Supply independent Schmitt trigger RC oscillator

Embodiments of the present invention provide an oscillator circuit having a steady output frequency that is independent of the supplied voltage. This oscillator includes a Schmitt trigger circuit which may be implemented within an integrated circuit of a wireless terminal or other like portable electronic device. The Schmitt trigger circuit receives a threshold voltage input and a second voltage input. The Schmitt trigger circuit generates an output voltage equal to either a first output voltage or a second output voltage based on the results of comparing the threshold voltage input to the second voltage input. An RC network may be coupled to the output of the Schmitt trigger circuit and is operable to supply the second voltage input to the Schmitt trigger circuit. A voltage divider network also couples to the output of the Schmitt trigger circuit wherein the threshold voltage input is proportional to the first output voltage reduced by the voltage divider network based on the output voltage of the Schmitt trigger circuit.

BACKGROUND

1. Technical Field

The present invention relates generally to oscillators, and more particularly to the type of oscillator utilized in high frequency power converters where initial and final frequencies are required to be well defined.

2. Related Art

Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.

Cellular wireless networks include a “network infrastructure” that wirelessly communicates with wireless terminals within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.

In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and a serving base station. The MSC routes voice communications to another MSC or to the PSTN. Typically, BSCs route data communications between a servicing base station and a packet data network that may include or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as “forward link” transmissions while transmissions from wireless terminals to base stations are referred to as “reverse link” transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals.

To conserve power, the wireless terminal may sleep when not actively communicating with a servicing base station. However, to ensure no communications are missed, the wireless terminal awakens periodically to receive a page burst that indicates if the wireless terminal must service a communication from the servicing base station. Various other electronic devices may enter a sleep mode as well in order to conserve power. To realize this advantage, the timing associated with the sleep mode should be accurately controlled in order to allow the wireless telephone to awaken at predetermined intervals to check for received messages or pages. Thus, it is important to have an accurate low power oscillator for timing when to awaken from or enter into the sleep mode and effectively conserve power.

One such low power oscillator is a Schmitt trigger RC oscillator.FIG. 1depicts a general Schmitt trigger RC oscillator10. Schmitt trigger RC oscillator10includes an operational amplifier12that receives a first voltage input or threshold voltage input VPand a second voltage input VN. Operational amplifier12generates an output voltage, VOUT, equal to a first output voltage, VDD, when VPis greater than VN; or a second output voltage, such as ground, when VPis less than VN. A resistive capacitive (RC) network14couples to the output of operational amplifier12and supplies VNto operational amplifier12. Additionally a voltage divider16also couples to the output of operational amplifier12. As shown here, VPis supplied from the voltage divider16, as the voltage seen at the node between resistors R1and R2.

When VP>VN, VOUTgoes to VDDand begins to charge capacitor CX. This increases the voltage VN. During this charging period, VP1=VREF+R1/(R1+R2)*(VDD−VREF). When VNexceeds the switching point VP, VOUTgoes to ground and begins to discharge capacitor CX. This decreases the voltage VN. During this discharging period, VP2=R2/(R1+R2)*VREF. Then, the switching point defined by VPis a function of VDDand VREF. During Charging Period, VNmay be defined as

VN=VDD⁡(1-ⅇ-t1RX⁢⁢CX)+VP⁢⁢2⁡(ⅇ-t1RX⁢⁢Cx)=VP⁢⁢1.
During the discharging period, VNmay be defined as

VN=VP⁢⁢1*ⅇ-(t2-t1)RX⁢CX=VP⁢⁢2.
Solving these two equations, where for example R1=R2, yields an expression for the period of the oscillator be defined

as⁢⁢t2=-RX*CX*ln(⁢12⁢(VDD-VREF)VDD-12⁢VREF*VREFVDD+VREF).
Thus, the frequency of the oscillator may be defined as

1t2,
which is a function (RX,CX,VDD, VREF).

The output of the operational amplifier may be a continuous square wave as shown inFIG. 2. The frequency of this square way depends on the values of R and C and the threshold points of the Schmitt trigger. The Schmitt trigger RC oscillator circuit may be easily incorporated within an integrated circuit (IC). However, it should be noted that the frequency stability is lacking as the frequency is dependent on the input voltage VDDand VREFfor the reasons shown above. As the input voltage can vary as much as +/−10 percent, the frequency may also vary +/−10 percent. This level of variation makes the Schmitt trigger oscillator unacceptable as an accurate timing source for determining when to awaken from or enter into the sleep mode and effectively conserve power.

BRIEF SUMMARY OF THE INVENTION

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 3is a system diagram illustrating a portion of a cellular wireless communication system100that supports wireless terminals operating according to the present invention. The cellular wireless communication system100includes a Mobile Switching Center (MSC)101, Serving GPRS Support Node/Serving EDGE Support Node (SGSN/SESN)102, base station controllers (BSCs)152and154, and base stations103,104,105, and106. The SGSN/SESN102couples to the Internet114via a GPRS Gateway Support Node (GGSN)112. A conventional voice terminal121couples to the PSTN110. A Voice over Internet Protocol (VoIP) terminal123and a personal computer125couple to the Internet114. The MSC101couples to the Public Switched Telephone Network (PSTN)110.

Each of the base stations103-106services a cell/set of sectors within which it supports wireless communications. Wireless links that include both forward link components and reverse link components support wireless communications between the base stations and their serviced wireless terminals. These wireless links support digital data communications, VoIP communications, and other digital multimedia communications. The cellular wireless communication system100may also be backward compatible in supporting analog operations as well. The cellular wireless communication system100supports the Global System for Mobile telecommunications (GSM) standard and also the Enhanced Data rates for GSM (or Global) Evolution (EDGE) extension thereof. The cellular wireless communication system100may also support the GSM General Packet Radio Service (GPRS) extension to GSM. However, the present invention is also applicable to other standards as well, e.g., TDMA standards, CDMA standards, etc. In general, the teachings of the present invention apply to digital communications that combine Automatic Repeat ReQuest (ARQ) operations at Layer2, e.g., LINK/MAC layer with variable coding/decoding operations at Layer1(PHY).

Wireless terminals116,118,120,122,124,126,128, and130couple to the cellular wireless communication system100via wireless links with the base stations103-106. As illustrated, wireless terminals may include cellular telephones116and118, laptop computers120and122, desktop computers124and126, and data terminals128and130. However, the cellular wireless communication system100supports communications with other types of wireless terminals as well. As is generally known, devices such as laptop computers120and122, desktop computers124and126, data terminals128and130, and cellular telephones116and118, are enabled to “surf” the Internet114, transmit and receive data communications such as email, transmit and receive files, and to perform other data operations. Many of these data operations have significant download data-rate requirements while the upload data-rate requirements are not as severe. Some or all of the wireless terminals116-130are therefore enabled to support the GPRS and/or EDGE operating standard as well as supporting the voice servicing portions the GSM standard.

FIG. 4is a block diagram functionally illustrating a wireless terminal200constructed according to the present invention. The wireless terminal200ofFIG. 4includes an RF transceiver202, digital processing components204, and various other components contained within a housing. The digital processing components204includes two main functional components, a physical layer processing, speech COder/DECoder (CODEC), and baseband CODEC functional block206and a protocol processing, man-machine interface functional block208. A Digital Signal Processor (DSP) is the major component of the physical layer processing, speech COder/DECoder (CODEC), and baseband CODEC functional block206while a microprocessor, e.g., Reduced Instruction Set Computing (RISC) processor, is the major component of the protocol processing, man-machine interface functional block208. The DSP may also be referred to as a Radio Interface Processor (RIP) while the RISC processor may be referred to as a system processor. However, these naming conventions are not to be taken as limiting the functions of these components.

The RF transceiver202couples to an antenna203, to the digital processing components204, and also to a battery224that powers all components of the wireless terminal200. The physical layer processing, speech COder/DECoder (CODEC), and baseband CODEC functional block206couples to the protocol processing, man-machine interface functional block208and to a coupled microphone226and speaker228. The protocol processing, man-machine interface functional block208couples to a Personal Computing/Data Terminal Equipment interface210, a keypad212, a Subscriber Identification Module (SIM) port213, a camera214, a flash RAM216, an SRAM218, a LCD220, and LED(s)222. The camera214and LCD220may support either/both still pictures and moving pictures. Thus, the wireless terminal200ofFIG. 4supports video services as well as audio services via the cellular network.

FIG. 5is a block diagram illustrating in more detail the wireless terminal ofFIG. 4, with particular emphasis on the digital processing components of the wireless terminal. The digital processing components204include a system processor302, a baseband processor304, and a plurality of supporting components. The supporting components include an external memory interface306, MMI drivers and I/F308, a video I/F310, an audio I/F312, a voice band CODEC314, auxiliary functions316, at least one clock or oscillator circuit317, a modulator/demodulator322, ROM324, RAM326and a plurality of processing modules. In some embodiments, the modulator/demodulator322is not a separate structural component with these functions being performed internal to the baseband processor304.

The processing modules are also referred to herein as accelerators, co-processors, processing modules, or otherwise, and include auxiliary functions316, an equalizer module318, an encoder/decoder module320, and an Incremental Redundancy (IR) processing module328. The interconnections ofFIG. 5are one example of a manner in which these components may be interconnected. Other embodiments support additional/alternate couplings. Such coupling may be direct, indirect, and/or may be via one or more intermediary components.

RAM and ROM service both the system processor302and the baseband processor304. Both the system processor302and the baseband processor304may couple to shared RAM326and ROM324, couple to separate RAM, coupled to separate ROM, couple to multiple RAM blocks, some shared, some not shared, or may be served in a differing manner by the memory. In one particular embodiment, the system processor302and the baseband processor304coupled to respective separate RAMs and ROMs and also couple to a shared RAM that services control and data transfers between the devices. The processing modules316,318,320,322, and328may coupled as illustrated inFIG. 5, but may also be coupled in other manners in differing embodiments.

The system processor302services at least a portion of a serviced protocol stack, e.g., GSM/GPRS/EDGE protocol stack. In particular the system processor302services Layer1(L1) operations330, a portion of Incremental Redundancy (IR) GSM protocol stack operations332(referred to as “IR control process”), Medium Access Control (MAC) operations334, and Radio Link Control (RLC) operations336. The baseband processor304in combination with the modulator/demodulator322, RF transceiver, equalizer module318, and/or encoder/decoder module320service the Physical Layer (PHY) operations performed by the digital processing components204.

A clock module or oscillator module317may service both the system processor302and the baseband processor304. This module may produce timing information which when accurate may be used to significantly conserve battery power. For example, to conserve power, the wireless terminal may sleep when not actively communicating with a servicing base station. However, to ensure no communications are missed, the wireless terminal awakens periodically to receive a page burst that indicates if the wireless terminal must service a communication from the servicing base station. A description of this process will be described with reference toFIG. 6. Various other electronic devices known to those having skill in the art may also enter a sleep mode as well in order to conserve power. To realize this advantage, the timing associated with the sleep mode should be accurately controlled based on timing signals produced by oscillator module317in order to allow the wireless telephone to awaken at predetermined intervals to check for received messages or pages.

FIG. 6depicts the various stages associated with forming and interpreting paging channel (PCH) downlink transmissions. The original pages for the individual wireless terminals or mobile stations are initially divided into a series of pages to be transmitted according to a predetermined schedule to the wireless terminals. This predetermined schedule allows the individual wireless terminals, when not actively transmitting, to enter a sleep mode and merely awaken when it is necessary to receive their respective page bursts. As shown here, the original page undergoes two stages of encoding. First, the original pages undergo a block coding operation that is typically referred to as outer encoding. The block coding stage, allows for the detection of errors within the data block. In addition, the Data blocks may be supplemented with tail bits or block code sequence. Since Block Coding is the first or external stage of channel coding, the block code is also known as an external or outer encoding scheme. Typically, two kinds of codes are used, a cyclic redundancy check (CRC) or a Fire Code. The Fire Codes allow for either error correction or error detection. Error detection with the Fire Code, verifies connectivity.

Next, the pages undergo a second level of encoding that typically is a convolutional coding referred to as inner encoding. The pages may be optionally interleaved to form paging bursts. These paging bursts are what the wireless terminal receives according to the predetermined schedule.

FIG. 7is a timeline illustrating the receipt and decoding of paging bursts particularly comparing full decoding to partial decoding according to the present invention. Illustrated inFIG. 7are a series of paging bursts400that are received according to paging groups received approximately every 0.5 to 2.0 seconds. The paging bursts carry either a page or a null page for each wireless terminal assigned to a corresponding paging group. When carrying a page, the paging burst400signal the wireless terminal to respond to the servicing base station. This may involve servicing a voice call, data or text. When the paging burst400is sent, individual wireless terminals that are assigned to the paging group awaken for a period of time indicated by the awake portion of timeline402to receive the paging burst.

Typically, 4 paging bursts makeup every paging message and traditionally all 4 paging bursts need to be received before decoding can begin. By making use of the Null page template a sufficiently reliable indication of whether or not the paging message contains any useful information for the mobile can be obtained from only the 1st paging burst of the 4 paging bursts without waiting the 4 paging bursts. If after receiving the 1st paging burst and performing the null pattern match the result is inconclusive then the 2nd paging burst can be received and tested for conformity to the null paging message, and so on until all 4 bursts have been received. As one can appreciate, each paging burst which does not have to be received over the air-interface provides measurable and useful power consumption benefits. If all 4 paging bursts of the block are received and decoded, this constitutes normal paging message reception/decoding. The benefits result from reducing the time that the radio (RF) portion of the receiver is employed (receiving 1 or 2 bursts instead of 4 bursts) and bypassing a large amount of unnecessary baseband message decoding and further processing to understand the contents of the message.

Timeline402shows that the wireless terminal's processors are either awake or asleep. When the wireless terminal awakens it may fully decode the paging burst. Alternatively, according to the present invention, when there is a favorable pattern comparison between the paging burst and a null page pattern, the wireless terminal determines that the paging burst is a null page. However, one should note that a null page might be required to be fully decoded. Time segments404and406show that the time required to fully decode the paging burst is much greater than that required to merely perform a pattern comparison on the processed paging burst with an existing pattern. Therefore one can appreciate that the wireless terminal will remain awake much longer when a full decode of the paging burst is required. This means that additional power will be consumed and processing resources will be utilized to fully decode the paging burst when compared to merely conducting a pattern comparison as indicated in block406.

FIG. 8depicts a general Schmitt trigger RC oscillator800. Schmitt trigger RC oscillator800, like Schmitt trigger RC oscillator10ofFIG. 1includes an operational amplifier12that receives a first voltage input or threshold voltage input, VP, and a second voltage input, VN. Operational amplifier12generates an output voltage, VOUT, equal to a first output voltage, VDD, when VPis greater than VN; or a second output voltage, such as ground, when VPis less than VN. However, as shown here, a defined relationship exists between VPand VDD. Resistive capacitive (RC) network14couples to the output of operational amplifier12and supplies VNto operational amplifier12. Additionally a voltage divider16also couples to the output of operational amplifier12. The relationship between VPand VDDis defined by the values of the resistances of the voltage divider16. As shown here, VPis supplied from the voltage divider16, as the voltage seen at the node between resistors R1and R2.

In order to simplify analysis, one can assume R1=R2=R3. When VP>VN, VOUTgoes to VDDand begins to charge capacitor CX. This increases the voltage VN. During this charging period, VP1=(⅔)*VDD. When VNexceeds the switching point VP, VOUTgoes to ground and begins to discharge capacitor CX. This decreases the voltage VN. During this discharging period, VP2=(⅓)*VDD. Thus, the switching point, VP, is no longer defined as a function of VREF. Rather, the switching point is a predetermined portion of VDD. During Charging Period, VNmay be defined as

VN=VDD⁡(1-ⅇ-t1RX⁢CX)+13⁢VDD⁡(ⅇ-t1RX⁢Cx)=23⁢VDD.
During the discharging period, VNmay be defined as

VN=23⁢VDD*ⅇ-(t2-t1)RX⁢CX=13⁢VDD.
Solving these two equations, where R1=R2=R3, yields an expression for the period of the oscillator be defined as t2=2 ln(2*RXCX). Thus, the frequency of the oscillator may be defined as

1t2,
which is a function Rx and Cx.

The output of the operational amplifier may be a continuous square wave as previously described with reference toFIG. 2. The frequency of this square way depends only on the values of RXand CXas the dependence on the threshold points of the Schmitt trigger and reference voltage have been canceled out by replacing the reference with ground and making the threshold points a function of VDD. The Schmitt trigger RC oscillator circuit may be easily incorporated within an integrated circuit (IC) such as that containing or coupled to oscillator module317that may service both the system processor302and the baseband processor304. This oscillator provides greater frequency stability as the frequency is not dependent on the input voltage VDDfor the reasons shown above. As the input voltage can vary as much as +/−10 percent, the possibility of frequency variations is reduced or eliminated. This reduced level of variation makes this Schmitt trigger RC oscillator an acceptable and accurate timing source for determining when to awaken from or enter into the sleep mode and effectively conserve power.

FIG. 9is a logic flow diagram illustrating one method of utilizing the Schmitt Trigger RC Oscillator, for which one embodiment is described inFIG. 8, in order to provide a timing signal having a steady frequency within a portable electronic device such as a wireless terminal. Beginning with Step802, the portable electronic device, such as but not limited to a wireless terminal, enters a sleep mode for a sleep period. The Schmitt Trigger RC Oscillator circuit is used to monitor the duration of the sleep mode in Step804. Because the Schmitt Trigger RC Oscillator Circuit is a lower power oscillator, battery power of the portable electronic device is able to be conserved. Additionally, by eliminating the frequency variations as seen when comparing the Schmitt Trigger RC Oscillator circuits ofFIG. 1andFIG. 8, a more accurate determination of the time to awaken the portable electronic device may be determined such that in Step806the portable electronic device may be awakened from the sleep mode in order to receive an encoded paging burst. Previous timing inaccuracies would have decreased the actual time spent in the sleep mode or required higher power oscillator circuits to be employed. This burst is processed in Step808. A comparison of the results of processing the encoded paging burst are made with a null page pattern, or other like means, to determine whether or not the encoded paging burst received is a null page. At Decision Point812, when the comparison made to determine whether or not the received encoded paging burst corresponds to a null page pattern is favorable, the process may continue to Step814wherein the portable electronic device is able to enter or reenter the sleep mode. In this way additional battery power may be conserved during the next sleep mode. However, if the comparison is unfavorable at Decision Point812, it may be necessary to fully awaken the portable electronic device in order to respond to the page/service call at Step816.

In summary, embodiments of the present invention provide an oscillator circuit having a steady output frequency that is independent of the supplied voltage. This oscillator includes a Schmitt trigger circuit which may be implemented within an integrated circuit of a wireless terminal or other like portable electronic device. The Schmitt trigger circuit receives a threshold voltage input and a second voltage input. The Schmitt trigger circuit generates an output voltage equal to either a first output voltage or a second output voltage based on the results of comparing the threshold voltage input to the second voltage input. An RC network may be coupled to the output of the Schmitt trigger circuit and is operable to supply the second voltage input to the Schmitt trigger circuit. A voltage divider network also couples to the output of the Schmitt trigger circuit wherein the threshold voltage input is proportional to the first output voltage reduced by the voltage divider network based on the output voltage of the Schmitt trigger circuit.

Additional embodiments may employ the Schmitt trigger RC oscillator circuit within portable electronic devices or devices where it is desirable to conserve power, such as a wireless terminal, such that a low power oscillator circuit may be used to provide a steady frequency timing signal for the purpose of determining when to awaken from and enter into a sleep mode in order to conserve power.