Abstract:
A power management unit monitors current drawn by a fingerprint sensor circuit and generates a “heartbeat” signal during normal operation. If a latchup event occurs, with attendant increase in current drawn by the fingerprint sensor circuitry, the heartbeat signal terminates and an interrupt is subsequently triggered to start a latchup recovery routine. Power to the fingerprint sensor circuitry is switched off and the interrupt is then cleared by writing appropriate values to control bits within the power management register.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention is directed, in general, to power management for integrated circuits and, more specifically, to power management with electrostatic discharge latchup event recovery for fingerprint sensor devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrostatic discharge techniques normally employed for integrated circuits are often not entirely suitable or satisfactory for fingerprint sensor devices. Because such sensor devices must, of necessity, allow contact in proximity to the sensor circuitry, electrostatic discharge (ESD) protection to prevent latchup, circuit damage and other problems must be implemented in a different manner.  
           [0003]    A primary requirement of power management for fingerprint sensor devices is the ability to restore operation of the fingerprint sensor circuitry following a latch-up condition triggered by an electrostatic discharge event. However, the same electrostatic discharge event occurring on the fingerprint sensor device and propagating inside the power management unit should not affect functionality of the power management unit.  
           [0004]    Fingerprint sensor devices may be required to operate at a nominal 5V supply), although optional operation at 3.3 volts supply may also be desired. Whether the dual power supply voltage capability accomplished by a charge pump receiving 3.3 volts and generating the 5 volt power supply voltage level or by a power regulator running out of 5 volts to produce both voltage levels, the power management unit should control the power of the charge pump or power regulator, if present, as well as shutting down the power supply of the fingerprint sensor device at will.  
           [0005]    There is, therefore, a need in the art for a power management system for fingerprint sensor devices which allows control over power to the fingerprint sensor device and enables recovery from electrostatic discharge events.  
         SUMMARY OF THE INVENTION  
         [0006]    To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a fingerprint detection system, a power management unit which monitors current drawn by a fingerprint sensor circuit and generates a “heartbeat” signal during normal operation. If a latchup event occurs, with attendant increase in current drawn by the fingerprint sensor circuitry, the heartbeat signal terminates and an interrupt is subsequently triggered to start a latchup recovery routine. Power to the fingerprint sensor circuitry is switched off and the interrupt is then cleared by writing appropriate values to control bits within the power management register.  
           [0007]    The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.  
           [0008]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which:  
         [0010]    [0010]FIG. 1 depicts a power management system for a fingerprint sensor device according to one embodiment of the present invention;  
         [0011]    [0011]FIGS. 2 and 3 are each a series of related plots illustrating operation of a power management system for a fingerprint sensor device according to one embodiment of the present invention; and  
         [0012]    [0012]FIG. 4 depicts in greater detail control of an interrupt signal within a power management system for a fingerprint sensor device according to one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    [0013]FIGS. 1 through 4, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device.  
         [0014]    [0014]FIG. 1 depicts a power management system for a fingerprint sensor device according to one embodiment of the present invention. Power management unit  100 , an error check and correction (ECC) device, controls power flow from a power supply HV through an internal power switch  101 . Power supply HV may be either a 5 volt or 3.3 volt power source. When connected to the power supply HV, power flows through an external sensing resistor R connected between nodes P 2  and P 3  within power management unit  100  to the fingerprint sensor device  102 .  
         [0015]    In the case of a latchup event, the fingerprint sensor device  102  will experience a substantial and permanent (as long as the power is applied) increase in current flowing through sensing resistor R. An “electrocardiogram” heartbeat unit ECG detects the attendant voltage drop increase between nodes P 2  and P 3 . Implemented by a dynamic differential comparator with the R+, R−, and IN+ inputs all connected to node P 2  and the IN− input connected to node P 3 , heartbeat unit ECG outputs a square wave synchronous with the system clock (the “heartbeat”) when the fingerprint sensor device is powered and draws only the nominal current associated with normal operation. This heartbeat signal is rectified by diode D and filtered through a low pass filter LPF to provide a logical high or 1 signal at node P 4  during normal operation.  
         [0016]    When a latchup event occurs, the heartbeat unit ECG output stops “beating,” providing, eventually, a logical low or zero output. After rectification and filtering of the ECG output, node P 4  gradually discharges to the ground voltage, asserting the interrupt signal INT output at logic gate  103 , assuming the interrupt enable bit X is on (a logical 1) and the power switch bit Y is also on. The interrupt signal is passed to the fingerprint sensor device  102  and/or any processor or controller controlling operation of the fingerprint sensor device  102  to initiate a reset or latchup recovery routine.  
         [0017]    Grounding of node P 4  also causes a bus interface  104  for the fingerprint sensing device  102 , to ground the data bus  105  coupled to the fingerprint sensing device. The signal at node P 4  may also be optionally routed, through the interrupt signal INT, to drive directly an ECC reset input for the fingerprint sensing device  102  and/or any associated processor or controller, starting a latchup recovery routine.  
         [0018]    [0018]FIGS. 2 and 3 are each a series of related plots illustrating operation of a power management system for a fingerprint sensor device according to one embodiment of the present invention, and specifically the power management unit  100  depicted in FIG. 1. In FIG. 2, the top graph represents current through the sensing resistor R. At 300 nanoseconds (ns) an induced spike is shown and at 600 ns a latchup event is simulated, causing the current through sensing resistor R to increase to a high direct current (DC) value of 120 milliamps (mA) in the example shown. The induced spike, by not generating a false interrupt, shows that the monitor unit is not affected by random disturbances on the power supply (i.e. current through resistor R) while it triggers an interrupt when a real latch-up event occurs. The middle graph shows the corresponding output of the heartbeat unit ECG at node Pi, as well as the signal at node P 4 . Activity at node Pi stops as soon as a latchup event occurs. After approximately 400 ns, node P 4 —which is the signal used to recover from the latchup event—goes to zero. The bottom graph shows the corresponding power supply voltage signal.  
         [0019]    [0019]FIG. 3 shows a complete sequence of latchup and  5  recovery. The top graph shows current through the sensing resistor R, with a latchup event occurring at about 600 ns with peak current of 80 mA. The bottom graph illustrates the corresponding signal at node P 4 , while the middle graph illustrates the voltages across sensing resistor R (at nodes P 2  and P 3 ) and the power control bit Y command to open the power switch  101 . When the switch  101  is open, nodes P 2  and P 3  gradually discharge, causing a false transition to logic state  1  for node P 4  but not affecting the operation since the interrupt signal INT has already been asserted.  
         [0020]    Referring back to FIG. 1, in a preferred embodiment the latchup recovery routine triggered by the interrupt signal INT includes the following sequence of events, programmed into a read-only memory (ROM) within the ECC unit: (1) the fingerprint sensor power switch  101  is turned off by the power switch control bit Y within the power management unit  100 ; (2) the fingerprint sensor data lines are automatically grounded by the bus interface  104 ; and (3) after an appropriate delay, the startup sequence is launched.  
         [0021]    The startup sequence in the exemplary embodiment is controlled by a boot program and includes a zero logic state for the power control bit Y, leaving power control switch  101  temporarily open, with the interrupt signal INT being disabled through the enable interrupt control bit X. These states are necessary since the heartbeat unit ECG would not produce a heartbeat while there is no power supplied to nodes P 2  and P 3 . Through the power control bit Y, the power switch  101  is closed and, after an appropriate delay, the interrupt signal INT is enabled by writing a logic one to the interrupt enable control bit X.  
         [0022]    [0022]FIG. 4 depicts in greater detail control of an interrupt signal within a power management system for a fingerprint sensor device according to one embodiment of the present invention. Logic gate  103  receives the inverted state of node P 4  and the pwr_switch_en signal corresponding to the power switch enable bit Y within the power management register. Logic gate  110  receives the raw_interrupt signal output of logic gate  103  and an early_interrupt signal from the output of a first flip-flop  112 . Logic gate  111  receives the set_interrupt signal output by logic gate  110  and an interrupt_enable signal corresponding to interrupt enable bit X within th power management register. Logic gate  113  also receives the interrupt_enable signal and the output of edge-triggered flip-flop  112  reflecting whether node P 4  has discharged to ground (before transitioning to a false logic one as described above). Edge-triggered flip-flop  114  generates the power management interrupt signal pwr_mgmt_int.  
         [0023]    At power up, the system reset clears the pwr_mgmt_int signal by writing zeros to the X and Y control bits of the power management register. When the power switch  101  is turned on by writing a logical 1 into control bit Y and subsequently enabling the interrupt by writing a logical 1 into control bit X, the circuit is ready to detect a change from high to low of node P 4 , caused by a latchup event in the fingerprint sensor device. At the next SYSTEM_CLOCK cycle, this transition is latched into pwr_mgmt_int. The routine responding to the interrupt, after taking care to ground the data bus lines, will turn the power switch  101  off by writing a zero into control bit Y, and will eventually clear the interrupt by writing a zero in control bit X.  
         [0024]    Although the present invention has been described in detail, those skilled in the art will understand that various changes, kiralgnev, substitutions and alterations herein may be made without departing from the spirit and scope of the invention in its broadest form.