Configurable multiplexing circuit and method

This invention relates to signal processing, and more particularly to a multiplexing circuit for phase locked loop applications.

Phase locked loops are used for a variety of communication applications. Some phase locked loop circuits employ a multiplexer for configuring the loop to different modes of operation. Conventional multiplexing arrangements have limited options in configuring the loop for different applications. On the other hand, using multiple loops can cause power penalty, and may consume too much area in a semiconductor chip. Further, there may be performance degradation associated with using more than one loop, if not properly implemented.

These and other problems with conventional multiplexing arrangements therefore create a need for a high-performance, low-jitter configurable multiplexing circuit.

FIG. 1 is a configurable multiplexing circuit according to one embodiment.

FIG. 2 shows a functional diagram for a configuration method according to the invention.

This invention provides a configurable multiplexer (CMUX). The CMUX is particularly well-suited for phase locked loop applications, offering a number of multiplexing options for one or more of a number of input signal types. For example, the present invention may be employed in a multi-loop PLL circuit of the type described in U.S. Pat. application titled, “Configurable Triple Phase-Locked Loop Circuit and Method, ” assigned to Applied Micro Circuits Corp., San Diego Calif. and the assignees of this invention, the contents of which are incorporated by reference for all purposes.

In one embodiment, this invention provides a phase locked loop the capability to operate with multiple input types, i.e. a reference clock application using a phase frequency detector, a no-reference clock application using a bang-bang phase frequency detector with nonreturn-to-zero (NRZ) data, and a clock recovery or synthesis application using a phase detector. Accordingly, various implementations are possible, for many other configurable applications.

The invention, embodied in hardware, firmware, or even software, includes a CMUX having an EX-OR element, a multiplexer element, a summer element, and a selection signal input. The selection signal input is preferably coupled to the EX-OR element, the multiplexer element, and the summer element. The selection signal input receives a selection signal that enables one or more of the EX-OR element, the multiplexer element, and the summer element. The selection signal may also disable any nonselected element, to eliminate or minimize jitter feedthrough, cross-signal noise, or other performance penalties.

In another embodiment of the invention, a CMUX includes a common output, an EX-OR configured to accept a first input signal, a multiplexer configured to accept a second input signal, and a summer configured to accept a third input signal. The CMUX further includes a selector configured to receive a selection signal for independent enabling of at least one, or a combination, of the EX-OR, the multiplexer, and the summer. The enabled EX-OR, multiplexer, and/or summer will execute its operation on the accepted input signal and provide an output signal to the common output. The non-enabled EX-OR, multiplexer, and/or summer is powered down, or disabled.

The invention may be embodied in hardware as a circuit or combination of circuits. In one such embodiment, a CMUX includes a plurality of inputs and a common output. The CMUX further includes an EX-OR circuit having an input to accept a first input signal, and configured to provide a first output to the common output. A multiplexer circuit has an input to accept a second input signal, and is configured to provide a second output to the common output. Furthermore, the CMUX includes a summer circuit having an input to accept a third input signal, and configured to provide a third output to the common output. According to a preferred embodiment, the EX-OR circuit, the multiplexer circuit, and the summer circuit are independently enabled.

FIG. 1 illustrates a CMUX 100 according to one embodiment of the invention. The CMUX 100 includes an EX-OR element 102 , a multiplexer element 104 , and a summer element 106 . The CMUX further includes a common output 108 having an UP output line 110 and a DOWN output line 112 . The EX-OR element 102 includes an input 103 configured to receive, when the EX-OR element 102 is enabled, HIGH and LOW signals corresponding to an output from a first detector type. The multiplexer element 104 includes an input 105 configured to receive, when the multiplexer element 104 is enabled, HIGH and LOW signals corresponding to an output from a second detector type. The summer element 106 includes an input 107 configured to receive, when the summer element 106 is enabled, HIGH and LOW signals corresponding to an output from a third detector type.

In a preferred embodiment, the first detector type is a phase detector, such as a Hogge phase detector used in a clock recovery loop. Further according to the embodiment, the second detector type is a phase frequency detector used in a clock synthesis loop. Still further according to the embodiment, the third detector type is a bang-bang (i.e. binary) phase frequency detector used in a no-reference clock application for clock recovery.

Each of the EX-OR element 102 , multiplexer element 104 , and summer element 106 can be implemented as simple well-known circuits including solid state logic gates, or may be implemented in silicon within the same or within different chips. The elements 102 , 104 and 106 may still further be implemented in firmware, as an application specific integrated circuit (ASIC). Those having relevant skill in the pertinent art would recognize various possible implementations of these elements.

The CMUX 100 further includes a selection signal input 120 for receiving a selection signal. The selection signal is configured to enable one or more of the EX-OR element 102 , the multiplexer element 104 , and/or summer element 106 , for receiving signals according to one or more multiplexing modes. In a preferred exemplary embodiment, the first detector type can be a phase detector, the second detector type can be a phase frequency detector, and the third detector type can be a bang-bang phase frequency detector. According to this embodiment, the selection signal input 120 can include a SEL_PD input 122 connected to the EX-OR element 102 for enabling the EX-OR element to receive signals from the phase detector in a first multiplexing mode. The selection signal input 120 can further include a SEL_PFD input 124 connected to enable the multiplexer element 104 to receive signals from the phase frequency detector in a second multiplexing mode. Furthermore, the selection signal input 120 can include a SEL_BBPFD input 126 connected to enable the summer element 106 to receive signals from the bang-bang phase frequency detector. The signal received on each of these inputs can be of any type, and have any format.

Each of the enabled EX-OR element 102 , multiplexer element 104 and/or summer element outputs a result of its operation to the common output 108 . The UP and DOWN signal lines 110 , 112 may be connected to drive a filter/charge pump 114 in phase locked loop systems, for example. The CMUX 100 is accordingly well-suited for the phase locked loops that can be configured for multiple applications.

FIG. 2 is a functional diagram of the various configurations of the CMUX 100 , to illustrate a method of the invention according to one embodiment. In this embodiment, the CMUX 100 can be configured to one, or more, of at least three multiplexing modes. A selector 201 is configured to receive a selection signal which specifies a multiplexing mode. In a first multiplexing mode, a PD mode 202 , the CMUX circuit is configured to receive inputs from the phase detector when the SEL_PD signal is asserted. In a second multiplexing mode, a PFD mode 204 , the CMUX circuit is configured to receive inputs from the phase frequency detector when the SEL_PFD signal is asserted. In a third multiplexing mode, BBPFD mode 206 , the CMUX circuit is configured to receive inputs from the bang bang phase frequency detector when the SEL_BBPFD signal is asserted.

Each of the multiplexing modes corresponds to a particular phase locked loop application. The unselected, or non-enabled portions of the CMUX circuit can be powered down to eliminate jitter feed-through, or other adverse interference effects. FIG. 2 merely illustrates one level of possible configurations, incorporating multiple inputs and common outputs.