Abstract:
An integrated circuit with power-up-warning circuitry wherein time integration and voltage level testing are done sequentially instead of simultaneously. A reference voltage is generated by current sourced to a reference voltage circuit, and this reference voltage is used as follows: An inverter receives the reference voltage as an input, and switches when the power supply becomes high enough that the reference voltage appears as a &#34;low&#34; level. When this inverter switches, current begins to be sourced to a timing capacitor. After the timing capacitor has charged up to a predetermined level, the current source to the reference-voltage node is turned off, and the power-up-warning signal (which has been driven high by output buffers) is turned off.

Description:
PARTIAL WAIVER OF COPYRIGHT 
     All of the material in this patent application is subject to copyright protection under the copyright laws of the United States and of other countries. As of the first effective filing date of the present application, this material is protected as unpublished material. 
     Portions of the material in the specification and drawings of this patent application are also subject to protection under the maskwork registration laws of the United States and of other countries. 
     However, permission to copy this material is hereby granted to the extent that the owner of the copyright and maskwork rights has no objection t the facsimile reproduction by anyone of the patent document or patent disclosure, as it appears in the United States Patent and Trademark Office patent file or records, but otherwise reserves all copyright and maskwork rights whatsoever. 
     CROSS-REFERENCE TO OTHER APPLICATIONS 
     The following application of common assignee contains related subject matter, and is believed to have an effective filing data identical with that of the present application: 
     Ser. No. 628,691, filed Dec. 14, 1990, entitled &#34;TRANSFER CIRCUIT FOR PREVENTING COLLISION ON USER READ,&#34; (DSC-187); which is hereby incorporated by reference. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to low-power integrated circuits generally, and particularly to battery-backed integrated circuits. 
     Power-Up Warning Signal 
     The disclosed embodiments provide a new circuit for generating a power-up warning signal, and a new way of operating an integrated circuit (using such a power-up warning signal). 
     Need to Reach a Known Logic State on Power-Up 
     If power were suddenly applied to all parts of a complex logic circuit, the logical state of the circuit might be unpredictable. For example, a flip-flop might come up to one state on one power-up, and in a different state on the next power-up. This is undesirable for several reasons: 
     Self-checking operations generally require that the circuit be in a known state to start with. 
     It may take many cycles to force complex logic into a known state, if the starting state is unknown. 
     Even worse, some of the states of a complex logic circuit (unless the circuit is specifically designed to avoid this) may be &#34;stuck&#34; states, from which normal operation of the logic cannot recover. 
     A different possible problem can arise in a CMOS circuit when the supply voltage is marginally low (e.g. greater than V TM , but less than V TM  +V TP ). At such marginally low voltages, some of the logic nodes will typically be floating, and the logic state of other nodes may depend on the (unpredictable) analog voltage of the floating node. As the supply voltage further increases (above V TM  +V TP ), some of the anomalous node values which occurred in the low-voltage regime may be &#34;frozen in,&#34; resulting, again, in an unknown logic state. 
     Thus, two conditions are required before the circuit can safely enter normal operation: a certain minimum time must have elapsed; and the supply voltage must have reached a certain minimum level. 
     Use of the Power-Up Signal 
     To prevent such problems, many prior art integrated circuits will generate a &#34;power-up&#34; signal for a brief period when power is first turned on. This logic signal is connected to various gates around the chip, so that, when power has just been turned on, these gates will prevent the logic from commencing normal operation, and will force logic elements into a known state. 1  After a certain minimum time delay, the logic will reliably have reached a known state. At that point, the logic can be allowed to begin its normal operation, where propagation of digital logic states is allowed. 
    
     Thus, the power-up signal is used not only for resetting circuits, but also for disabling normal operation for long enough to provide a safety margin. If the power-up signal terminates too soon, the circuit may be unreliable; if the power-up signal lasts longer than it needs to, time will be wasted on every power-up; and if the power-p signal never terminates, the circuit will certainly be operative. 
     Generation of the Power-Up Signal 
     As noted, two conditions must be satisfied before the circuit can safely enter normal operation. Two circuits are normally used to test for these two conditions: an integrating capacitor, with a relatively long time constant, is used to ensure that the desired minimum time has elapsed; and a reference voltage element, driven by a load, is used to assure that the supply voltage has reached the desired minimum level. The two corresponding output signals are (effectively) ANDed to assure that the power-up transition is complete. 
     FIG. 8 shows an example of such a previously-used circuit. Note that the pull-up resistor sources current to ground whenever the power is on. 
     Innovative Power-Up Circuit 
     According to the present invention, time integration and voltage level testing are done sequentially instead of simultaneously. A reference voltage is generated by a reference voltage circuit. 2  An inverter receives the reference voltage as an input, and will switch when the power supply becomes high enough. After this inverter switches, current is then sourced to a timing capacitor. After the capacitor has charged up to a predetermined level, the current source to the reference voltage node is turned off, and the power-up warning signal (which had been driven high by output buffers) is turned off. 
    
     Thus, no power is consumed by this feature except during the transitional period immediately after power is applied. 
     Sequential Stages of Operation 
     In the presently preferred embodiment, several sequential stages of operation can be identified, as voltage V DD  increases from zero: 3   
    
     1. As V DD  comes up above V TP , a strong PMOS pull-up will begin to source current to a reference-voltage node, until the reference voltage node comes up to its regulated value (V TM , in the presently preferred embodiment). 4   
    
     2. A chain of very weak inverters 5  is connected to drive an output buffer chain 6  which provides the power-up warning signal (signal &#34;PWRUP&#34;, in the presently preferred embodiment). The chain of weak inverters provides an output which will initially be low, so that, after the output buffer chain has been powered up, signal PWRUP will go high. 
    
    
     3. As the power supply comes above V TM  +V TP , the inverter following the reference voltage node will begin to turn on, and eventually this inverter will switch to drive its output high. 7   
    
     4. A weak pull-up will then begin to supply charging current to the integration capacitor. 
     5. After the integration capacitor has charged up to the following inverter&#39;s trip point (which is about 1/2V DD , in the presently preferred embodiment), two things happen: 
     5.1. This gate drives the chain of weak inverters which lead to the output buffer chain. Thus, after a small further delay, the power-up signal will go low. 
     5.2. This gate also is connected back to turn off the PMOS pull-up which is sourcing current to the reference voltage node. 
     Greater detail may be seen from FIGS. 1 and 2, as discussed below. (FIG. 2 is a SPICE simulation of voltages at marked nodes of FIG. 1, as the power supply V DD  comes up.) 
     The present invention provides several advantages: First, there can be no current burn during the low-voltage analog period before device threshold voltages are reached. Second, since continuing current burn is not a problem, the resistor can be made of lower value. This saves space in the layout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein: 
     FIG. 1 shows the presently preferred embodiment of a power-up circuit according to the present invention. 
     FIG. 2 shows a simulation of the voltages appearing at various nodes of the circuit of FIG. 1 as the power supply voltage increases from zero. 
     FIG. 3 shows the main elements of the serial timekeeper chip embodiment. 
     FIGS. 4-7 illustrate data transfer operations in the serial timekeeper chip embodiment of FIG. 3: FIG. 4 shows single byte transfer; FIG. 5 shows burst mode transfer; FIG. 6 shows write data transfer; and FIG. 7 shows a read data transfer. 
     FIG. 8 shows an example of a previously-used power-up circuit. Note that the pull-up resistor sources current to ground whenever the power is on. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment, which provides a serial timekeeping chip, which can be accessed by a user, over a serial bus, to get clock and calendar information. The disclosed innovations will therefore be described in the context of that chip. However, it must be appreciated that the disclosed innovations are not by any means limited to such a context, and this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. 
     Preferred Embodiment of the Invention 
     FIG. 1 shows the presently preferred embodiment of a power-up circuit according to the present invention, and FIG. 2 shows a simulation of the voltages appearing at various nodes of the circuit of FIG. 1 as the power supply voltage increases. 
     Note that inverter 120 is highly asymmetrical: its W p   8  is 50 microns, but its W N  is only 3.2 microns. (this helps to assure that node P13 will initially track the rising voltage V DD ./) Thus, as the power supply voltage V DD  begins to rise, inverter 120 will begin to source current to resistor 104 and NMOS depletion load 105, thereby pulling up the voltage of node A. 
    
     The presently preferred embodiment has been realized with a minimum nominal (drawn) dimension of 1.2 microns. Of course, as is well known to processing engineers, the actual dimensions may vary from the nominal dimensions, and the nominal dimensions can readily be varied according to known scaling laws. 
     As node A comes up to about V TM , transistor 106 will turn on and hold the node voltage reasonably constant. (See curve &#34;a&#34; in FIG. 2). Transistor 106 has nominal dimensions, in the presently preferred embodiment, of W N  =50 μm and L N  =10 μm. 
     Node B is driven by a gate 108, which has nominal dimensions, in the presently preferred embodiment, of W P  =10 μm, W N  =25 μm, L P  =10 μm, and L N  =10 μm. This gate is a Schmitt trigger, which also includes a feedback transistor P1 having nominal dimensions of W p1  =10 μm and L P1  =10 μm. 
     Node C is driven by an inverter 110, which has nominal dimensions, in the presently preferred embodiment, of W P  =20 μm, W N  =3.2 μm, L P  =1.2 μm, and L N  =1.2 μm. 
     Node D is driven by weak gate 112, which has nominal dimensions, in the presently preferred embodiment, of W P  =3.2 μm, W N  =3.2 μm, L P  =50 μm, and L N  =1.2 μm. The current sourced to node D must charge capacitor 114 (which, in the presently preferred embodiment, is a MOs capacitor of about 525 μm 2 , with a capacitance, at 5 Volts, of about 1 Pf). The long L P  of gate 112 reduces the current sourced to capacitor 114. Thus, as shown by curve &#34;d&#34; in FIG. 2, this node comes up fairly slowly, even after gate 112 has turned on. 
     Node E is driven by a gate 116, which has nominal dimensions, in the presently preferred embodiment, of W P  =6.8 μm, W N  =3.2 μm, L P  =1.2 μm, and L N  =1.2 μm. This gate is a Schmitt trigger, which also includes a feedback transistor P1 having nominal dimensions of W P1  =3.2 μm and L P1  =5 μm. 
     Signal PWRUP is driven, in the presently preferred embodiment, by a chain of 10 inverters following node E. Seven weak inverters 122 (each having W P  =7 μm, W N  =5 l μm, L P  =40 μm, and L N  =40 μm) are followed by successively larger inverters 124, 126, and 128, to produce signal PWRUP. 
     Node E is also followed by inverter 118, which drives inverter 120. Thus, when node E is driven low, node P13 will be driven low soon thereafter. (The low level on node E will also bring signal PWRUP low, after a delay determined by weak gates 122.) Thus, when the power-up transition is completed, current to resistor 104 is turned off, and essentially no DC current burn occurs. 
     In the case where V DD  rises very slowly, the circuit shown will typically trip at a DC level of V DD  =3.5 V; but the circuit trip point may be as low as 2.1 Volts (at 100° C., where processing variation has produced strong NMOS devices and weak PMOS devices) or a high as 4.5 Volts (at -55° C., where processing variation has produced weak NMOS devices and strong PMOS devices). 
     Asymmetrical Device Sizes 
     Note that several of these gates have asymmetrical device sizes. A CMOS gate will usually be dimensioned with W P  about two to three times as large as W N , which will typically give a gate threshold voltage of about half the supply voltage. However, note that several of the gates shown depart markedly from this range of ratios. 
     Behavior of Node Voltages as V DD  Rises 
     FIG. 2 shows a detailed simulation of the voltages appearing at various nodes of the circuit of FIG. 1 as the power supply voltage V DD  increases from zero. This simulation was made using SPICE, a software package which is widely used for detailed analysis of circuit behavior. 
     Initially (while V DD  is still less than |V TN  | or |V TP  |), node A will be low, and node B will (approximately) follow V DD  up. 
     When V DD  exceeds |V TP  |, the strong P-channel device of gate 120 will turn on and pull node P13 high. 
     Node A will then be pulled up (with a slew rate limited by resistor 104), until it reaches V TN . (node A is clamped at V TN  by transistor 106.) 
     When node B reaches V TN  (at approximately the same time as node A reaches V TN ), it will go to ground. 
     Note that node C (which initially follows V DD  up) starts to fall as node A starts to rise; but very soon thereafter, as node B falls, node C returns to F DD . 
     At about the time V DD  reaches 2.2V (in this example), the PWRUP signal rises sharply. (The timing of this is largely determined by the delay of gates 122. Note that node E, which provides the input to gates 122, follows V DD  up during the early stages.) 
     When V DD  exceeds V TN  plus the threshold voltage of gate 108 (about 3.5 V, in the presently preferred embodiment), gate 108 switches, and drives node B high. This drives node C low, and turns on the weak pullup transistor of gate 112. This slowly drives node D high. When node D has reached the threshold voltage of gate 116, node E goes low, and node P13 goes low. After a propagation delay (due to gates 122), signal PWRUP then goes low. 
     Preferred Subsystem Context for Using the Invention 
     The preferred integrated circuit embodiment, in which the disclosed innovative circuitry is used to provide a serial timekeeper chip, will now be described in detail. These details merely represent a contemplated example of making and using the claimed inventions, and are not all necessary for practicing the claimed inventions. 
     Features 
     Some distinctive features of the preferred serial timekeeper chip include the following: 
     Real Time clock counts seconds, minutes, hours, date of the month, month, day of the week and year with Leap Year compensation 
     24×8 RAM for scratch pad data storage 
     Serial I/O for minimum pin count 
     2.5 volt clock operation 
     Uses less than 1 μA at 3 volts 
     Single byte or multiple byte (burst mode) data transfer for read or write of clock or RAM data 
     8-pin DIP or optional 16-pin SOIC for surface mount 
     Simple 3-wire interface 
     TTL compatible (V cc  =5V) 
     Pin Connections 
     The presently preferred best mode integrated circuit embodiment uses an 8-pin DIP package, with the following pins. An asterisk (*) represents a complemented or inverted signal. 
     
         ______________________________________N.C.           No ConnectionX1, X2         32.768 kHz Crystal InputsGND            GroundRST*           ResetI/O            Data Input/OutputSCLK           Serial ClockVCC            Power Supply Pin______________________________________ 
    
     General Features 
     The chip contains a RealTime Clock/Calendar, 24 bytes of static RAM, and communicates with a microprocessor via a simple serial interface. The RealTime Clock/Calendar provides seconds, minutes, hours, day, date, month, and year information. The end of the month data is automatically adjusted for months with less than 31 days, including corrections for leap Year. The clock operates in either the 24-hour or 12-hour format with an AM/PM indicator. Interfacing the Serial timekeeper chip with a microprocessor is simplified using synchronous serial communication. Only three wires are required to communicate with the Clock/RAM: (1)RST* (Reset), (2) I/O (Data line), and (3) SCLK (Serial Clock). Data can be transferred to and from the Clock/RAM one byte at a time or in a burst of up to 24 bytes. The serial timekeeper chip is designed to operate on very low power and retain data and clock information on less than 3 microwatts. 
     Operation 
     The main elements of the serial timekeeper chip are shown in FIG. 3, namely, shift register, control logic, oscillator, real time clock and RAM. To initiate any transfer of data, RST* is taken high and eight bits are loaded into the shift register providing both address and command information. Each bit is serially input on the rising edge of the clock input. The first eight bits specify which of 32 bytes will be accessed, whether a read or write cycle will take place, and whether a byte or burst mode transfer is to occur. After the first eight clock cycles have occurred which load the command word into the shift register, additional clocks will output data for a read, or input data for a write. The number of clock pulses equals eight plus eight for byte mode or eight plus up to 192 for burst mode. 
     
                                           TABLE 1__________________________________________________________________________ ##STR1## ##STR2## ##STR3## ##STR4##DATE86W01-310010DATEDATE01__________________________________________________________________________ 
    
     Command Byte 
     Each data transfer is initiated by a one byte input called the address/command byte. The format of the address/command byte is shown in Table 1. 
     As defined, the MSB (Bit 7) must be a logical one. If zero, further action will be terminated. Bit 6 specifies a clock/calendar register if logic zero or a RAM location if Logical One. Bits one through five specify the designated registers to be input or output and the LSB (Bit 0) specifies a write operation (input) if logical zero or read operation output if logical one. To initiate any write operation, the write protect bit must be set to zero. 
     Burst Mode 
     Burst Mode may be specified for either the clock/calendar or the RAM registers by initiating a burst mode command. Bit 6 specifies clock or RAM and bit 0 specifies read or write. There is not data storage capacity in the Clock/Calendar or in the RAM for command bytes. 
     Write Protect Command Byte 
     Before any write operation to clock/calendar or RAM, the write protect bit must be set to zero. This operation requires driving the RST* high and the write protect command, 8E H , is loaded into the shift register followed by eight bits, 00 H . RST* must be driven before any other command can be initiated. To restore write protect, drive RST* high followed by WRITE command byte, 8E H , and data byte 80 H . Sixteen clock cycles are required for this operation. Complete the operation by driving RST* low. 
     Reset And Clock Control 
     All data transfers are initiated by driving the RST* input high. The RST* input serves two functions. First, RST* turns on the control logic which allows access to the shift register for the address/command sequence. Second, the RST* signal provides a method of terminating either single byte or multiple byte data transfer. A clock cycle is a sequence of a falling edge followed by a rising edge. For data inputs, data must be valid during the rising edge of the clock and data bits are output on the falling edge of clock. All data transfer terminates if the RST input i low and the I/O pin goes to a high impedance state. Data transfer is illustrated in FIGS. 4-7; FIG. 4 shows single byte transfer; FIG. 5 shows burst mode transfer; FIG. 6 shows write data transfer; and FIG. 7 shows a read data transfer. 
     Data Input 
     Following the eight SCLK cycles that input the write mode command byte a data byte is read on the rising edge of the next eight SCLK cycles. Additional SCLK cycles are ignored should they inadvertently occur. 
     Data Output 
     Following the eight SCLK cycles that input the read mode command byte a data bit is read out on the falling edge of the next eight SCLK cycles. Note that the first data bit to be transmitted from the clock/RAM occurs on the first falling edge after the last bit of the command byte is written. Additional SCLK cycles retransmit the data bytes should they inadvertently occur so long as RST remains high. This operation permits continuous burst read mode capability. 
     Clockcalendar 
     The Clock/Calendar is contained in eight writable/readable registers as shown in Table 2. Data contained in the clock/calendar registers is in binary coded decimal format (BCD). 
     
                                           TABLE 2__________________________________________________________________________ ##STR5##A. CLOCK ##STR6## ##STR7## ##STR8## ##STR9## ##STR10## ##STR11## ##STR12## ##STR13## ##STR14##B. RAM ##STR15## ##STR16## ##STR17##__________________________________________________________________________ 
    
     Clock Halt Flag 
     Bit 7 of the seconds register is defined as the clock halt flag. When this bit is set to logic one, the clock oscillator is stopped and the serial timekeeper chip is placed into a low power standby mode with a current drain of not more than 1.0 microamp. When this bit is written to logical zero, the clock will start. 
     AM-PM/12-24 Mode 
     Bit 7 of the hours register is defined as the 12- or 24-hour mode select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5 is the AM/PM bit with logic high being PM. In the 24-hour mode, bit 5 is the second 10 hour bit (20+hours). 
     Write Protect Register 
     Bit 7 of write protect register is the write protect bit. The first seven bits (bits 0-6) are forced to zero and will always read a zero when read. Bit 7 is set to logical one on power up and before any write operation to CLK or RAM, bit 7 must be set to zero. When high, the write protect bit prevents a write operation to any other register. 
     Clock/Calendar Burst Mode 
     The clock/calendar command byte specifies burst mode operation. In this mode the eight clock/calendar registers may be consecutively read or written. 
     RAM 
     The Static RAM is 24×8 bytes addressed consecutively in the RAM address space. 
     RAM Burst Mode 
     The RAM command byte specifies burst mode operation. In this mode, the 24 RAM registers may be consecutively read or written. 
     Register Summary 
     A register data format summary is shown in Table 2, above. 
     Crystal Selection 
     A 32,768 kHz crystal, Daiwa part No. DT26S, Seiko Part No. DS-VT-200 or equivalent, can be directly connected to the serial timekeeper chip via pins 2 and 3 (X1,X2). The crystal selected for use should have a specified load capacitance (C L ) of 6 pF. 
     Detailed Parameters 
     The following tables give specific values for some of the voltage and timing parameters just referred to, as used in the specific context of the presently preferred embodiment. It must be understood that these specific values are given merely to provide a wealth of detail regarding the described chip embodiment, and do not by any means delimit necessary features of the invention. 
     
         __________________________________________________________________________ABSOLUTE MAXIMUM RATINGS:__________________________________________________________________________VOLTAGE ON ANY PIN RELATIVE TO GROUND                      -0.5 V TO +7.0 VOPERATING TEMPERATURE      0° C. TO +70° C.STORAGE TEMPERATURE        -55° C. TO +125° C.SOLDERING TEMPERATURE      260° C. FOR 10 SEC__________________________________________________________________________PARAMETER  SYMBOL            MIN TYP                   MAX  UNITS                             NOTES__________________________________________________________________________RECOMMENDED D.C. OPERATING CONDITIONS (0° to ±70° C.)Supply Voltage      V.sub.CC            4.5 5.0                   5.5  VOLTS                             1Standby    V.sub.CC1            3.0    5.5  VOLTS                             1Supply VoltageLogic 1 Input      V.sub.IH            2.0    V.sub.CC                        VOLTS                             1Logic 0 Input      V.sub.IL            -0.5   0.8  VOLTS                             1DC Electrical Characteristics (0° to +70° C., V.sub.CC = 5V ± 10%)Input Leakage      I.sub.LI     +500 μA                             6I/O Leakage      I.sub.LO     +500 μA                             6Logic 1 Output      V.sub.OH            2.4         VOLTS                             2Logic 0 Output      V.sub.OL     0.4  VOLTS                             3Active Supply      I.sub.CC      4   mA   4CurrentStandby Supply      I.sub.CC1     1   μA                             5CurrentStandby Supply      I.sub.CC2    100  nA   10CurrentCAPACITANCE (T.sub.A = 25° C.)Input Capacitance      C.sub.I       5   pFI/O Capacitance      C.sub.I/O     10  pFCrystal Capacitance      C.sub.X       6   pFAC Electrical Characteristics (V.sub.CC = +5 V ± 10%,0°-70° C.)Data To CLK Setup      t.sub.DC             50         ns   7CLK To Data Hold      t.sub.CDH             70         ns   7CLK To Data Delay      t.sub.CDD    200  ns   7,8,9CLK Low Time      t.sub.CC            250         ns   7CLK High Time      t.sub.CH            250         ns   7CLK Frequency      f.sub.CLK D.C.    2.0 MHz                             7CLK Rise &amp; Fall      t.sub.F      500  nsRST To CLK Setup      t.sub.CS             1          us   7CLK To RST Hold      t.sub.CCH             60         ns   7RST Inactive Time      t.sub.CWH             1          us   7RST To I/O High Z      t.sub.CDZ     70  ns   7__________________________________________________________________________ NOTES TO THE TABLES: 1 All voltages are referenced to ground. 2 Logic one voltages are specified at a source current of 1 mA. 3 Logic zero voltages are specified at a sink current of 4 mA. 4 I.sub.CC is specified with the I/O pin open. 5 I.sub.CC1 is specified with V.sub.CC at 3.0 volts and RST*, I/O, and SCLK are open. 6 RST*, SCLK and I/O all have 40 kQ pull down resistors to ground. 7 Measured at VIH = 2.0 V or VIL = 0.8 V and 10 ms maximum rise and fall time. 8 Measured at VOH = 2.4 V or VOL = 0.4 V. 9 Load capacitance = 50 pF. 10 I.sub.CC2 is specified with V.sub.CC at 3.0 volts and RST*, I/O, and SCLK are open. The clock halt flag must also be set to logic one. 
    
     Further Modifications and Variations 
     It will be recognized by those skilled in the art that the innovative concepts disclosed in the present application can be applied in a wide variety of contexts. Moreover, the preferred implementation can be modified in a tremendous variety of ways. Accordingly, it should be understood that the modifications and variations suggested below and above are merely illustrative. These examples may help to show some of the scope of the inventive concepts, but these examples do not nearly exhaust the full scope of variations in the disclosed novel concepts. 
     Of course, various of the logic gates which are shown as inverters in the presently preferred embodiment could alternatively be configured as NAND gates, NOR gates, or other structures. This might be done, for example, if it is desired to be able to force the power-up signal high from an external pin, in a system where some components may be powered down while others remain active. 
     As will be recognized by those skilled in the art, the innovative concepts described in the preset application can be modified and varied over a tremendous range of applications, and accordingly their scope is not limited except by the allowed claims.