conductive state and the reset inhibit signal at a second logic level to said reset terminal when said zener diode is operating in said non conductive state; characterised in that; said regulator circuit includes a capacitor and resistor delay circuit (32,34) connected to the said electrode of the zener diode (node B), said reset circuit includes a transistor circuit (36,38) including a first transistor (36) having a base electrode connected to be biased on and off by the delay circuit (32,34) and a second transistor (38) having a base electrode controlled by the first transistor, the delay time of the bias to the base electrode of the said first transistor (36) is controlled by the time constant defined by the delay circuit thereby to delay the timing of the reset inhibit signal, and the second transistor (38) is forced by the first transistor (36) to supply the reset inhibit signal at said second logic level, prior to the said supply voltage falling below the said predetermined threshold value. A circuit as claimed in claim 1, wherein the threshold value is dependent upon the zener voltage of said zener diode (30). A circuit as claimed in claim 2, wherein said zener diode (30) controls the level of said regulated power supply voltage (Vcc).

Inventors: MACKS HAROLD RYAN, MACKS, HAROLD RYAN
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1996
Main classification: G06F1
IPC codes: G06F1