Abstract:
A threshold personalization circuit for a reset or supervisor chip includes personalization fuses, which shift a resistor divider to provide a variety of selectable voltage thresholds. The personalization fuses may provide hundreds of millivolts of adjustment. The threshold personalization circuit further includes trim fuses to fine tune the threshold to within a few millivolts of the target threshold voltage. The threshold personalization circuit includes a test mode to cycle through to a particular personalization trim, such that at prelaser testing the personalized value is found (the fuse blow for personalization is emulated) and then the trim fuse amount can be based on the actual final personalized voltage. This results in very accurate threshold voltages for all personalized values.

Description:
RELATED APPLICATION 
     The present application claims priority from U.S. Provisional Application No. 60/744,563, filed Apr. 10, 2006. The disclosure of the foregoing United States Patent Application is specifically incorporated herein by this reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a circuit and method to accurately predict which fuses to blow to meet a particular voltage threshold suitable for use in reset chips, supervisor chips, and the like. 
     One problem for such chips is that some devices have a number of thresholds, varying over a wide range. For example, from 1.6V to 4.6V. One device can have as many as 24 different threshold offerings. It is difficult to provide one chip that can accurately provide so many different threshold offerings while remaining cost effective to produce. 
     What is desired, therefore, is a cost effective circuit and method that can provide a selectable, accurate voltage threshold for use in a reset or supervisor chip. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the invention a threshold personalization circuit for a reset or supervisor chip includes personalization fuses, which shift a resistor divider to provide a plurality of main trip points. The threshold personalization circuit of the present invention further comprises trim fuses to fine tune the threshold to within a few millivolts of the target threshold voltage. The personalization fuses may provide hundreds of millivolts of adjustment. During testing, a default threshold exists (before any fuses are blown). According to an embodiment of the present invention, the default threshold may be set to 4.6 volts, for example. The chip would be tested at a 4.6 volt threshold and trimmed accordingly. For a particular application, however, the threshold may be 1.6 volts, for example. In this case, a personalization fuse (in addition to the trim fuse) needs to is blown. The amount of trim needed to achieve a 4.6 volt threshold is not necessarily the same as the amount of trim required for a 1.6 volt threshold. This is because the resistor divider varies slightly over voltage. The method of the present invention includes a test mode to cycle through the particular personalization trim, such that at prelaser testing the personalized value is found (the fuse blow for personalization is emulated) and then the trim fuse amount can be based on the actual final personalized voltage. This results in very accurate threshold voltages for all personalized values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic/block diagram of a first circuit portion of the present invention including a resistor divider and trim fuses, a selectable passgate circuit including four selectable passgates, a bandgap reference voltage circuit, and a comparator for providing a reset signal; 
         FIG. 2  is a schematic/block diagram of a second circuit portion of the present invention including four personalization fuse circuits that are coupled to four corresponding two-to-one multiplexers for providing selection signals to the selectable passgate circuit, a test pad for receiving a test mode signal coupled to the multiplexers, and a load resistor coupled to the test pad; and 
         FIG. 3  is a schematic/block diagram of a third circuit portion of the present invention including a test mode shift register circuit comprising four D-type flip-flops in serial connection having an input for receiving a data input signal, and four output for providing four shift signals to the multiplexers. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a first portion  100  of the circuit of the present invention including a bandgap reference voltage circuit  102 . Bandgap circuits are well known in the art and many different bandgap designs can be used based upon the requirements of a particular application. Bandgap reference voltage circuit  102  generates a voltage that is constant over temperature of about 1.25 volts. The circuit  102  shown in  FIG. 1  is an one example of a bandgap circuit and its exact implementation details are not critical to the method and circuit of the present invention, and other known circuits can be used. The output is labeled VREF and is about 1.25 volts. As is known in the art, bandgap circuit  102  can be designed to include current source outputs to be provided to other circuits, such as comparators, and the like. As is also known in the art, bandgap circuit  102  can be designed to include a startup circuit as bandgap circuits are known to have two stable states, one of which is not operational. 
       FIG. 1  also includes a comparator  104  having a negative input for receiving the VREF reference voltage from the bandgap circuit  102 , a positive output that is selectively coupled to a resistor divider  106  through selectable passgate circuitry  110  as is explained in further detail below, and an output for providing the RESET output signal. The exact implementation detail of comparator  104  of  FIG. 2  is not critical to the method and circuit of the present invention, and many types of comparator circuits can be used for the particular implementation of comparator  104 . 
       FIG. 1  further shows a segmented resistor divider  106 , according to the present invention. Resistor divider is coupled between VCC and ground. Resistor divider  106  includes a middle segmented portion of resistors having multiple output taps. In the example of  FIG. 1 , four such output taps can be seen. Resistor divider  106  also includes upper and lower segmented portions  108 , wherein the resistors are each in parallel with individual trim fuses. The trim fuses at the top and bottom  108  of the resistor divider  106  are used to provide the fine trim up and trim down capability, respectively. The four taps off the resistor are the personalization taps (and the associated hysteresis taps). These taps feed into the personalization selection circuit shown in greater detail in  FIG. 2 . 
     Hysteresis is provided by the circuit of  FIG. 1 . Each one of the SEL passgates are actually a pair of passgates, that tap into two different points on the resistor, for example 20 mv or 50 mv apart (for a 20 mv or 50 mv hysteresis). The RESET output (output of the comparator) controls which of the pair of passgates is conducting, hence providing hysteresis. 
     Finally,  FIG. 1  includes the selectable passgate circuitry  110 , previously referred to.  FIG. 1  shows four individual passgates, each passgate being controlled by a separate digital control signal labeled SEL 1 , SEL 2 , SEL 3 , and SEL 4 . Each passgate can be any type required by a particular application. Passgates can include P-channel transistors, N-channel transistors, or a combination of P-channel transistors, N-channel transistors and diodes as is known in the art. The passgate circuitry  110  is used to selectively couple the output voltages on the various resistor divider taps to the positive input of comparator  104 . The passgate circuitry  110  is used to select amongst many personalization options such that a particular desired voltage threshold required by a specific application can be implemented. 
       FIG. 2  contains multiplexing circuitry  200 , including passgates on the right, that select one of the many personalization options. Multiplexers  206 A,  206 B,  206 C, and  206 D are all two-to-one multiplexers having a first input coupled to personalization fuse circuitry  204 A,  204 B,  204 C, and  204 D, and a second input coupled a testmode shift register  300  shown in  FIG. 3 . Both the personalization fuse circuitry and the testmode shift register are described in further detail below. Muliplexers  206 A- 206 D also include a selection input controlled by the state of the logic signal found on test pad  202 . The selection inputs of all of the multiplexers is also coupled to a load resistor R L . 
     The test mode shift register  300  includes four serially coupled D-type flip-flops  302 ,  304 ,  306 , and  308 . The “D” input of flip-flop  302  receives the input DATA signal, and each of the outputs of flip-flops  302 - 308  provide the SHIFT 1 , SHIFT 2 , SHIFT 3 , and SHIFT 4  output signals. The SHIFT 1 -SHIFT 4  output signals are received by the second input of multiplexers  206 A- 206 D. Shift register  300  shown in  FIG. 3  allows clocking through all of the personalization options, stopping at the one of interest for the voltage threshold required, and thereafter measuring the threshold based on the personalization option selected. Then, the correct trim fuses can be blown for resistor divider  106  shown in  FIG. 1 . The testmode is entered by bringing a logic high signal on test bondpad  202  (not bonded in the package). The pad is tied low via a load resistor R L . When the voltage on the test pad is low, normal operation occurs and the personalization is controlled by the fuse circuitry. 
       FIG. 2  shows the personalization fuse circuits  204 A,  204 B,  204 C, and  204 D that control the generation of the SEL 1 , SEL 2 , SEL 3 , and SEL 4  signals. Each circuit is the same and so only one will be described. Personalization fuse circuit  204 A includes a fuse, a first transistor having a gate for receiving an INT signal, an inverter, and a second transistor having a gate controlled by the output of the inverter. The INT signal is an initialization signal that is generated on power up (power on reset). It is a pulsed signal that is generated when VCC rises above a certain threshold. In the example of  FIG. 2 , the fuse is coupled between VCC on one end, and the two transistors and inverter on the other end. When the fuse is blown, the output signal of the inverter is a logic high. When the fuse is intact, the output signal of the inverter is a logic low. The fuse in the personalization fuse circuit can be blown by a laser as is known in the art. The output of the inverter is coupled to the first input of multiplexer  206 A. 
     There are passgates/transmission gates connected between these sets of trim up and trim down fuses. The conducting passgate determines the threshold personalization. The passgates are formed in pairs, with the additional pass-gate being used for hysteresis. Once the threshold is reached, the output of the comparator turns off the conducting passgate and turns on it&#39;s “pair”, which is about 20 mV higher. Thus, to switch back the other way, VCC with have to go 20 mV higher than the threshold reached when VCC was falling (hence hysteresis, to avoid noise and oscillations). 
     While there have been described above the principles of the present invention in conjunction with specific implementations and device processing technology, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.