Abstract:
A digital phase locked loop (DPLL) for providing clock synchronization in backplane bus systems has a loop filter with selectable high and low bandwidth modes. The DPLL is thus capable of respectively attenuating or tracking jitter from an input reference clock.

Description:
BACKGROUND OF INVENTION 
   This invention relates to the field of digital communications, and in particular clock synchronization in systems employing a backplane bus. 
   In switching systems where several cards are connected to a common data bus, it is important that clock distribution and synchronization be done such that re-arrangements in the clock distribution do not affect data transport over the common data bus. Standards, such as the ECTF H.110 standard, describe the implementation of a common data bus and a clocking scheme to ensure that no data disruptions occur due to clocking re-arrangements. 
   The ECTF H.110 standard defines timing devices within the system as “primary master”, “secondary master” or “slave”. The “primary master” and “secondary master” devices are capable of synchronizing to a network reference clock and driving one of the two independent backplane bus clock/frame pulse pairs. The backplane bus clock is 8.192 MHz while the backplane bus frame pulse is 8 kHz. The “slave” devices use the main backplane bus clock and frame pulse, the “A” clocks for synchronization to the backplane bus. The “A” clocks are driven by the “primary master” and are derived from a local network reference or a system wide network reference. A second pair consisting of a backplane bus clock and frame pulse, the “B” clocks are used as a backup clock on the backplane. The “B” clocks are driven by the “secondary master”. During normal operation, the “secondary master” and “slaves” are synchronized to the “A” clocks. When the “A” clocks becomes unreliable, the “slave” devices switch from using the “A” clocks to the “B” clocks for synchronization to the backplane bus. To ensure that during the switch from the “A” clocks to the “B” clocks, no data is lost, the “B” clocks must be phase locked with a minimum phase offset to the “A” clocks when the “A” clocks are reliable. When the “B” clocks are the main clocks on the backplane, the “secondary master” device that drives the “B” clocks switches from using the “A” clocks to using a network clock as its reference without disrupting data traffic on the backplane bus. 
   In order to meet all the requirements of synchronization and clock switching without the loss of data, the “secondary master” device must use a Phase Locked Loop (PLL) that can track the “A” clocks closely even in the presence of jitter. Therefore the “secondary master” should pass the jitter on the “A” clocks on to the “B” clocks. While not strictly required by the ECTF H.110 standard, it is also preferable for the “primary master” to attenuate jitter from the network reference clock while driving the “A” clocks. However, in order for the “Secondary master” to track the jitter on the “A” clocks, the PLL driving the “B” clocks may not attenuate the jitter. Since a “secondary master” or “slave” can become a “primary master”, the DPLLs in these devices must track the “A” clocks continuously. Also, they must be able to switch from one input reference to another and at the same time switch from no jitter attenuation to jitter attenuation without disrupting the backplane clock and frame pulse. 
   Prior art implementations have not been able to provide these mixed jitter attenuation requirements without disrupting data during clock re-arrangements. 
   SUMMARY OF INVENTION 
   According to the present invention there is provided a digital phase locked loop (DPLL) for providing clock synchronization in backplane bus systems, which has a loop filter having selectable high and low bandwidth modes for respectively attenuating or tracking jitter from an input reference clock. 
   The invention allows one single device to be employed to perform both main roles required by the H.110 standard, namely synchronizing to a network clock with jitter attenuation, and synchronizing to a backplane clock without jitter attenuation. The invention also allows automatic reference switching from a reference where jitter attenuation is required, to a reference where jitter must not be attenuated and vice versa, without any significant output clock phase jump. The invention is thus capable of both attenuating and tracking incoming clock jitter while maintaining output clock integrity (preventing any significant change in the output phase) during and after switch from one input reference to another. 
   The Digital Phase Locked Loop (DPLL), containing this invention, used in combination with a digital switch, can form a device that is fully compliant to the H.110 timing specifications. The same device can be used to perform the roll of any of the three H.110 required devices: “primary master”, “secondary master” and “slave”, without any additional logic and with good performance, thereby drastically reducing the cost for building such part or system. 
   The loop filter of the DPLL can be switched between a low bandwidth mode, thereby attenuating jitter, and a high bandwidth mode, thereby passing the jitter on the input clock on to the output clock of said DPLL. A non-linear phase transfer in the high bandwidth mode ensures that the output clock of said DPLL has no significant phase jumps when the filter is switched from a high bandwidth to a low bandwidth. The non-linear phase transfer in the high bandwidth mode also improves the holdover accuracy of said DPLL in the high bandwidth mode. 
   The divider circuit of the DPLL that generates the 8 kHz output frame pulse from the DPLL output clock has a synchronization circuit built in that ensures that the output frame pulse is aligned with the input frame pulse of the DPLL in high bandwidth mode. In this way, both the output clock and frame pulse of said DPLL can be aligned with the input clock and frame pulse. 
   In another aspect the invention provides a clock generation circuit, comprising a multiplexer for receiving selecting one of a plurality of input reference clock signals; and a digital phase locked loop including a maximum time interval error (MTIE) detector having first and second inputs, said first input being coupled to an output of said multiplexer; a phase detector coupled to an output of said MTIE detector; a loop filter coupled to an output of said phase detector, said loop filter having selectable high and low bandwidth modes for respectively attenuating or tracking jitter from an input reference clock signal; a digital controlled oscillator coupled to an output of said loop filter; and an output divider coupled to an output of said digital controlled oscillator for generating output clocks and an output frame pulse, said output divider having an output connected to an input of said MTIE detector. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagram of a typical H.110 timing control configuration; 
       FIG. 2  is a top-level block diagram of a DPLL circuit according to a preferred embodiment of the invention; 
       FIG. 3  is a block diagram of the loop filter module from  FIG. 2 ; 
       FIG. 4  is a block diagram of the phase multiplier from  FIG. 3 ; 
       FIG. 5  is the phase transfer diagram for the phase multiplier from  FIG. 4 ; and 
       FIG. 6  is a block diagram of the divider from  FIG. 3 . 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , a typical H.110 compliant system consists of a primary master  10 , a secondary master  12  and multiple slave devices  14 . A DPLL (Double Phase Locked Loop) in the primary master  10  is locked to either a locally derived network reference or a system wide network reference (CT_NETREF_ 1  or CT_NETREF_ 2 ) and drives the “A” clocks (CT_C 8 _A and CT_FRAME_A). The system wide network references are typically driven by two of the slave devices  14 . The CT_NETREF clocks are derived from the slaves″ local network references and do not have a specific phase with respect to each other or the bus clocks CT_C 8 _A/B and CT_FRAMEA/B. A DPLL of the secondary master  12  is locked to the “A” clocks and generates the “B” clocks. The slaves  13  monitor the “A” and “B” clocks, and the DPLL of each of the slaves  14  is locked to the “A” clocks. 
   If the primary reference for the DPLL in the primary master  10  becomes unreliable, the DPLL in the primary master  10  continues driving the “A” clocks in stable holdover mode until it makes a Stratum 4 Enhanced compatible switch to a secondary reference being a local network reference or a CT_NETREF clock for its network timing. In stable holdover mode, the DPLL just maintains a stable output frequency. 
   The secondary master device  12  drives the B clocks (CT_C 8 _B/CT_FRAME_B), by locking to the “A” clocks generated by the primary master  10 . The “B” clocks have to be phase locked and edge synchronous to the “A” clocks even in the presence of jitter on the “A” clocks. If the “A” clocks become unreliable, the secondary master  12  continues driving the “B” clocks in stable holdover mode until it makes a Stratum 4 Enhanced compatible switch to a local network reference or a CT_NETREF for its timing. The slave  14  devices are synchronized to the “A” clocks, and if the “A” clocks become unreliable, the slaves  14  make a switch to the “B” clocks. 
   Referring to now  FIG. 2 , the DPLL circuit that is capable of the functionality described above consists of the following modules: an input reference multiplexer or selector  20 , an MTIE (Maximum Time Interval Error) module  21 , a phase detector  22 , a loop filter  23 , a digitally controlled oscillator (DCO)  24 , a frequency converter  25  and an output divider  26 . 
   The input reference selector  20  selects the proper clock and/or frame pulse from the local network references, the two CT_NETREFs, the “A” clocks or the “B” clocks as the input clock for the DPLL. In case the DPLL is a secondary master or slave, it also selects the right frame pulse to synchronize the DPLL output frame pulse to. 
   The reference monitor modules  27  monitor the timing of the input references and notify the State Machine module  28  whether the reference is reliable or not. The State Machine  28  controls the input reference selector  20 , the loop filter bandwidth and switches the DPLL automatically into holdover mode when required. 
   The MTIE module  21  compensates for the phase offset between the new reference input and the DPLL output clock in case of a reference switch or a recover from holdover mode. 
   The phase detector  22  converts phase difference between the input reference clock and output feedback clock to a signed binary number representation. 
   The loop filter  23  performs output phase slope limiting, filters the phase error output of the phase detector to a frequency offset signal for the DCO. The bandwidth of the DPLL is determined in the loop filter  23 . 
   The DCO  24  is the actual clock generator. It generates a clock that is phase locked to the input reference clock. The frequency of the generated clock is determined by the system clock, the center frequency setting inside the DCO and the frequency offset value from the loop filter. 
   The frequency converter  25  multiplies the current phase of the DCO by a constant fraction and thereby generates a clock of a frequency that is a fraction of the DCO output clock frequency. 
   The output divider  26  divides the output clocks of the DCO and the frequency converter down to the required clocks and an 8 kHz frame pulse. 
   When the DPLL is a primary master it locks to the input clock selected by the system configuration and attenuates the jitter on the input clock. The bandwidth of the DPLL is approximately 1.5 Hz. The DPLL drives the “A” clocks CT_C 8 _A and CT_FRAME_A. The input clock frequency and output clock frequency are not necessarily of the same frequency. The input clock can be a 1.544 MHz clock while the output clock is the 8.192 MHz clock CT_C 8 _A as prescribed by the H.110 standard. The output frame pulse CT &#39; FRAME_A is generated by dividing the 8.192 MHz clock down to 8 kHz. 
   When the DPLL is a secondary master  12 , it locks to the 8.192 MHz “A” clock CT_C 8 _A as it has to generate the 8.192 MHz “B” clock CT_C 8 _B that is edge synchronous to the “A” clock. The DPLL does not attenuate but tracks the jitter on the CT_C 8 _A clock. The bandwidth of the DPLL is approximately 100 kHz. The frame pulse from the DPLL CT_FRAME_B must closely track the “A” frame pulse CT_FRAME_A. Therefore, the divider module  26  of the DPLL synchronizes the CT_C 8 _B frame pulse to the incoming CT_C 8 _A frame pulse. 
   When the DPLL is a slave  14 , it locks to the 8.182 MHz “A” clock CT_C 8 _A. The DPLL does not attenuate but tracks the jitter on the CT_C 8 _A clock. The bandwidth of the DPLL is approximately 100 kHz. The output clocks of the DPLL are used for the switch in the slave device and the serial TDM streams. 
   The role of devices in the system is not fixed but may change over time. When the “A” clocks from the primary master become unreliable, the “B” clocks from the secondary master become “A” clocks and the secondary master is promoted to primary master. The primary master is demoted to slave and a slave is promoted to secondary master. 
     FIG. 3  is a block diagram of the loop filter  23 . The phase error value from the phase detector is passed through a limiter  30  to achieve a phase slope limiting on the DPLL output clocks when the low bandwidth is selected. When the high bandwidth is selected, there is no phase slope limiting. The limiter output is multiplied by a power of two by a barrel shifter phase multiplier  31 . The multiplied output, the P(roportional)-value, is integrated in an accumulator integrator  32 . The integrator value is attenuated to keep the DPLL stable and the peaking in the DPLL&#39;s jitter transfer function under control. The attenuated integrator output is the I-value in  FIG. 3 . The P-value and the I-value are added in adder  34  to provide the loop filter output. 
   The I-value is stored in two alternating locations in holdover memory  33  at regular intervals. When the DPLL is switched into holdover mode by multiplexer  35 , the oldest holdover memory value is restored as the loop filter output. 
   The loop filter circuit  23  implements a first order low pass filter. The shift value of the barrel shifter determines the cut-off frequency. In the preferred embodiment, two shift values, 0 and 16, are implemented. The corresponding jitter cut-off frequencies are: 1.5 Hz and 100 kHz. When the DPLL is a primary master, the 1.5 Hz setting is used and the DPLL will attenuate jitter above 1.5 Hz. When the DPLL is secondary master or slave, the 100 kHz setting is used and the DPLL will pass jitter below 100 kHz on to the DPLL&#39;s output clocks. 
   The output of the phase detector  22  is configured so that it outputs values that are always odd. When the DPLL is locked, the phase detector output toggles between −1 and +1. The barrel shifter multiplier  31  has a threshold built in such that it does not perform a shift when the phase error from the phase detector is −1 or +1, thereby creating a nonlinear transfer for shift values larger than zero (multiply by more than 1). This is indicated in  FIG. 4 , which shows the multiplier  31  consisting of phase comparator  37 , AND gate  38 , and barrel shifter  39 . 
   The advantage of this arrangement is that when the DPLL is locked, the values fed into the integrator are +1 or −1 even when a shift value of  16  is used. Effectively this means that the noise of one LSB (Least Significant Bit) from the phase detector falls below the threshold and is not amplified by the barrel shifter. That reduces the noise integrator and improves the accuracy of the holdover mode dramatically. Also, without the barrel shifter threshold, a phase jump on the output clock is observed when the loop filter is switched from the 100 kHz setting to the 1.5 Hz setting. With the barrel shifter threshold in place, this output phase jump no longer exists, thereby maintaining phase integrity of the output clock. 
   Besides the application of a master/slave DPLL, the non-linear phase transfer loop filter is also useful for a DPLL with a relative high bandwidth in general as it improves the holdover accuracy. 
   The divider module  26  divides the high speed clock provided by the DCO  24  and the frequency converter  25  to provide the necessary output clocks and also to generate the feedback reference for the MTIE module  21  and the phase detector  22 . 
   When the DPLL is a primary master and in low bandwidth mode, the output clocks may not be aligned with the input clock due to jitter on the input clock. When the DPLL is a secondary master or slave, the DPLL&#39;s output clock and frame pulse must be aligned to the backplane clock and frame pulse. In order to track the jitter on the backplane clock, the DPLL must lock to the backplane clock, but that means that the backplane frame pulse and the DPLL output frame pulse may not be aligned. Therefore, as shown in  FIG. 6 , the divider  26  has a synchronization circuit built in. This comprises edge detector  40 , AND gate  41 , counter  42 , and zero detector  43 . 
   At the backplane frame pulse, the counter  42  in the divider is loaded with a fixed value. The counter counts down on the DPLL output clock, and when the counter reaches zero the output frame pulse is generated. The constant value with which the counter is loaded is such that the output frame pulse is aligned with the input frame pulse. Since the DPLL output clock closely tracks the backplane input clock, this synchronization scheme ensures that the DPLL output frame pulse is phase aligned with the backplane frame pulse. The synchronization scheme is only enabled by the AND gate  41  when the high bandwidth is selected. When the low bandwidth is selected, the counter just counts down and rolls over when it reaches zero. 
   The described DPLL&#39;s output clock and frame pulse have no significant phase jump when the DPLL is switched from a high bandwidth to a low bandwidth. It also permits an 8 kHz output frame pulse to be generated from the DPLL&#39;s jittery output clock, which allows it to always be properly aligned to the input frame pulse. 
   Persons skilled in the art will appreciate that many variants of the invention are possible within the scope of the appended claims.