Abstract:
An integrated circuit device may have an internal oscillator for generating a system clock, a trimming logic with a trimming register for adjusting an oscillation frequency of the internal oscillator; a serial data receiver, wherein a serial data stream includes a synchronization signal. The synchronization signal is operable to indicate that the system clock correct, too fast or too slow. The device may further have a circuit for decoding the synchronization signal operable to re-adjust a value stored in the trimming register upon evaluation of the synchronization signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/777,678 filed on Mar. 12, 2013, which is incorporated herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a method and apparatus for clock recovery, in particular clock recovery of a Controller Area Network (CAN) bus signal. 
     BACKGROUND 
     CAN signals are asynchronous serial communication signals that do not provide for a separate clock signal. Hence, CAN uses an asynchronous transmission that embeds the clock signal in the transmitted serial data stream. A receiving device must then analyze this signal and reconstruct the clock to synchronize the device to the received serial stream. The Controller Area Network (CAN) protocol is an asynchronous serial bus with Non-Return to Zero (NRZ) bit coding designed for fast, robust communications in harsh environments, such as automotive and industrial applications. The CAN protocol allows the user to program the bit rate, the sample point of the bit, and the number of times the bit is sampled. With these features, the network can be optimized for a given application. The specific bit timing of the CAN protocol is discussed in the application note “Understanding Microchip&#39;s CAN Module Bit Timing”, AN754, by Pat Richards, published in 2001 which is hereby incorporated by reference. 
     Timing within the receiving device is generally controlled by an internal or external oscillator of the receiving device. Such oscillators generally provide for limited timing resolution within the device. However, oscillator tolerance in a CAN system needs to be tighter than what is normally achievable with an internal oscillator across operating temperature ranges. This requires CAN modules to be clocked with an external source (crystal, clock, etc.). It is more desirable to have an internal clock source. 
     SUMMARY 
     Hence, a need exists for an improved device operable to receive a CAN signal with an internal clock source. 
     According to an embodiment, an integrated circuit device may comprise: an internal oscillator for generating a system clock, a trimming logic comprising a trimming register for adjusting an oscillation frequency of the internal oscillator; a serial data receiver, wherein a serial data stream includes a synchronization signal, wherein the synchronization signal is operable to indicate that the system clock correct, too fast or too slow; a circuit for decoding the synchronization signal operable to re-adjust a value stored in the trimming register upon evaluation of the synchronization signal. 
     According to a further embodiment, the integrated circuit device is can be a CAN protocol controller. According to a further embodiment, the serial data stream can be a CAN data stream. According to a further embodiment, the synchronization signal can be a synchronization jump width value in the serial data stream. According to a further embodiment, internal oscillator can be an RC oscillator. According to a further embodiment, the integrated circuit device furthermore may comprise a PLL coupled with the RC oscillator to provide the system clock. According to a further embodiment, the trimming register can be a configuration register of a microcontroller or CAN protocol controller. According to a further embodiment, the integrated circuit device may further comprise a CAN unit comprising a sync jump width processor generating a control signal dependent on the sync jump width value coupled with an oscillator tuning unit configured to increment or decrement the trimming register. According to a further embodiment, the integrated circuit device may further comprise a temperature sensor coupled with temperature register, wherein the temperature register is coupled with the oscillator tuning unit. 
     According to another embodiment, a method for operating an integrated circuit device may comprise generating a system clock by an internal oscillator, loading a trimming register to adjust an oscillation frequency of the internal oscillator; receiving a serial data stream by a serial data receiver, wherein the serial data stream includes a synchronization signal, wherein the synchronization signal is operable to indicate that the system clock correct, too fast or too slow; and evaluating the synchronization signal and depending on the synchronization signal re-adjusting a value stored in the trimming register. 
     According to a further embodiment of the method, the integrated circuit device can be a CAN protocol controller and the serial data stream can be a CAN data stream. According to a further embodiment of the method, the synchronization signal can be a synchronization jump width value in the serial data stream. According to a further embodiment of the method, the internal oscillator can be an RC oscillator. According to a further embodiment of the method, the method may further comprise multiplying an output frequency of the RC oscillator by a PLL to provide the system clock. According to a further embodiment of the method, the trimming register can be a configuration register of a microcontroller. According to a further embodiment of the method, the method may further comprise increment or decrement the trimming register by a predefined step value depending on the synchronization jump width value. According to a further embodiment of the method, the method may further comprise measuring a temperature of the integrated circuit device by an internal temperature sensor and trimming the oscillator frequency according to the temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  show a timing diagram of the time quantum bit segments and bit period of a typical CAN signal 
         FIG. 2  is a timing diagram of scenarios of a typical received CAN signal. 
         FIG. 3  is another timing diagram including logic signals generated according to various embodiments; 
         FIG. 4  is a general block diagram of a programmable oscillator according to various embodiments. 
         FIG. 5  is a detailed block diagram of a receiving device. 
         FIG. 6  shows a plurality of possible packages of integrated circuit devices according to various embodiments. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents. 
     DETAILED DESCRIPTION 
     Various embodiments allow for the recovery of the CAN clock in an asynchronous received signal so the internal oscillator can be dynamically calibrated. An internal oscillator of a receiving device operating according to the CAN protocol can be dynamically calibrated, for example, by monitoring the synchronization jump width (SJW) phase adjustments signals and calibrating the oscillator as needed. 
     Integrated oscillators typically operate with an internal resistor-capacitor oscillation circuit. Various internal circuitry may be used, in particular phase lock loop circuits and other circuitry to stabilize clock signals generated by such circuits. To reduces costs, microcontrollers often use such internal circuits thereby avoiding external oscillators such as crystals. Integrated oscillators of, for example, such microcontrollers drift over time, voltage and temperature, in particular substantially more than external crystal oscillators. Internal oscillators drift too much over temperature to allow use with the CAN module. However, according to various embodiments, the need for an external clock with low PPM variation is avoided. 
     According to various embodiments, the synchronization jump width (SJW) mechanism in the CAN module, which automatically adjusts a given bit time when in receive mode to stay in sync with the transmitting node, as discussed in more detail in the above mentioned application note AN754, can be used to dynamically trim the internal oscillator to match the transmitting node&#39;s oscillator. 
     As mentioned above, the clock signal is not sent as a separate part of the CAN signal. CAN specifies a worst case oscillator tolerance of 1.58% and many systems require much tighter tolerance, particularly automotive systems which can require a 0.3% tolerance across automotive temperature ranges (−40° C. to +125° C.). Thus, according to various embodiments, to meet these specifications the internal oscillator of the receiving device can be dynamically calibrated to compensate for internal oscillator drift over temperature and voltage, to eliminate the need for an external clock source. 
     The clock signal of the transmitting node can be revealed by monitoring the phase errors using the Synchronization Jump Width (SJW) mechanism built into the CAN engine. The internal oscillator can be adjusted up/down based on the phase error of the SJW signal.  FIGS. 2 and 3  show how the SJW signal is used to add/subtract Time Quanta (TQ) on a received bit depending on the phase error polarity of the actual transition edge. 
     As shown in  FIG. 1 , a CAN bit time is made up of non-overlapping segments. Each of these segments are made up of integer units called Time Quanta (TQ). The Nominal Bit Rate (NBR) is defined in the CAN specification as the number of bits per second transmitted by an ideal transmitter with no resynchronization and can be described with the equation:
 
NBR= f   bit =1 /t   bit  
 
     The Nominal Bit Time (NBT), or t bit , is made up of nonoverlapping segments SS, PropSeg, PS1, and PS2, therefore, the NBT is the summation of the following segments:
 
 T   bit   =t   SyncSeg   +t   PropSeg   +t   PS1   +t   PS2  
 
     Associated with the NBT are the Sample Point, Synchronization Jump Width (SJW), and Information Processing Time (IPT). The Synchronization Segment (SyncSeg) is the first segment in the NBT and is used to synchronize the nodes on the bus. Bit edges are expected to occur within the SyncSeg. This segment is fixed at 1TQ. The Propagation Segment (PropSeg) exists to compensate for physical delays between nodes. The propagation delay is defined as twice the sum of the signal&#39;s propagation time on the bus line, including the delays associated with the bus driver. The PropSeg is programmable from 1-8TQ. SJW overlays the following segments PS1 and PS2 and can be programmed from 1-4 TQ. PS1 is programmable from 1-8TQ and PS2 is programmable from 2-8TQ. The sample point is the point in the bit time in which the logic level is read and interpreted. The sample point is located at the end of phase segment 1. The exception to this rule is, if the sample mode is configured to sample three times per bit. In this case, the bit is still sampled at the end of PS1, however, two additional samples are taken at one-half TQ intervals prior to the end of PS1 and the value of the bit is determined by a majority decision. The Information Processing Time (IPT) is the time required for the logic to determine the bit level of a sampled bit. The IPT begins at the sample point, is measured in TQ and is fixed for the CAN module, for example in some cases it may assume 2TQ. Since phase segment 2 also begins at the sample point and is the last segment in the bit time, it may be required that PS2 minimum is not less than the IPT. Therefore, according to one scenario:
 
PS2 min =IPT=2TQ
 
     The Synchronization Jump Width (SJW) adjusts the bit clock as necessary by 1-4TQ as configured) to maintain synchronization with the transmitted message. Each of the segments that make up a bit time are made up of integer units called Time Quanta (TQ). The length of each Time Quantum is based on the oscillator period (t OSC ). The base TQ equals twice the oscillator period. 
       FIG. 1  shows how the bit period is derived from t OSC  and TQ. The TQ length equals one TQ Clock period (t BRPCLK ), which is programmable using a programmable prescaler named the Baud Rate Prescaler (BRP). This is shown in the following equation: 
     
       
         
           
             TQ 
             = 
             
               
                 2 
                 · 
                 BRP 
                 · 
                 
                   T 
                   OSC 
                 
               
               = 
               
                 
                   2 
                   · 
                   BRP 
                 
                 
                   F 
                   OSC 
                 
               
             
           
         
       
     
     All nodes on the CAN bus must have the same nominal bit rate. Noise, phase shifts, and oscillator drift can create situations where the nominal bit rate does not equal the actual bit rate in a real system. Therefore, the nodes must have a method for achieving and maintaining synchronization with bus messages. 
     The bit timing for each node in a CAN system is derived from the reference frequency (f OSC ) of its node. This creates a situation where phase shifting and oscillator drift will occur between nodes due to less than ideal oscillator tolerances between the nodes. The CAN specification indicates that the worst case oscillator tolerance is 1.58% and is only suitable for low bit rates (125 kb/s or less). 
     The CAN protocol has defined a recessive (logic 1) and dominant (logic 0) state to implement a non-destructive bit-wise arbitration scheme. It is this arbitration methodology that is affected the most by propagation delays. Each node involved with arbitration must be able to sample each bit level within the same bit time. For example, if two nodes at opposite ends of the bus start to transmit their messages at the same time, they must arbitrate for control of the bus. This arbitration is only effective if both nodes are able to sample during the same bit time. Extreme propagation delays (beyond the sample point) will result in invalid arbitration. This implies that bus lengths are limited at given CAN data rates. A CAN system&#39;s propagation delay is calculated as being a signal&#39;s round trip time on the physical bus (t bus ), the output driver delay (t drv ), and the input comparator delay (t cmp ). Assuming all nodes in the system have similar component delays, the propagation delay is explained mathematically as:
 
 t   prop =2( t   bus   +t   cmp   +t   drv )
 
     All nodes on a given CAN bus must have the same NBT. The NRZ bit coding does not encode a clock into the message. The receivers must synchronize to the transmitted data stream to insure messages are properly decoded. There are two methods used for achieving and maintaining synchronization. Hard Synchronization only occurs on the first recessive-to-dominant (logic “1” to “0”) edge during a bus idle condition, which indicates a Start-of-Frame (SOF) condition. Hard synchronization causes the bit timing counter to be reset to the SyncSeg which causes the edge to lie within the SyncSeg. At this point, all of the receivers will be synchronized to the transmitter. Hard synchronization occurs only once during a message. Also, resynchronization may not occur during the same bit time (SOF) that hard synchronization occurred. 
     Resynchronization is implemented to maintain the initial synchronization that was established by the hard synchronization. Without resynchronization, the receiving nodes could get out of synchronization due to oscillator drift between nodes. Resynchronization is achieved by implementing a Digital Phase Lock Loop (DPLL) function which compares the actual position of a recessive-to-dominant edge on the bus to the position of the expected edge (within the SyncSeg) and adjusting the bit time as necessary. The phase error of a bit is given by the position of the edge in relation to the SyncSeg, measured in TQ, is shown in  FIG. 2  and is defined as follows: 
     Example  210  in  FIG. 2 : e=0; the edge lies within the SyncSeg. 
     Example  220  in  FIG. 2 : e&gt;0; the edge lies before the sample point. (TQ added to PS1). 
     Example  230  in  FIG. 2 : e&lt;0; the edge lies after the sample point of the previous bit. (TQ subtracted from PS2) 
       FIG. 2  shows how phase errors, other than zero, cause the bit time to be lengthened or shortened, wherein 1) Only recessive-to-dominant edges will be used for synchronization; 2) Only one synchronization within one bit time is allowed; 3) An edge will be used for synchronization only if the value at the previous sample point differs from the bus value immediately after the edge; 4) A transmitting node will not resynchronize on a positive phase error (e&gt;0). This implies that a transmitter will not resynchronize due to propagation delays of it&#39;s own transmitted message. The receivers will synchronize normally; and 5) If the absolute magnitude of the phase error is greater than the SJW, then the appropriate phase segment will be adjusted by an amount equal to the SJW. 
     Thus, as shown in  FIG. 3 , if the SJW controller detects a positive phase error, the receiver&#39;s oscillator is too fast with respect to the transmitting node. The SJW will add the pre-configured time quantum (TQ) as normally occurs in CAN as mentioned above. However, in addition, according to various embodiments, as shown in  FIG. 3  the phase error signal will also trigger a state machine to slow the oscillator in the trim register. To this end, as shown with signal  310  the SJW controller sends a signal dec_osc_trim to the state machine which causes the trim register to be adjusted accordingly, for example by subtracting a predefined step value. The trim register controls the actual oscillation frequency of the internal RC oscillator. 
     Likewise, if the phase error is negative, as shown with signal  320  the receiver is too slow with respect to the transmitting node. The SJW controller shortens the bit in question by the TQ, and also adjusts the oscillator trim register using signal inc_osc_trim accordingly. Thus, the trim register will be incremented by a predefined step value. There may be separate control signals for incrementing and decrementing as shown, for example, the signals may use the transition from high to low to signal that a respective adjustment must be made. Other signals may be used to increment or decrement the oscillator frequency. It is to be noted that the internal oscillator is stable at constant temperatures. Adjustments would only be needed if the temperature changes enough to cause phase errors in the SJW due to oscillator drift. 
     Most effort in a microcontroller unit (MCU) is to develop a more stable internal oscillator to address the need for multiple peripherals. While a CAN peripheral is not realized in every microcontroller, other peripheral device may also benefit from a more stable oscillator. The focus has most likely been the 80/20 rule. 
       FIG. 4  shows a CAN peripheral module  400  in an integrated circuit device, for example a microcontroller, that provides for an additional output signal for re-calibrating the internal oscillator of a microcontroller. The internal RC oscillator of the device comprises an adjustment unit including, for example, a register which is programmable to fine adjust the output frequency of the internal oscillator. Thus, trimming data can be usually programmed by a user to adjust the oscillator when its output frequency has drifted from an initial factory adjustment. According to an embodiment as shown in  FIG. 4 , the adjustment signal provided by the CAN transceiver  410  may be stored in a volatile or non-volatile memory  430  which may operate as a trimming register for oscillator  420 . According to an embodiment, the CAN module may integrate respective logic and directly write into the trimming register  420 . Thus, a user does not need to program the device to readjust the trimming value. 
       FIG. 5  shows a more detailed block diagram of a CAN module  500  that can be integrated within a microcontroller according to various embodiments. Here, the system clock is provided by unit  510  which can be fine adjusted by oscillator tuning block  550 . This tuning block  550  may be coupled with register OSCTUNE  560 . According to some embodiments, an additional temperature sensor  570  may be integrated with the microcontroller. Temperature data can be retrieved from an optional temperature register  580  which is also coupled with tuning block  550 . Tuning block  550  is configured to automatically adjust the system clock oscillator  510  when an up or down adjustment is received from the CAN unit  520 ,  530 ,  540  in particular, sync jump width processor  525  which may be part of the bit stream processor  520  coupled with bit timing logic  530  which again is coupled with the pins TX and RX. A shift register  540  is also shown which may be used to assemble the incoming data signal. According to some embodiments, tuning circuitry  550  may further automatically adjust the oscillating frequency according to an internal algorithm depending on the present temperature measured by an optional temperature sensor  570 . 
       FIG. 6  shows various embodiments  610 ,  620 ,  630  of housings comprising integrated CAN controllers and the capabilities according to the present disclosure using different packages such as dual in-line and various surface mount packages as known in the art. The different packages are merely examples. 
     While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.