Patent Publication Number: US-9426016-B2

Title: Self track scheme for multi frequency band serializer de-serializer I/O circuits

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. §111(a) continuation of PCT international application number PCT/US2013/066194 filed on Oct. 22, 2013, incorporated herein by reference in its entirety, which claims priority to, and the benefit of, U.S. provisional patent application Ser. No. 61/723,284 filed on Nov. 6, 2012, incorporated herein by reference in its entirety. Priority is claimed to each of the foregoing applications. 
     The above-referenced PCT international application was published as PCT International Publication No. WO 2014/074301 on May 15, 2014, which publication is incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED IN A COMPUTER PROGRAM APPENDIX 
     Not Applicable 
     NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION 
     A portion of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. §1.14. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to chip-to-chip communications, and more particularly to a self tracking serializer de-serializer. 
     2. Description of Related Art 
     Conventional serializer de-serializer I/O is based on multiplexing and demultiplexing digital communications. Using such conventional schemes to increase communications bandwidth requires increasing clock rate. 
     Attempting to use multi-frequency bands in a traditional scheme for modulation and demodulation to overcome the above problems brings up additional issues. Current multi-frequency serializers and de-serializers for use with chip-to-chip I/O include modulation and demodulation that are complicated and dependent on external factors, such as silicon process, connection conditions, power supply quality, and the like. In these conventional systems, a complicated scheme, such as error correction or base band processing is required to achieve reliable modulation and demodulation with low bit error rate. When the latency of modulation and demodulation becomes critical in the I/O connection, the traditional base band processing, used to ensure low bit-error-rate (BER), in modulation and demodulation becomes impractical. 
     The use of base band processing techniques can in some cases provide for reliable data transmission and reception, yet it comes with a high cost penalty regarding circuit complexity and unnecessarily long delays for data processing. Although the typical multi-frequency approach may be suitable for high throughput operations, it is not well suited when short latencies are required to perform mission critical operations. 
     Accordingly, a need exists for chip-to-chip multi-frequency communication circuits that have short latencies and are readily implemented. The present invention fulfills these needs, and overcomes shortcomings of previous multi-frequency chip-to-chip communication topologies. 
     BRIEF SUMMARY OF THE INVENTION 
     A chip-to-chip serializer and de-serializer are described which utilize a self tracking method based on track pulse generation on the transmitter (TX) and track pulse restoration on the receiver (RX). The data to be transmitted is synchronized with the generated track pulse on TX, with the transmitted data and track pulse being modulated at the same time in the TX. All signals communicated chip-to-chip utilizing the present invention are processed under the same conditions, including silicon process variation, power noise, critical path delay, and so forth, thus eliminating/reducing the impact of these variables on operation of the serializer de-serializer. 
     Utilizing the inventive self-tracking serializer de-serializer, one can reach the performance limitation for any given technology by not only increasing the data throughput, but in response to also reducing data transfer latency. 
     The signals are serialized, modulated and transmitted through a short I/O connection from transmitter (TX) to receiver (RX). The RX also demodulates all signals under the same conditions and characteristics of the receiver. The track pulse is restored in the RX after demodulation. Because the data and track pulse are synchronized in the TX, the signals should likewise be synchronized in the RX when all signals are processed with identical demodulation. Once the track pulse is restored, the self track scheme can sample the restored data at the correct timing. The sample timing tracks the external factors even under different operating conditions, use of different integrated circuit chips or different process technology. The next level of synchronization with the system bus after the signal is sampled in the analog-to-digital converter (ADC) which can also be processed based on the timing of restored track pulse. 
     The restored track pulse can experience significant jitter in practice. By further use of over-sampling techniques to build a restored track pulse, the implementation provides an improved large jitter tolerance. 
     The inventive self tracking serializer de-serializer provides significant improvements to the yield of chip-to-chip I/O circuits. The inventive system can be ported to fabricate devices compatible with future silicon process advancements, such as from 28 nm nodes to 14 or 20 nm nodes with minor effort. 
     Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: 
         FIG. 1A  and  FIG. 1B  is a block diagram of a self-tracking multi-frequency band serializer and de-serializer according to an embodiment of the present invention. 
         FIG. 2  is a timing diagram of track pulse generation and restoration within a self-tracking multi-frequency band serializer and de-serializer according to an embodiment of the present invention. 
         FIG. 3  is a flow diagram of a self tracking serializer de-serializer method according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A self tracking serializer de-serializer is described in which a track pulse is generated which travels along with the digital data through serialization and modulation at the transmitter (i.e., first chip) and the demodulation and de-serialization at the receiver (i.e., second chip). This self tracking ability allows one to build a circuit with minimum timing overhead without complicated base band processing, while performance and device yield are achieved with low circuit overhead. Integrated circuits can be fabricated using this self track mechanism in any desired device technology or process, including 28 nm or use of advanced silicon process technologies. 
       FIG. 1A  and  FIG. 1B  illustrate an example embodiment  10  for a self tracking multi-frequency band serializer and de-serializer, shown for communicating between a transmitter (TX) in a first chip  12  seen in  FIG. 1A  over an I/O channel  14 , to a receiver (RX) in a second chip  16  seen in  FIG. 1B . 
     Digital data  18  (DQ_TX) and byte mask  19  (DM_TX) is seen received in the TX in  FIG. 1A . The byte mask is utilized when the data word to be communicated is sent in multiple sections (e.g., 16 bits over the 8 bit path shown in  FIG. 1A ,  FIG. 1B ). Although the example embodiment often describes a structure allowing sending of 8 bits of digital data (e.g., [7:0]), it will be appreciated that the present invention can be configured with any desired number of bits (e.g., 16 bits, 32 bits, and so forth). However, this requires adding more data buffers, converters (DAC, ADC), and modulators and demodulators operating at more frequencies or with higher order encoding. 
     The transmitter controller asserts a synchronization signal  20  (DQS_TX) flag to trigger tracking and serialization. In any embodiment of the invention, the DQS_TX pulse traverses the whole propagation path to which the data is subject. A track pulse generator  24  receives synchronization signal  20  (DQS_TX) and a clock signal  22 . The track pulse generator is configured to generate a synchronized pulse as the flag for data synchronization. It is shown in the figure that DQS_TX is also applied to a buffer for data mask (DM_TX)  19  and as a first input of the data buffers  26   a ,  26   b ,  26   c , on through to  26   m  and  26   n , which also receive bits within data DQ_TX  18  so that data synchronizes with the track pulse. 
     The track pulse and digital data word are serialized and modulated for transmission to the receiver. Bits of data DQ_TX  18  along with a track pulse from track pulse generator  24  at modulators  28   a ,  28   b , through  28   n . Each of these modulators is configured for converting the digital data to analog data which is encoded over multiple frequency channels, such as using multiple modulators which are each configured for operation at a different frequency (e.g., different carrier frequencies). It should be appreciated that one of these modulation frequencies can be zero, that is DC. Using DC as one modulation frequency can reduce the number of frequency generation circuits needed, including phase-locking circuits (e.g., PLL) which are needed. As these signals are well known with analog modulation and demodulation the circuits for generating them are not shown within controllers  62  and  74  in  FIG. 1A  and  FIG. 1B , respectively. In one example, each modulator utilizes quadrature amplitude modulation (QAM) (e.g., QAM16) and has mixers which encode both an I channel and a Q channel of information into a given modulation frequency. 
     It will be appreciated that QAM, as described in this embodiment, is an analog modulation mechanism, which differs from digital multiplexing used in a digital serialization scheme. In analog QAM, two analog message signals are communicated on each frequency channel by changing (modulating) two carrier waves. The two carrier waves (typically sinusoids), are out of phase with each other by 90° and are thus called quadrature carriers. Output over a frequency channel is the sum of the modulated waves of phase modulation (PM) and amplitude modulation (AM). For the sake of simplicity of description, the internal circuitry for analog QAM is not described. It will be noted that a large number of QAM circuits are available and the technology is well known to one of ordinary skill in the art. It should be appreciated that a variety of forms of QAM are available and can be utilized with the present invention, some of the more common forms that can be selected for use include: QAM8, QAM16, QAM32, QAM64, QAM128, and QAM256. It will be appreciated that QAM distributes information in the I-Q plane evenly, and the higher orders of QAM involve information spaced more closely in the constellation. Thus, higher order QAM allows transmitting more bits per symbol, but if the energy of the constellation is to remain the same, the points on the constellation are closer together and the transmission becomes more susceptible to noise. It should also be appreciated that other forms of modulation (demodulation) can be utilized in the present invention without departing from the teachings herein. Examples of other forms of multi-frequency modulation which can be utilized include pulse-width modulation (PWM), frequency-shift keying (FSK), frequency-hopping, spread spectrum, and so forth. 
     During serialization and modulation in  FIG. 1A , digital data is converted by digital-to-analog converter (DAC)  30   a ,  30   b , resulting in analog outputs received at mixers  32   a ,  32   b , through  32   m ,  32   n  along with 90 degree out-of-phase modulation carriers f i  and f q , respectively. In the example figures, each of the DACs is shown comprising a 2 bit DAC. It should be appreciated, however, that embodiments can be readily implemented using DACs with a different number of bits, such as 4 or more bits. Each of the modulators  28   a ,  28   b  through  28   n  operate at a different frequency, with the output of each being summed at the I/O channel  14  and thus travel through the same I/O connection for receipt at second chip  16 . This configuration assures a close tracking of the data path through the same modulation process as the tracking signal, regardless of the various channel conditions of I/O channel  14 . 
     In the receiver (RX)  16  seen in  FIG. 1B , all signals are demodulated through the same path, exemplified with demodulators  34   a ,  34   b  through  34   n , each operating at a different frequency within the multiple frequencies (e.g., which can include zero frequency (DC)). Using QAM demodulators, each demodulator receives the incoming analog signal at two mixers (e.g.,  36   a  and  36   b ,  36   c  and  36   d , on through to  36   m  and  36   n ) which also receives 90 degree out-of-phase modulation carriers f i  and f q , respectively. Demodulated output from each of the mixers is received by analog-to-digital converters (ADC)  38   a ,  38   b  through  38   m ,  38   n . The output from the ADC is only collected in synchronous with the track pulse. By continuously sampling the ADC of the track pulse at the TX, this signal can be restored in the RX. 
     One of the ADCs is seen outputting digital data to a track pulse restore circuit  40 , which operates in combination with output data buffers  42   a ,  42   b ,  42   c , through  42   m  and  42   n . A first output  44 , comprises the restored track pulse itself used to synchronize the output from the ADCs to the data buffers. Track pulse restore circuit  40  receives a clock  46  (CLK_SYS), and outputs a synchronization signal (pulse)  48  for controlling the latching of data from the ADCs at each of the data buffers whose output is de-serialized digital data (DQ_RX)  52 . Once data from the ADCs is latched to the data buffer, the track pulse restoration circuit sets a flag DQS_RX  50 , signaling that the receiver controller can now read the DQ_RX data word. A byte mask (DM_RX)  51  is also generated for masking purposes when handling multiple bytes. 
     It will be seen that the track pulse and all data follow through the same path and time, thus allowing the track pulse restoration circuit to generate signals for controlling ADC output and latching of the data into the buffers with proper timing. 
     The above serializer and de-serializer are particularly well-suited for operation on a first chip and a second chip between which communication is to be established. It will be appreciated that a second I/O channel can be utilized for establishing a communication path in the opposite direction between the first and second chips. This first and second chips seen in  FIG. 1A  and  FIG. 1B  are each configured with a controller  62 ,  74 , respectively, for controlling the transmit and receive side operations and generating the signals for operating the serializer and de-serializer. By way of example, and not limitation, the TX controller is shown having at least one processor circuit  64 , comprising CPU  68  with memory  70 , and utilizing a clock circuit  66 . In a similar manner RX controller  74  is shown having at least one processor circuit  76 , comprising CPU  78  with memory  80 , and utilizing a clock circuit  82 . It will be appreciated, however, that controller circuits can be implemented with various combinations of discrete and programmable logic circuits and processors, without departing from the teachings of the present invention. 
       FIG. 2  illustrates an embodiment  90  of signal timing according to the invention as shown in  FIG. 1  for serializing D_TX (e.g., D_TX [7:0]), for transmission from the first chip and upon receipt at the second chip performing deserialization back into D_RX (e.g., D_RX [7:0]). The upper portion of the figure shows signals on the transmitter (TX) while the lower section shows signals on the receiver (RX). A clock signal (CLK_SYS) is utilized for timing the various actions in the transmitter circuit. The rising edge of CLK_SYS is utilized to strobe DQS_RX and D_RX. A transmitter enable signal (TX_EN) enables data serialization and transmission. Upon asserting DQS_TX, the TRACK_TX pulses are generated to which all D_TX data operations are synchronized during modulation. 
     The lower portion of  FIG. 2  depicts TRACK_RX and data D_RX being restored. A clock signal (CLK_SYS) is utilized for timing the various actions in the receiver circuit. A receiver enable signal (RX_EN) enables data deserialization. The TRACK_RX signal is shown experiencing jitter after demodulation. Data output from the ADCs is seen (D_after_ADC) after demodulation. The track pulse restore circuit will perform the restoration of data from the desired eye window and then synchronizes the data bus with the system clock. The DQS_RX flag signal is seen being generated as data words D_RX and are ready to be read by the receiver controller. 
       FIG. 3  illustrates an example embodiment of the tracked method of serializing and de-serializing according to the invention. Beginning with step  110 , a track pulse start signal is used for starting track pulse generation and storing  112  a digital data word into buffers for modulation and transmission. The track pulses and digital data word are serialized and modulated  114  into a multi-frequency analog output for receipt, such as on another chip, by a de-serializer. In the de-serializer, the multi-frequency analog is converted  116  back to a digital data word, and the track pulse is also restored  118 . Synchronization signals are generated in response to the restored track pulse for collecting  120  the digital data word for output. It will be noted that synchronization signals are generated in response to the track pulse for: (1) triggering the digital output to the buffers from the ADCs, (2) latching digital data onto the buffers; and (3) signaling that a digital data word is ready. 
     From the discussion above it will be appreciated that the invention can be embodied in various ways, including the following: 
     1. An apparatus for serializing and de-serializing chip-to-chip communications, comprising: a serializer configured for serializing and modulating digital data bits, said serializer having a track pulse generator and data buffers whose digital data bit outputs are modulated by multiple modulators into multiple analog frequency signals configured for communication over an I/O channel to an off chip de-serializer; and a de-serializer having multiple demodulators configured for receiving said multiple analog frequency signals over the I/O channel and demodulating these signals for receipt by a track pulse restoration circuit and data buffers; wherein said track pulse restoration circuit generates synchronization signals for triggering output from said demodulators to said receiver data buffers, latching data into said receiver data buffers, and signaling that digital data bits can be read from said receiver data buffers. 
     2. The apparatus of any of the previous embodiments, wherein said modulator and said demodulator operate utilizing quadrature amplitude modulation (QAM). 
     3. The apparatus of any of the previous embodiments, wherein said modulator and said demodulator operate utilizing quadrature amplitude modulation (QAM), selected from the group of QAM orders consisting of QAM8, QAM16, QAM32, QAM64, QAM128 or QAM256. 
     4. The apparatus of any of the previous embodiments, wherein said quadrature amplitude modulation (QAM) encodes two analog message signals into carrier waves at its carrier frequency. 
     5. The apparatus of any of the previous embodiments, wherein said digital data bits comprises at least 8 bits. 
     6. An apparatus for serializing and de-serializing chip-to-chip communications, comprising: a track pulse generator, configured for generating track pulses, within a serializer configured for serializing words of digital data into multiple analog frequency signals for communication to an off chip de-serializer; multiple transmitter data buffers configured for receiving digital data bits; wherein said multiple transmitter data buffers load data bits in response to receipt of a synchronization signal which is also received at said track pulse generator; multiple modulator circuits, each modulator circuit operating at a different modulation frequency; said modulator circuits are configured for converting said track pulses and outputs from said multiple transmitter data buffers into an analog signal modulated into carriers signals at multiple frequencies for transmission to a de-serializer; and multiple demodulator circuits within a de-serializer configured for demodulating and de-serializing data from multiple analog frequency signals received from a serializer, and outputting digital data; wherein each demodulator circuit operates at a different modulation frequency; wherein said demodulator circuits are configured for converting analog signals modulated into carriers signals at multiple frequencies into words of digital data; a track pulse restoration circuit, configured for restoring the track pulse from the transmitter and generating multiple synchronization signals; multiple receiver data buffers configured for receiving digital data bits from said demodulator circuits; wherein output of digital data from each said demodulator is triggered in response to a first synchronization signal from said track pulse restoration circuit; wherein said digital data from each said demodulator is latched into each of said multiple receiver data buffers in response to receipt of a second synchronization signal from said track pulse restoration circuit; and wherein said digital data is read from said data buffers in response to a third synchronization signal from said track pulse restoration circuit. 
     7. The apparatus of any of the previous embodiments, wherein said modulation and demodulation utilizes quadrature amplitude modulation (QAM). 
     8. The apparatus of any of the previous embodiments, wherein said QAM is selected from the group of QAM orders consisting of QAM8, QAM16, QAM32, QAM64, QAM128 or QAM256. 
     9. The apparatus of any of the previous embodiments, wherein said quadrature amplitude modulation (QAM) encodes two analog message signals into carrier waves at each output frequency. 
     10. The apparatus of any of the previous embodiments, wherein said digital data bits comprises at least 8 bits. 
     11. A method for serializing and de-serializing chip-to-chip communications, comprising: generating track pulses within a serializer of a first device chip; storing a digital data word for transmission from the first device chip; serializing said track pulses and said digital data word and modulating them into multiple frequency analog signals for communication a de-serializer in a second chip; demodulating the multiple frequency analog signals received at a second device chip; performing restoration of the track pulse received from said first device chip; and controlling the collection of said digital data word into output buffers in response to synchronization signals timed in relation to restoration of the track pulse. 
     12. The method of any of the previous embodiments, wherein said storing a digital data word for transmission is performed with multiple transmitter data buffers which load data bits in response to receipt of a synchronization signal which is also received at a track pulse generator which generates said track pulses. 
     13. The method of any of the previous embodiments, wherein said serializing and modulating into multiple frequency analog signals is performed by multiple modulator circuits, each modulator circuit operating at a different modulation frequency. 
     14. The method of any of the previous embodiments, wherein each said modulator circuit contains digital-to-analog converters (DACs) coupled to modulating mixers. 
     15. The method of any of the previous embodiments, wherein said modulating and demodulating is performed in response to a quadrature amplitude modulation (QAM) technique. 
     16. The method of any of the previous embodiments, wherein said quadrature amplitude modulation (QAM) encodes two analog message signals into carrier waves at each output frequency. 
     17. The method of any of the previous embodiments, wherein said quadrature amplitude modulation (QAM) is selected from the group of QAM orders consisting of QAM8, QAM16, QAM32, QAM64, QAM128 or QAM256. 
     18. The method of any of the previous embodiments, wherein said de-serializing and demodulating from multiple frequency analog signals is performed by multiple demodulator circuits, each demodulator circuit operating at a different modulation frequency. 
     19. The method of any of the previous embodiments, wherein each said demodulator circuit contains analog-to-digital converters (ADCs) coupled to demodulating mixers. 
     20. The method of any of the previous embodiments, wherein said synchronization signals timed in relation to restoration of the track pulse, comprises: a first synchronization signal which triggers demodulator output of digital data to data buffers; a second synchronization signal which latches said digital data onto the data buffers; and a third synchronization signal which indicates that the latched digital data can be read from the buffers. 
     Although the description herein contains many details, these should not be construed as limiting the scope of the disclosure but as merely providing illustrations of some of the presently preferred embodiments. Therefore, it will be appreciated that the scope of the disclosure fully encompasses other embodiments which may become obvious to those skilled in the art. 
     In the claims, reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the disclosed embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed as a “means plus function” element unless the element is expressly recited using the phrase “means for”. No claim element herein is to be construed as a “step plus function” element unless the element is expressly recited using the phrase “step for”.