Digital watch with oscillator/divider power selection circuitry
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to digital electronic watches, and more particularly to a digital watch having power selection circuitry for the oscillator and initial divider, which supplies power from only one battery when the battery voltage is high (current is low) and which supplies power from two series batteries when the battery voltage is low (current is high). 
     2. Description of the Prior Art 
     In the art, the LED digital watch and the liquid crystal display digital watch with an illumination light have employed two batteries (each supplying 1.5 volts) and a series resistor to power their oscillator/divider. Use of two batteries wasted power, i.e., only one battery, 1.5 volts, was needed when in the case of the LED digital watch, the LED display elements were not illuminated and in the case of the liquid crystal display digital watch when the illumination light was not lit. Such digital watches have been disclosed in application Ser. No. 515,797,  filed Oct. 17, 1974, entitled &#34;Digital Watch with Liquid Crystal Display,&#34; in patent application Ser. No. 558,183, filed Mar. 13, 1975, entitled &#34;Digital Watch with Liquid Crystal and Light Emitting Diode Display&#34; both of which are commonly assigned. The subject matter of these cross-references are incorporated herein in their entirety. A drawback with these digital watches is that two betteries and a resistor connected in series are being used to supply voltage to the oscillator/divider all of the time when the voltage from only one battery would be sufficient most of the time. This technique was used because one battery alone with a high load current only put out 1.2 V, which was so low that requiring operation at that voltage caused very low production yields. In order that 1.5 V be supplied during the short time that current was high, about 2.0 V was supplied the rest of the time. Current increases faster than supply voltage, so power was wasted by running at such a high voltage. 
     SUMMARY OF THE INVENTION 
     The power selection circuitry in a digital watch, in accordance with the invention, consists of a plurality of batteries connected in series and a plurality of switching means which connect varying numbers of batteries, depending upon the amount of voltage needed, to the oscillator/divider supply terminal. Typically only one battery is needed to power the crystal oscillator when the LED display elements are not illuminated. But the voltage from the series combination of two batteries, diminished by a series resistor, is needed when the LED display elements are illuminated. 
     Accordingly, it is an object of this invention to provide a power selection circuit in a digital watch which selects whether one battery, or two batteries in series are needed to power the crystal oscillator depending upon whether or not the batteries are carrying a high or low amount of current at the particular time. 
     It is a further object to provide power selection circuitry which increases the life of the batteries in a digital watch. 
     It is a further object to provide a circuit which uses only one battery to power the oscillator/divider when the LED display elements are not illuminated and which uses two batteries to power the oscillator/divider when the LED display elements are illuminated. 
     It is a further object to employ a power selection circuit which uses one battery to power the oscillator/divider of the digital watch when the illumination light in a liquid crystal display watch is off and to use two batteries when the illumination light is on. 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may be understood best by reference to the following description, taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the circuitry of an LED digital watch; 
     FIG. 2 is a schematic drawing of the power selection circuitry of the present invention; 
     FIG. 3 is a schematic drawing of the battery powered selection circuitry for an illumination light in a liquid crystal display digital watch. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, the digital watch 10 includes power selection circuitry 12, which has output 14 to crystal oscillator/initial divider 16 and input 34 which is connected from multiplex control 30. The crystal oscillator oscillates at a predetermined and substantially constant frequency. The output from the oscillator/divider 16 is driven into another CMOS divider 20 which results in an output pulse of one cycle per second. This 1 Hertz pulse is then driven into a number of counters 22. The sequence of counters, which is described in patent application Ser. No. 515,797, filed Oct. 17, 1974, entitled &#34;Digital Watch With Liquid Crystal Display,&#34; can be thought of as a consecutive chain of dividers from which various points in the divider chain may be tapped to produce the desired seconds, minutes, hours, and date outputs. All outputs from the counters 22 are standard binary coded decimal. Seven-segment decoder 25 is a device which receives the binary coded decimal signals from the counters 22 and outputs seven signals which correspond to the seven segments of the LED display devices so that, when turned on to be visibly distinctive, the segments represent the digits corresponding to the counter state. 
     Multiplex control 30 has an input 31 from the final divider and three outputs, lines 32, line 34, and lines 38. When a push button (e.g., 36) is depressed, multiplex control 30 signals through lines 32 the respective counters 22 to deliver their information through seven segment decoder 24 and the LED segment driver 26 to all the LED display elements 40, 42, 44, and 46. Also, multiplex control 30 signals through line 34 the power selection circuitry 12 to supply voltage from two batteries to oscillator/divider 16. Finally, multiplex control 30, through lines 38, allows illumination of LED characters in sequence in synchronism with data on lines 32 so that horological information is properly positioned. But when a push button is not depressed, and any timer to lengthen the display time has counted out, lines 38 prevent the LED digit drivers 28 from illuminating the LED display devices 40 through 46, and line 34 causes the oscillator/divider to receive power from one battery. 
     FIG. 2 shows a detailed schematic of the power selection circuitry 12 of FIG. 1. The negative terminal of battery 60 is connected to the positive terminal of battery 62 and to N type MOSFET 64. The negative terminal of battery 62 is connected through resistor 66 to N type MOSFET 68. Resistor 66 is most easily included by suitable geometric sizing of MOSFET 68. The output of multiplex control 30 goes to the gate of MOSFET 68 and to the input of inverter 72. The source of MOSFET 64 is connected to the negative terminal of battery 60. The output of inverter 72 is connected to the gate of MOSFET 64, and the drains of MOSFET&#39;s 68 and 64 are both connected to the negative power terminal of oscillator/divider 16. All circuitry except the oscillator/divider is always powered by both batteries. 
     In operation, when all lights are off, a low or zero binary level signal is outputted via line 34 to MOSFET 68, turning if off. The low binary level via line 34 is also inputted into inverter 72 which outputs a high binary signal, and thereby allows MOSFET 64 to conduct delivering 1.5 volts to oscillator/divider 16. 
     When a light is on, multiplex control 30 delivers a high binary level signal to the gate of MOSFET 68 thereby allowing a higher voltage from the series combination of batteries and resistor to be delivered to oscillator/divider 16. Therefore, during low current conditions, only one battery 60 is used to power the oscillator/divider 16. But when high current is needed and battery voltage is thus reduced, the voltage from the two batteries 60 and 62 and a resistor 66 is used to power oscillator/divider 16. Another advantage of this circuit is evident here. Since MOSFET 64 is in a N-type well tied to one battery, the series combination of MOSFET 68 and resistor 66 cannot pull the output voltage more than 1 forward biased diode drop from the single battery voltage. This provides further regulation of the oscillator/divider supply voltage, and improves timekeeping stability and manufacturing yield. 
     FIG. 3 shows a schematic drawing of the power selection circuitry in a liquid crystal display digital watch. This power selection circuitry works in the same way as that circuit of FIG. 2 except for the fact that when push button 36 is depressed, an incandescent light is turned on which is used in the liquid crystal watch to illuminate the liquid crystal numerals when insufficient ambient light is available. 
     Although the device which has just been described appears to afford the greatest advantages for implementing the invention, it will be understood that various modifications may be made thereto without going beyond the scope of the invention, it being possible to replace certain elements by other elements capable of fulfilling the same technical functions therein.