Abstract:
A low headroom oscillator operates at low supply voltages without the use of monostable circuits or flip flops. The oscillator operates in multiple states which allow for the charging and discharging of the capacitors alternately to enable the proper operating of the oscillator at low supply voltages without locking up.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a low headroom oscillator. The present invention further relates to a method of operating an oscillator at low voltage supplies while preventing the oscillator from reaching a lock up state. 
       BACKGROUND INFORMATION 
       [0002]    An electronic oscillator is a device that allows for the generation of electronic signals at varying frequencies. One significant problem with oscillators is that, when coupled with a low supply voltage, the timing of the digital elements in the oscillator circuit becomes unreliable. Previous attempts to correct the timing of these elements have focused on the design of oscillator circuits which use latches, flip flops or monostable circuits, a two state circuit where one of the states remains stable but the other state is unstable for a fixed period of time before returning to a stable state (also known as “one-shot” circuits), to discharge the capacitors in the integrated circuits of the oscillators. The use of latches, flip flops or monostable circuits may operate sufficiently at standard operating supply voltage ranges, but are not reliable to control the charge and discharge of the oscillator capacitors for wide supply ranges. At low supply voltages, the oscillator may “lock up”, where one of the capacitors in the integrated circuit is continually charged, and the circuit does not switch over to charge the other capacitor. 
         [0003]    At low supply voltages, the use of monostable circuits or other latches to control the charging and discharging of the capacitors in the oscillator circuit may be problematic. Monostable circuits are used to create specific time periods where one of the states of the circuit is unstable, and at low supply voltages, a determination of the timing becomes unpredictable. The use of monostable circuits in oscillators may therefore become unreliable. In oscillator configurations that use flip flops, the flip flops may startup in an invalid state and remain in this locked state during operation of the oscillator. 
         [0004]    Other attempts to correct the timing of the integrated circuit elements of an oscillator at low supply voltages have centered on the use of multiple reference voltages. The use of two reference voltages presents multiple problems because it requires the use of two separate and distinct references that have a large voltage range, but still operate in the common mode range of the comparators in the oscillator circuit. The selection of two reference voltages to accomplish this is neither practical nor feasible at low supply voltages. 
         [0005]    Thus there is a need in the art, particularly for oscillators on silicon chips that have a large voltage supply range, for an oscillator that may perform at low supply voltages without locking up. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a circuit diagram of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    Embodiments of the present invention provide an oscillator having a plurality of comparators, a current source, at least one inverter, and at least two capacitors. The current source in the oscillator circuit may be used to charge either capacitor in the integrated circuit. This may present the distinct advantage that only a single reference voltage may be needed for operation at low supply voltage ranges. Using the current source to alternately charge the capacitors in the integrated circuit may also allow for the design of an integrated circuit of an oscillator without the use of monostable circuits, flip flops, or other combinational logic. 
         [0008]      FIG. 1  illustrates an integrated circuit for an oscillator which may allow for capacitors  100  and  102  to be charged and discharged alternately. In particular, the integrated circuit may be designed to have a first state where capacitor  102  is discharging when capacitor  100  is charging, and a second state, where capacitor  100  is discharging when capacitor  102  is charging. 
         [0009]    Each capacitor  100 ,  102  may be connected at its capacitor plates to a three terminal device  120 ,  122 . In a preferred embodiment, three terminal devices  120 ,  122 , may be nMOS devices. The source and the drain terminals of nMOS device  120  may be connected to the plates of capacitor  100 . The source and the drain terminals of nMOS device  122  may be connected to the plates of capacitor  102 . The source terminals of nMOS devices  120 ,  122  and one of the plates of capacitors  100 ,  102  may be connected to Vss  180 . Devices  120 ,  122  may be arranged in a manner that they turn on in alternate states. NMOS device  120  may be off when capacitor  100  is charging. Alternatively, nMOS device  122  may be turned off when capacitor  102  is charging. 
         [0010]    The integrated circuit may also include comparators  110  and  112  that are connected to capacitors  100 ,  102 . The positive input terminal of comparator  112  may be connected to one of the plates of capacitor  102 , and may also be connected to the drain of nMOS device  122 . The negative terminal of comparator  112  may be connected to a reference voltage  160 , therefore comparing the voltage at the drain of nMOS device  122  to the reference voltage. The output of comparator  112  may be tied to the gate of nMOS device  120 . 
         [0011]    In a preferred embodiment, comparator  110  may be a hysteresis comparator. Selecting comparator  110  to be a hysteresis comparator may allow for tuning of the frequency and duty cycle of the oscillator. 
         [0012]    The positive terminal of comparator  110  may be connected to one of the plates of capacitor  100  and the drain terminal of nMOS device  120 . The negative terminal of comparator  110  may be connected to reference voltage  160 , wherein the reference voltage  160  may be compared to the voltage at the drain of nMOS device  120 . The output of comparator  110  may be connected to a three terminal device  140  and an inverter  130 , which inverters the output signal to three terminal device  142 . 
         [0013]    The integrated circuit of the oscillator may contain three terminal devices  140 ,  142 , which may preferably be pMOS devices. The gate of the pMOS device  140  may be connected to the output of hysteresis comparator  110 . The connection of the gate terminal of pMOS device  140  to comparator  110  may allow for the pMOS devices to be turned on by the output of the comparator. The presence of inverter  130  between the output of comparator  110  and pMOS device  142  may allow pMOS device  142  to be turned on when device  140  is turned off. When the output of comparator  110  is high (“1”), the input to pMOS device  140  may be high (“1”), whereas the input to the gate of pMOS device  142  may be low (“0”). Conversely, when the output of comparator  110  is low (“0”), the input to pMOS device  140  may be low (“0”), whereas the input to the gate of pMOS device  142  may be high (“1”). This may allow pMOS device  142  to be off when device  140  is on, and device  142  may be on when pMOS device  140  is off. 
         [0014]    The gate terminal of pMOS device  142  may also be connected to an inverter  132 . The gate terminal of pMOS device  142  may also be coupled to the gate of nMOS device  122 . The output of inverter  132  may be the voltage output of the entire oscillator. 
         [0015]    A current source  150  may be used to bias the pMOS devices  142  and  140  to alternately charge capacitors  100  and  102 . The input terminal of current source  150  is connected to Vdd  170  and the output terminal is connected to the source terminals of pMOS devices  142  and  140 . In an embodiment, current source  150  may be a cascoded current source. 
         [0016]    During operation, the oscillator may operate in two states: a first state where capacitor  100  is charging and a second state where capacitor  102  is charging. In the first state, comparator  110  may compare the voltage at the node connecting the top plate of capacitor  100  and the drain of nMOS device  120  to reference voltage  160 . If capacitor  100  is not charged, the reference voltage  160  may be greater than the voltage at this node, which represents the charge on capacitor  100 . If the reference voltage  160  is greater than the voltage on the top plate of capacitor  100 , than comparator  110  outputs a “0”. The logic “0” output from comparator may be input to the gate of pMOS device  140 . PMOS device  140  may turn on because the source-gate voltage of the device is less than the threshold voltage, and current is pulled up through the drain of pMOS device  140  to charge capacitor  100 . 
         [0017]    When comparator  110  outputs a “0”, inverter  130  may logically negate this input and output a “1” to the gate terminal of pMOS device  142 . PMOS device  142  may turn off because the source-gate voltage of the pMOS device is greater than the threshold voltage of the pMOS device. Because pMOS device  142  may be off in the first state, capacitor  102  may not be charging. 
         [0018]    As the gate terminal of nMOS device  122  is connected to the gate terminal of pMOS device  142 , when a “1” is input to pMOS device  142  in the first state, a “1” is also input to the gate of nMOS device  122 . NMOS device  122  may turn on because the gate-source voltage of the device is greater than the turn-on voltage for the nMOS device. When nMOS device  122  is on, the device may be connected to capacitor  102 , and charge may be dissipated through nMOS  122 . Capacitor  102  may be discharging in the first state. 
         [0019]    Comparator  112  may compare the drain voltage of nMOS device  122  (the charge on capacitor  102 ) to reference voltage  160 . As capacitor  102  is discharged, the charge on the capacitor may reach a level below the reference voltage  160 , and therefore the input to the positive terminal of comparator  112  may be less than the reference voltage  160 . If the input to comparator  112  is less than the reference voltage  160 , the comparator may output a “0” to nMOS device  120 . 
         [0020]    If comparator  112  outputs a logic “0” to the gate of nMOS device  120 , the nMOS device may turn off. NMOS device  120  may turn off because the gate-source voltage of the device is less than the turn-on voltage for the nMOS device and nMOS device  120  may operate in cut-off mode. Since nMOS device  120  is off, no current may flow through this device and capacitor  100  may not be discharged. This may be consistent with capacitor  100  being charged by pMOS device  140 . 
         [0021]    When the charge on capacitor  100  is greater than the reference voltage  160 , the oscillator may move to the second state. In the second state, comparator  110  may compare the voltage on the top plate of the capacitor  100  to reference voltage  160 . Since the voltage at capacitor  100  may be greater than the reference voltage  160 , the comparator  110  may output a “1”. The logic “1” output from comparator may be input to the gate of pMOS device  140 . PMOS device  140  may turn off because the source-gate voltage of the device may be greater than the threshold voltage of the device. If pMOS device  140  is turned off, capacitor  100  may not be charged. 
         [0022]    If comparator  100  outputs a “1” to inverter  130 , inverter  130  may logically negate this input and output a “0” to the gate terminal of pMOS device  142 . PMOS device  142  may turn on because the source-gate voltage of the pMOS device may be less than the threshold voltage of the pMOS device. When pMOS device  142  turns on in the second state, capacitor  102  may begin charging. 
         [0023]    When inverter  130  outputs a “0” to pMOS device  142 , a “0” may also be output to the connected gate of nMOS device  122 . NMOS device  122  may turn off because the gate-source voltage of the device may be less than the turn-on voltage for the nMOS device. If nMOS device  122  is turned off, the nMOS device may be disconnected from capacitor  102 , and charge is no longer dissipated through nMOS device  122 . 
         [0024]    In the second state, comparator  112  may compare the drain voltage of nMOS device  122  to reference voltage  160 . If pMOS device  142  is on and nMOS device  122  is off, the capacitor  102  is charging and may reach a level greater than the reference voltage  160 . Therefore, when nMOS device  122  is off and pMOS device  142  is on, the input to the positive terminal of comparator  112  may be greater than the reference voltage  160 , and comparator  112  may output a “1” to the gate terminal of nMOS device  120 . 
         [0025]    If comparator  112  outputs a logic “1” to the gate terminal of nMOS device  120 , the nMOS device may turn on. NMOS device  120  may turn on because the gate-source voltage of the device is greater than the turn-on voltage for the nMOS device. When nMOS device  120  is on, the device  120  may be connected to capacitor  100 , and the capacitor may be discharged through the nMOS device. 
         [0026]    Because comparator  110  may be a hysteresis comparator, in the second state, capacitor  100  may not be discharged to a voltage level equal to the reference voltage  160 , but may be discharged to where the voltage at the top plate of capacitor  100  may be equal to V REF -V HYS . Once capacitor  100  has a charge less than V REF -V HYS , comparator  112  outputs a “0” and the oscillator returns back to the first state. 
         [0027]    In preferred embodiments, the oscillator may operate for low supply voltages varying from 1.4 V to 3.6 V. However, the present invention is not limited to operating only with supply voltages within that range. 
         [0028]    Several embodiments of the invention are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.