Abstract:
A clock selection circuit for selecting one of a plurality of clocks as an output clock. When the selection circuit switches between two of the plurality of clocks for output, the currently output clock is removed from the output. The removal of the currently output clock is performed synchronously to the currently selected clock. The newly selected clock is then coupled to the output. Coupling of the newly selected clock is performed synchronously to the newly selected clock.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the field of clock selection, and in particular to glitch-free clock selection. 
     Digital electronic systems often rely upon a clock signal to synchronize and control the operation of the various circuit elements (e.g., gates, flip-flops, latches, etc.). In many present day digital electronic systems, such as microprocessor-based devices, there exist multiple clock sources and a concomitant need to switch between them. 
     When switching between clocks, it is preferable to avoid glitches and intermediate clock behavior on the clock output of the selection circuitry. FIGS. 1 a  and  1   b  help illustrate the occurrence of a glitch when switching between clock sources. FIG. 1 a  shows a typical circuit for switching between clock sources. As shown in FIG. 1 a , two clock signals, CLOCK_ 1  and CLOCK_ 2 , are provided as inputs to a switching circuit  100 , such as a multiplexor. Multiplexor  100  also receives a Select signal, which switches the output signal, CLOCK_OUT, between the input signals CLOCK_ 1  and CLOCK_ 2 . For instance, when the Select signal is high, CLOCK_ 1  is output on CLOCK_OUT and when the Select signal is low, CLOCK_ 2  is output on CLOCK_OUT. FIG. 1 b  illustrates a timing relationship between the Select signal, CLOCK_ 1  and CLOCK_ 2  that results in a glitch on CLOCK_OUT. As shown, the Select signal is initially high, resulting in CLOCK_ 1  being output on CLOCK_OUT. The Select signal then goes low while CLOCK_ 1  is high and CLOCK_ 2  is low. This results in a shortened pulse  102 , i.e. a glitch, output on CLOCK_OUT. 
     Generally, a glitch signal causes errors during execution of a microprocessor and other components because a glitch may erratically clock subsequent flip-flops, latches, etc. Therefore, there is a need for a switching circuit that enables switching of the clock source, dynamically and cleanly, without any perturbation on the logic driven by the clock. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a clock selection circuit for switching between a plurality of possible clocks is provided. A switch made from an existing clock to a new frequency clock is made in synchronization with both the existing and the new frequency clock. The clock selection circuit comprises a first clock input to receive the existing clock as input and a new frequency clock input to receive the new frequency clock. The circuit also comprises first synchronization logic associated with the first clock to enable/disable output of the existing clock and second synchronization logic associated with the new frequency clock to enable/disable output of the new frequency clock. The first synchronization and second synchronization logic cooperates to disable output of the existing clock synchronously to the existing clock and to enable output of the new frequency clock synchronously to the new frequency clock. 
     In another aspect of the present invention, a clock selection circuit for outputting an input clock signal selected from among a plurality of input clocks is provided. The circuit comprises enable logic responsive to a clock select input to generate, for each input clock, an associated select signal. Each select signal is indicative of whether or not its associated input clock is selected to be output. For each select signal, there is synchronization logic responsive to the select signal to generate an enable signal synchronously to the select signal&#39;s associated input clock. The enable signal is indicative of whether or not the select signal&#39;s associated clock is to be output. Output logic is responsive to the enable signals to output the selected input clock. 
     In another aspect of the present invention, a clock selection circuit for switching from a first clock signal coupled to an output to a second clock signal coupled to the output is provided. The circuit comprises enable logic responsive to a clock select signal to generate a first select signal that indicates the first clock is to be decoupled from the output and a second select signal that indicates the second clock signal is to be coupled to the output. First synchronization logic is responsive to the first select signal to generate a first enable signal synchronously to the first clock. The first enable signal indicates that the first clock is to be decoupled from the output. Second synchronization logic is responsive to the second select signal to generate a second enable signal synchronously to the second clock. The second enable signal indicates the second clock signal is to be coupled to the output. The first enable signal is generated before the second enable signal is generated. Output logic is responsive to the first enable signal to decouple the first clock signal from the output and responsive to the second enable signal to couple the second clock signal to the output. 
     In another aspect of the present invention, a method of switching from a first clock signal coupled to an output of a clock selection circuit to a second clock signal coupled to the output of the clock selection circuit is provided. An indication to switch from outputting the first clock signal to the second clock signal is received. The first clock is then removed from output synchronously to the first clock. The second clock signal is then coupled to the output synchronously to the second clock signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  illustrates a typical circuit for switching between clock sources; 
     FIG. 1 b  illustrates a timing relationship between the Select signal, CLOCK_ 1  and CLOCK_ 2  that results in a glitch on CLOCK_OUT for the circuit of FIG. 1 a;    
     FIG. 2 a  illustrates a clock selection circuit according to the principles of the present invention; 
     FIG. 2 b  illustrates a timing diagram for the circuit of FIG. 2 a;    
     FIG. 2 c  illustrates an embodiment of clock selection circuit of FIG. 2 a  that allows for a synchronous reset of the synchronization logic; 
     FIG. 2 d  illustrates an embodiment of clock selection circuit of FIG. 2 a  that allows for an asynchronous reset of synchronization logic; 
     FIG. 2 e  illustrates the clock selection circuit of FIG. 2 a  extended to select between three clock sources; 
     FIG. 3 illustrates the use of a clock selection circuit to for select between a base clock and a higher frequency clock created from a phase-locked loop (PLL) based frequency multiplier; 
     FIG. 4 a  illustrates a clock selection circuit  400  according to the principles of the present invention which is particularly suited to select between a base clock and a higher frequency clock created from a phase-locked loop (PLL) based frequency multiplier; 
     FIGS. 4 b - 4   c  illustrate timing diagrams for the selection circuit of FIG. 4 a;    
     FIG. 4 d  illustrates an embodiment of the clock selection circuit of FIG. 4 a  without the internal enable signals connected to reset inputs of the flip-flops; 
     FIG. 4 e  illustrates another embodiment of the clock selection circuit of FIG. 4 a  without the internal enable signals connected to reset inputs of the flip-flops; 
     FIG. 5 a  illustrates an arrangement in which a clock selection circuit according to the principles of the present invention is used to clock a processor that issues the clock selection circuit&#39;s control inputs; 
     FIG. 5 b  illustrates a clock selection circuit according to the principles of the present invention which is particularly suited for control signals that change synchronously with the selected clock; and 
     FIGS. 5 c - 5   f  illustrate timing diagrams for the selection circuit of FIG. 5 b.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 a  illustrates a clock selection circuit  200  according to the principles of the present invention. Selection circuit  200  generally comprises enable logic  203 , synchronization logic  204   a  clocked by CLOCK_ 1 , synchronization logic  204   b  clocked by CLOCK_ 2  and output logic  202 . 
     Enable logic  203  generates internal select signals SEL 1  and SEL 2  based upon the Select input and the current state of the clock selection, i.e. whether CLOCK_ 1  is output or not and whether CLOCK_ 2  is output or not. Internal selection signals indicate which clock, CLOCK_ 1  or CLOCK_ 2 , is to be output on CLOCK_OUT. Internal select signal SEL 1  is input to synchronization logic  204   a , while internal select signal SEL 2  is input to synchronization logic  204   b . Synchronization logic  204   a  generates an internal enable signal EN 1  synchronously to CLOCK_ 1  based upon internal select signal SEL 1 . Likewise, synchronization logic  204   b  generates an internal enable signal EN 2  synchronously to CLOCK_ 2  based upon internal select signal SEL 2 . Internal enable signals EN 1  and EN 2  are input to output logic  202  in addition to CLOCK_ 1  and CLOCK_ 2 . The states of enable signals EN 1  and EN 2  determine which clock, CLOCK_ 1  or CLOCK_ 2 , is output-by-output logic  202 . Enable signals EN 1  and EN 2  are also fed back to enable logic  203  via inverters  212  and  214 , respectively. 
     As shown, enable logic  202  comprises AND gates  218  and  216 , and inverters  212 . The output of AND gate  218  is SEL 1  and the output of AND gate  216  is SEL 2 . One of the inputs of AND gate  218  is connected directly to the Select input, while one of the inputs of AND gate  216  is connected to the Select input via inverter  220 . The other input of AND gate  216  is connected to EN 1  via inverter  212 . Similarly, the other input of AND gate  218  is connected to EN 2  via inverter  214 . 
     Synchronization logic  204   a  preferably comprises a plurality of cascaded memory elements or flip-flops, such as D flip-flops. The associated input clock, i.e. CLOCK_ 1 , clocks each of the flip-flops, for example, on the negative edge of the input clock. The first flip-flop of the cascade has its input connected to SEL 1  and the output of the last flip-flop of the cascade is EN 1 . In a similar fashion, synchronization logic  204   b  preferably comprises a plurality of cascaded flip-flops, such as D flip-flops. The associated input clock, i.e. CLOCK_ 2 , clocks each of the flip-flops, for example, on the negative edge of the input clock. The first flip-flop of the cascade has its input connected to SEL 2  and the output of the last flip-flop of the cascade is EN 2 . 
     While it is preferred to use a plurality of cascaded flip-flops, it is within the spirit of the present invention for synchronization logic  204   a  or  204   b  to be formed with a single flip-flop. The use of a plurality of cascaded flip-flops, however, is preferred as this decreases the possibility of a metastable condition. 
     Output logic  202  comprises an OR gate  206  with one input connected to the output of an AND gate  208  and the other input connected to the output of a second AND gate  210 . AND gate  208  has one of its inputs connected to CLOCK_ 1  and the other input connected to the EN 1 . Similarly, AND gate  210  has one of its inputs connected to CLOCK_ 2  and the other input connected to the EN 2 . The output of OR gate  206  is taken as CLOCK_OUT. 
     Discussion of the operation of selection circuit  200  for selecting between CLOCK_ 1  and CLOCK_ 2  will be made in conjunction with the timing diagram in FIG. 2 b  and is made starting from a state in which CLOCK  1  is output on CLOCK_OUT. Further, discussion of the operation of selection circuit  200  is made with respect to active high logic, while it is, however, within the spirit of the present invention to use active low logic. 
     Initially CLOCK_ 1  is output on CLOCK_OUT, EN 1  is high, EN 2  is low and Select is high. CLOCK_ 2  is chosen as the output clock by switching Select from high to low. When Select is switched low, this causes SEL 1  to go low. Flip-flops  204   a  are clocked by CLOCK_ 1 , causing the low input signal SEL 1  to propagate to the output of synchronization flip-flops  204   a , i.e. to EN 1 , synchronously with CLOCK_ 1 . The signal EN 1  going low disables the output of CLOCK_ 1  on CLOCK_OUT. The output EN 1  going low also causes SEL 2  to go high. Flip-flops  204   b  are clocked by CLOCK_ 2 , causing the high input signal SEL 2  to propagate to the output of synchronization flip-flops  204   b , i.e. to EN 2 , synchronously with CLOCK_ 2 . The output EN 2  going high enables the output of CLOCK_ 2  on CLOCK_OUT. Therefore, as can be seen, the disabling of CLOCK_ 1  on CLOCK_OUT is performed synchronously with CLOCK_ 1  and the enabling of CLOCK_ 2  on CLOCK_OUT is performed synchronously with CLOCK_ 2 , thereby preventing the occurrence of glitches during the switching of the clock output. 
     As will be illustrated below, the selection circuit according to the principles of the present invention can be expanded to include specific needs of the application, e.g. more control lines, power saving features, reduction of synchronization latency, etc. and to satisfy initialization requirements of synchronization logic  204 . 
     Generally, clock selection circuit  200  is used to select a clock signal that clocks various components in a system. Select is typically initialized to a default value by the system after power up to provide an initial clock output on CLOCK_OUT. For example, Select is designed to default high. When Select defaults high on power up, SEL 2  is initialized low. This low signal then propagates through synchronization logic  204   b  to EN 2  to make EN 2  take on a defined (low) value. After EN 2  becomes low, then SEL 1 , which is high, can propagate through synchronization logic  204   a  to EN 1 . EN 1  going high enables the output of CLOCK_ 1  on CLOCK_OUT. In some applications an output on CLOCK_OUT may be needed for the system to initialize Select. Or, in some applications the time needed to first propagate SEL 2  through synchronization logic  204   b  then to propagate SEL 1  through synchronization logic  204   a  may be too long. An ability to reset selection circuit  200  is desirable in these applications. In addition, a reset of selection circuit  200  may also be generally desirable so as to allow selection circuit  200  to be placed into a known state during normal operation. 
     FIG. 2 c  illustrates an embodiment of clock selection circuit  200  of FIG. 2 a  that allows for a synchronous reset of synchronization logic  204 . By synchronous reset, it is meant that a nReset input changes state synchronously to the clock output on CLOCK_OUT. Selection circuit  200  operates as described in relation to FIGS. 2 a  and  2   b , except that reset logic  222  causes the synchronization logic  204  to generate a particular state of the enable signals EN 1  and EN 2 , regardless of the state of Select and the currently selected clock. This results in a particular clock being output when the nReset input is activated. As shown, the nReset signal is supplied to reset logic  222  via an inverter, making nReset active low. However, the inverter may not be needed depending on the reset logic  222 . For instance, if nReset is desired to be active high, it can be provided straight to reset logic  222 . In the embodiment illustrated in FIG. 2 c , reset logic  222  comprises an OR gate  224 , inverter  226  and an AND gate  228 . OR gate  222  receives nReset and the output of AND gate  218  as inputs and its output is SEL 1  to synchronization logic  204   a . AND gate  228  receives nReset via inverter and the output of AND gate  216  as inputs and its output is SEL 2  to synchronization logic  204   b.    
     For the embodiment of FIG. 2 c , nReset in a high state has no effect on which clock is enabled on CLOCK_OUT. When nReset is placed in a low state, however, SEL 1  is forced high by reset logic  222 , while SEL 2  is forced low by reset logic  222 . This causes CLOCK_ 1  to be enabled on CLOCK_OUT, regardless of the state of Select. As will be apparent to one of skill in the art, reset logic can readily be designed to cause CLOCK_ 2  to be output instead of CLOCK_ 1 . For instance, interchanging the outputs of OR gate  224  and AND gate  228  such that the output of OR gate is SEL 2  and the output of AND gate is SEL 1 , while removing the inverter nReset is supplied through, provides for reset logic which results in CLOCK_ 2  output when nReset is in a high state. 
     FIG. 2 d  illustrates an embodiment of clock selection circuit  200  of FIG. 2 a  that allows for an asynchronous reset of synchronization logic  204 . By asynchronous reset, it is meant that a nReset input changes state asynchronously from the clock output on CLOCK_OUT. Selection circuit  200  operates as described in relation to FIGS. 2 a  and  2   b , except a nReset input causes the synchronization logic  204  to generate a particular state of the enable signals EN 1  and EN 2 , regardless of the state of Select and the currently selected clock. This results in a particular clock being output when nReset is activated. The synchronization logic corresponding to the particular clock to be output has a set input connected to nReset, while the other synchronization logic has reset inputs connected to nReset. As shown in FIG. 2 d , CLOCK_ 1  is the clock to be enabled on reset and, consequently, its synchronization logic  204   a  has a set input connected to nReset via an inverter. Synchronization logic  204   b  has a reset input connected to nReset via an inverter. A set input forces the output of its corresponding synchronization logic to go high when it is high. Conversely, a reset input forces the output of its corresponding synchronization logic to go low when it is high. For the set or reset inputs low, synchronization logic operate as normal. Therefore, when nReset is low, CLOCK_ 1  is enabled on CLOCK_OUT, while nReset high does not affect selection circuit  200 . 
     A clock selection circuit according to the principles of the present invention can be extended to select between more than two clocks. FIG. 2 e  illustrates clock selection circuit  200  extended to select between three clock sources, CLOCK_ 1 , CLOCK_ 2 , and CLOCK_ 3 . Selection circuit  200  is similar to the circuit of FIG. 2 a  and generally comprises enable logic  203 , synchronization logic  204   a  clocked by CLOCK_ 1 , synchronization logic  204   b  clocked by CLOCK_ 2  and output logic  202 . Selection circuit  200  has been extended by the addition of synchronization logic  204 c clocked by CLOCK_ 3  and the addition of logic in enable logic  203  to generate a third internal select signal SEL 3 . In addition, to provide selection between three clocks, the Select input is two Select lines, Select_ 1  and Select_ 2 . Thus, the extended selection circuit  200  of FIG. 2 e  operates similar to the two-clock implementation. Enable logic  203  generates internal select signals SEL 1 , SEL 2 , and SEL 3  based upon the Select input and the current state of the clock selection. Each of these signals is input to the corresponding synchronization logic  204   a ,  204   b , and  204   c , respectively. As with the two clock implementation, synchronization logic  204   a ,  204   b , and  204   c  generates enable signals EN 1 , EN 2 , and EN 3 , respectively. The enable signals are generated based on the internal select signals such that an enabled clock is disable synchronously to itself and then the clock to be enabled is enabled synchronously to itself. The enable signals are provided to output logic  202  to control which clock is output and are fed back to enable logic to indicate the current state of the clock selection. 
     In the three-clock implementation shown, both of the Select inputs in a low state causes CLOCK_ 1  to be enabled on CLOCK_OUT. A low on both Select inputs results in SEL 1  being high, while SEL 2  and SEL 3  are low. Depending upon which clock is already active, EN 2  or EN 3  goes low synchronously to its respective clock, disabling that clock. For instance, if CLOCK_ 2  is being output, EN 2  goes low synchronously to CLOCK_ 2  (in this case EN 3  would already be low so it does not change), causing CLOCK_ 2  to be disabled. Likewise, if CLOCK_ 3  is being output, EN 3  goes low synchronously to CLOCK_ 3  (in this case EN 2  would already be low so it does not change), causing CLOCK_ 3  to be disabled. After the previously enabled clock is disable, EN 1  goes high synchronously to CLOCK_ 1  (it was low previously), causing CLOCK_ 1  to be enabled on CLOCK_OUT. 
     A low on Select_ 1  and a high on Select_ 2  causes CLOCK_ 2  to be enabled on CLOCK_OUT. A low on Select_ 1  and a high on Select_ 2  results in SEL 2  being high, while SEL 1  and SEL 3  are low. Depending upon which clock is already active, EN 1  or EN 3  goes low synchronously to its respective clock, disabling that clock. For instance, if CLOCK_ 1  is being output, EN 1  goes low synchronously to CLOCK_ 1  (in this case EN 3  would already be low so it does not change), causing CLOCK_ 1  to be disabled. Likewise, if CLOCK_ 3  is being output, EN 3  goes low synchronously to CLOCK_ 3  (in this case EN 2  would already be low so it does not change), causing CLOCK_ 3  to be disabled. After the previously enabled clock is disable, EN 2  goes high synchronously to CLOCK_ 2  (it was low previously), causing CLOCK_ 2  to be enabled on CLOCK_OUT. 
     A high on Select_ 1  and a low on Select_ 2  causes CLOCK_ 3  to be enabled on CLOCK_OUT. A high on Select_ 1  and a low on Select_ 2  results in SEL 3  being high, while SEL 1  and SEL 2  are low. Depending upon which clock is already active, EN 1  or EN 2  goes low synchronously to its respective clock, disabling that clock. For instance, if CLOCK_ 1  is being output, EN 1  goes low synchronously to CLOCK_ 1  (in this case EN 2  would already be low so it does not change), causing CLOCK_ 1  to be disabled. Likewise, if CLOCK_ 2  is being output, EN 2  goes low synchronously to CLOCK_ 2  (in this case EN 1  would already be low so it does not change), causing CLOCK_ 2  to be disabled. After the previously enabled clock is disable, EN 3  goes high synchronously to CLOCK_ 3  (it was low previously), causing CLOCK_ 3  to be enabled on CLOCK_OUT. 
     Lastly, a high on Select_ 1  and a high on Select_ 2  causes all clocks to be disabled, driving CLOCK_OUT low. A high on Select_ 1  and a high on Select_ 2  results in SEL 1 , SEL 2 , and SEL 3  going low. In turn, any clock that was enabled will be disabled synchronously to itself and CLOCK_OUT will be driven low. 
     One exemplary application for a clock selection circuit according to the present invention is the selection between a base clock and a higher frequency clock created from a phase-locked loop (PLL) based frequency multiplier. This is generally illustrated in FIG.  3 . As shown, a base clock signal CLOCK_IN is provided (as CLOCK_ 1 ) to a clock switch and synchronization circuit  302  according to the present invention. CLOCK_IN is also provided to a PLL frequency multiplier  300 , which multiplies the frequency of CLOCK_IN to derive a clock signal CLOCK_ 2  having a higher frequency than CLOCK_ 1 . The second clock signal CLOCK_ 2  is also provided to clock switch and synchronization circuit  302 . A Select line is used to select between either CLOCK_ 1  or CLOCK_ 2  as the output on CLOCK_OUT. For instance, when Select is high, CLOCK_ 1  is output as CLOCK_OUT (i.e. the PLL frequency multiplier  300  is bypassed). When Select is low, however, CLOCK_ 2  is output as CLOCK_OUT. 
     Clock selection circuit  302  also has nReset, StopCK and CLOCK_VALID as control inputs. The control input nReset is an active low input that resets clock selection circuit  302 . The StopCK input is used to stop the clock on the CLOCK_OUT output. When StopCK is high, CLOCK_OUT is stopped. The CLOCK_VALID input is used to prevent switching to the PLL clock, CLOCK_ 2 , during the time when the PLL has not achieved lock. When the PLL has achieved lock, CLOCK_VALID goes high, allowing a switch to CLOCK_ 2 . 
     FIG. 4 a  illustrates a clock selection circuit  400  according to the principles of the present invention which is particularly suited to select between a base clock and a higher frequency clock created from a phase-locked loop (PLL) based frequency multiplier. Selection circuit  400  generally comprises enable logic  404 , synchronization logic  408  clocked by CLOCK_ 1 , synchronization  406  clocked by CLOCK_ 2  and output logic  402 . 
     Enable logic  404  generates internal select signals SEL 1  and SEL 2  based upon input signals nReset, Select, StopCK and CLOCK_VALID and the current state of the clock selection, i.e. whether CLOCK_ 1  is output or not and whether CLOCK_ 2  is output or not. Internal select signal SEL 1  is input to synchronization logic  408 , while internal select signal SEL 2  is input to synchronization logic  406 . Synchronization logic  408  generates an internal enable signal EN 1  synchronously to CLOCK_ 1  based upon internal select signal SEL 1 . Likewise, synchronization logic  406  generates an internal enable signal EN 2  synchronously to CLOCK_ 2  based upon internal select signal SEL 2 . Internal enable signals EN 1  and EN 2  are input to output logic  402  in addition to CLOCK_ 1  and CLOCK_ 2 . The states of enable signals EN 1  and EN 2  determine which clock, CLOCK_ 1  or CLOCK_ 2 , is output-by-output logic  402 . Enable signals EN 1  and EN 2  are also fed back to enable logic  404  via inverters  412  and  414 , respectively. 
     As shown, enable logic  404  comprises AND gates  424 ,  428  and  432 , inverters  420 , NOR gate  430  and OR gate  422 . The output of OR gate  218  is SEL 1 . One of the inputs of OR gate  218  is the output of inverter  420 , which has the nReset signal as its input. The other input of OR gate is the output of AND gate  424 . AND gate  424  receives as inputs the Select signal, EN 2  via inverter  412  and the output of AND gate  428  via inverter  426 . AND gate  428  receives the nReset signal and StopCK signal as inputs. The output of AND gate  432 . AND gate  432  has the CLOCK_VALID signal, EN 1  via inverter  416  and the output of NOR gate  430  as inputs. NOR gate  430  receives the Select signal and the output of AND gate  428  as inputs. 
     Similar to the embodiment of FIG. 2 a , synchronization logic  408  preferably comprises a plurality of cascaded flip-flops, such as D flip-flops. Each of the flip-flops is clocked by the associated input clock, i.e. CLOCK_ 1 , on the negative edge of the input clock as a result of inverter  434 . In addition, each of the flip-flops has its reset input connected to EN 2 . The first flip-flop of the cascade receives SEL 1  as its input. Synchronization logic  408  also comprises OR gate to facilitate the reset function of nReset. The output of inverter  420  is one input of OR gate  434 . The output of the last flip-flop of the cascade is the other input to OR gate  434 . The output of OR gate  434  is EN 1 . Similarly, synchronization logic  406  preferably comprises a plurality of cascaded flip-flops, such as D flip-flops. Each of the flip-flops is clocked by the associated input clock, i.e. CLOCK_ 2 , on the negative edge of the input clock as a result of inverter  438 . In addition, each of the flip-flops has its reset input connected to EN 1 . The first flip-flop of the cascade receives SEL 2  as its input and the output of the last flip-flop of the cascade is EN 2 . 
     As previously described above in conjunction with the embodiment of FIG. 2 a , the use of a plurality of flip-flops rather than a single flip-flop reduces the possibility of a metastable condition. 
     It is preferable to apply EN 1  and EN 2  to the reset inputs of the opposite set of flip-flops as shown to cause the opposite set of flip-flops to be in a reset state as described below. This insures that the opposite internal enable signal is low when one of the internal enable signals is high. 
     Output logic  402  comprises an OR gate  440  with one input connected to the output of an AND gate  442  and the other input connected to the output of a second AND gate  446 . AND gate  442  has one of its inputs connected to CLOCK_ 1  and the other input connected to EN 1 . Similarly, AND gate  446  has one of its inputs connected to CLOCK_ 2  and the other input connected to EN 2 . The output of OR gate  440  is taken as CLOCK_OUT. 
     Discussion of the operation of selection circuit  400  for selecting between CLOCK_ 1  and CLOCK_ 2  will be made in conjunction with the timing diagram in FIG. 4 b  and is made starting from a state in which CLOCK_ 1  is output on CLOCK_OUT. Further, discussion of the operation of selection circuit  400  is made with respect to active high logic, while it is, however, within the spirit of the present invention to use active low logic. 
     It should be noted that selection circuit  400  provides for the ability of the inputs to change asynchronously with respect to the selected clock. This is because, before becoming fully operative on the output, a change to any of the inputs passes through synchronization flip-flops  406  and  408 . 
     In the case of CLOCK_ 1  being output on CLOCK_OUT, Select and nReset are high, while StopCK is low. This results in internal selection signal SEL 1  being high, while SEL 2  is low. Internal enable signal EN 1  is consequently high, which holds synchronization flip-flops  406  in a reset state, insuring internal enable signal EN 2  is maintained in a low state. Because EN 1  is high and EN 2  is low, CLOCK_ 1  is output from output logic  402 . 
     When CLOCK_ 2  is to be selected for output on CLOCK_OUT, Select is switched low, which causes SEL 1  to switch low. As long as CLOCK_VALID is high, indicating PLL lock, switching Select low results in SEL 2  going high. At this point, EN 1  still holds flip-flops  406  in a reset state, preventing SEL 2  from propagating to EN 2 . 
     Flip-flops  408 , however, are not held in a reset state because EN 2  is low. Therefore, SEL 1  is propagated through flip-flops  408 . Flip-flops  408  are clocked by the negative edge of CLOCK_ 1 . This results in EN 1  synchronously disabling the CLOCK_ 1  output on CLOCK_OUT by going low after a falling edge, but prior to a rising edge, of CLOCK_ 1 . This synchronous disabling of CLOCK_ 1  on CLOCK_OUT prevents a glitch output. 
     Internal enable signal EN 1  going low removes flip-flops  406  from a reset state. Therefore, SEL 2  is propagated through flip-flops  406 . Flip-flops  406  are clocked by the negative edge of CLOCK_ 2 . This results in EN 2  synchronously enabling the CLOCK_ 2  output on CLOCK_OUT by going high after a falling edge, but prior to a rising edge, of CLOCK_ 2 . This synchronous enabling of CLOCK_ 2  on CLOCK_OUT prevents a glitch output. Further, internal enable signal EN 2  going high causes flip-flops  408  to enter a reset state, which maintains EN 1  low. 
     As previously described, operation of selection circuit also depends upon the inputs CLOCK_VALID, nReset and StopCK. CLOCK_VALID is a signal indicating the clock input CLOCK_ 2  is good or valid and that switching can proceed. In the present embodiment, when the PLL has not achieved lock and, therefore, CLOCK_ 2  is not valid, CLOCK_VALID is low causing SEL 2  to be low. This prevents CLOCK_ 2  from being output even if Select is low. Thus, CLOCK_VALID prevents the switching to CLOCK_ 2  while the PLL is not in lock (i.e., while CLOCK_ 2  is not valid). A similar signal could exist for CLOCK_ 1 , or any other clock to be switched. 
     StopCK stops the output on CLOCK_OUT and nReset places selection circuit  400  in a reset state. When StopCK goes high, both SEL 1  and SEL 2  go low, thereby causing both EN 1  and EN 2  to be low, which stops the output on CLOCK_OUT, as shown in FIG. 4 c . When nReset goes low, EN 1  and SEL 1  are forced high, thereby forcing EN 2  and SEL 2  low. This results in CLOCK_ 1  being output on CLOCK_OUT. 
     As would be apparent to one of skill in the art, arrangements in which the internal enable signals, EN 1  and EN 2 , are not applied to the reset inputs of the flip-flops are possible. This is illustrated in FIGS. 4 d  and  4   e . As can be seen, the embodiment of FIG. 4 d  is the same as the embodiment of FIG. 4 a , except the internal enable signals, EN 1  and EN 2 , are not connected to the reset inputs of the opposite flip-flops. 
     In the embodiment of FIG. 4 e , EN 1  and EN 2  are not connected to the resets of the opposite flip-flops. In this embodiment, however, nReset is connected to the set inputs of flip-flops  408  via inverter  404 . Similarly, nReset is connected to the reset inputs of flip-flops  406  via an inverter  450 . OR gates  434  and  422  are eliminated, with the output of AND gate  424  as SELL going directly to the first flip-flop of flip-flops  408 . In this embodiment, nReset going low causes EN 1  to go high because of the set inputs, while EN 2  goes low because of the reset inputs. 
     Another embodiment of the present invention is designed for control signals (i.e. Select, StopCK, and StopClockout) that change synchronously to the selected clock. This occurs, for example, when a clock selection circuit is used to clock a processor that issues the control signals, as shown in FIG. 5 a . A processor  501  is clocked by the CLOCK_OUT signal of a clock selection circuit  500  designed according to the principles of the present invention. Some of the control inputs of clock selection circuit  500 , i.e. Select and nReset are provided to selection circuit  500  by processor  501 . StopCK is generated as a combination of outputs from processor  501 , peripheral logic  503  and system logic  505 . Because CLOCK_OUT clocks processor  501 , Select, nReset and StopCK change synchronously to whichever clock, CLOCK_ 1  or CLOCK_ 2 , is selected for output on CLOCK_OUT. 
     As illustrated in FIG. 5 b , selection circuit  500  generally comprises enable logic  502 , synchronization logic  504  clocked by CLOCK_ 1 , synchronization logic  506  clocked by CLOCK_ 2 , output logic  508  and power control logic  510 . 
     Enable logic  502  generates internal select signals SEL 1  and SEL 2  based upon input signals nReset, Select, and StopCK. Internal select signal SEL 1  is input to synchronization logic  504 , while internal select signal SEL 2  is input to synchronization logic  506 . Synchronization logic  504  generates an internal enable signal EN 1  synchronously to CLOCK_ 1  based upon internal select signal SEL 1 . Likewise, synchronization logic  506  generates an internal enable signal EN 2  synchronously to CLOCK_ 2  based upon internal select signal SEL 2 . Internal enable signals EN 1  and EN 2  are input to output logic  502  in addition to CLOCK_ 1 , CLOCK_ 2  and StopClockout. The states of enable signals EN 1  and EN 2  determine which clock, CLOCK_ 1  or CLOCK_ 2 , is output-by-output logic  502 . Enable signals EN 1  and EN 2  are also input to power control logic  510 , in addition to CLOCK_ 1  and CLOCK_ 2 . Power control logic  510 , as described more fully below, controls the clocking of synchronization logic  504  and  506  based on the states of EN 1  and EN 2 . 
     As shown, enable logic  502  comprises OR gates  512 ,  522  and  520 , inverter  516 , NAND gate  514 , and AND gate  518 . The output of NAND gate  514  is SEL 1 . One of the inputs of NAND gate  514  is the Select signal. The other input of NAND gate  518  is the output of inverter  420 , which has the output of AND gate  518  as its input. AND gate  518  receives the nReset signal and StopCK signal as inputs. The output of OR gate  520  is SEL 2 . OR gate  520  receives the Select signal and the output of AND gate  428  as inputs. 
     The signal nReset is also provided to one of the inputs of OR gate  512 . The other input of OR gate  512  is the output of NAND gate  514 , i.e. SEL 1 . The output of OR gate  512  is provided to power control logic  510  and synchronization logic  504  in order to enable the function of nReset. Likewise, the signal nReset is provided to one of the inputs of OR gate  522 . The other input of OR gate  522  is the output of OR gate  514 , i.e. SEL 2 . The output of OR gate  522  is also provided to power control logic  510  in order to enable the function of nReset. 
     Synchronization logic  504  preferably comprises a plurality of cascaded flip-flops, such as D flip-flops. Each of the flip-flops is clocked by the associated input clock, i.e. CLOCK_ 1 , on the positive edge of the input clock. The first flip-flop of the cascade receives SELL as its input. In addition, SEL 1  is applied to the set input of each of the flip-flops. Synchronization logic  504  also comprises AND gate  524  and OR gate  526 . The last flip-flop of the cascade has its output connected to AND gate  526 , whose other input is the output of OR gate  512 . The output of AND gate  526  is input to OR gate  524 . The other input of OR gate  524  is SEL 1 . The output of OR gate  524  is EN 1 . 
     Likewise, synchronization logic  506  preferably comprises a plurality of cascaded flip-flops, such as D flip-flops. Each of the flip-flops is clocked by the associated input clock, i.e. CLOCK_ 2 , on the positive edge of the input clock. The first flip-flop of the cascade receives SEL 2  as its input. In addition, SEL 2  is applied to the set input of each of the flip-flops. Synchronization logic  506  also comprises AND gate  530  and OR gate  528 . The last flip-flop of the cascade has its output connected to AND gate  530 , whose other input is the output of OR gate  522 . The output of AND gate  530  is input to OR gate  528 . The other input of OR gate  524  is SEL 2 . The output of OR gate  528  is EN 2 . 
     It should be noted that, similar to the other embodiments, the use of a plurality of flip-flops rather than a single flip-flop reduces the possibility of a metastable condition, however, use of a single flip-flop is possible. 
     Output logic  508  comprises an AND gate  548  with one input connected to the output of an OR gate  544  and the other input connected to the output of a second OR gate  546 . OR gate  544  has one of its inputs connected to CLOCK_ 1  and the other input connected to EN 1 . Similarly, OR gate  546  has one of its inputs connected to CLOCK_ 2  and the other input connected to EN 2 . The output of AND gate  548  is input to OR gate  550 . The other input of OR gate  550  is the signal StopClockout. The output of OR gate  550  is taken as CLOCK_OUT. 
     Power control circuit  510  comprises NAND gate  532 , AND gates  536  and  538 , and OR gates  540  and  542 . NAND gate  532  receives EN 1  and EN 2  as inputs. The output of NAND gate  532  is input to AND gate  536 . AND gate  536  also receives the output of OR gate  512  as an input. The output of AND gate  536  is one of the inputs to OR gate  540 . The other input of OR gate  540  is CLOCK_ 1 . Each clock input of the flip-flops receives the output of OR gate  540 . The output of NAND gate  532  is also input to AND gate  538 . AND gate  538  also receives the output of OR gate  522  as an input. The output of AND gate  538  is one of the inputs to OR gate  542 . The other input of OR gate  540  is CLOCK_ 1 . Each clock input of the flip-flops receives the output of OR gate  542 . 
     Discussion of the operation of selection circuit  500  for selecting between CLOCK_ 1  and CLOCK_ 2  will be made in conjunction with the timing diagram in FIG. 5 c  and is made starting from a state in which CLOCK_ 1  is output on CLOCK_OUT. 
     In the case of CLOCK_ 1  being output on CLOCK_OUT, Select and nReset are high, while StopCK and StopClockout are low. This results in internal selection signal SEL 1  being low, while SEL 2  is high. Internal enable signal EN 1  is consequently low, while enable signal EN 2  is consequently high. Because EN 1  is low and EN 2  is high, CLOCK_ 1  is output on CLOCK_OUT from output logic  508 . 
     As previously described, the enable signals EN 1  and EN 2  are also input to power control logic  510 , in addition to CLOCK_ 1  and CLOCK_ 2 . Power control logic  510  controls the clocking of synchronization logic  504  and  506  based upon the states of EN 1  and EN 2  in order to reduce the power usage of selection circuit  500 . Therefore, when a clock is enabled for output, power control logic  510  prevents the clocking of synchronization logic  504  and  506 , while, during switching between clocks or upon reset, power control logic  510  allows clocking of synchronization logic  504  and  506 . As such, power control logic  510  prevents the clocking of synchronization logic  504  and  506  when CLOCK_ 1  is output. 
     When CLOCK_ 2  is to be selected for output on CLOCK_OUT, Select is switched low. Switching Select low results in SEL 2  going low, while SEL 1  and EN 1  go high. Because the Select signal is synchronized to the clock currently selected, the output of CLOCK_ 1  on CLOCK_OUT can be disabled when Select is changed. That is, because Select is synchronized to the currently selected clock, disabling CLOCK_ 1  when Select changes disables CLOCK_ 1  synchronously to itself. The enabling of CLOCK_ 2  on CLOCK_OUT, however, must still be synchronized to CLOCK_ 2  in order to prevent a glitch output. Therefore, synchronization logic  506  maintains EN 2  high. 
     As both EN 1  and EN 2  are high (indicating switching of the clocks), power control circuit  510  allows the clocking of synchronization logic  504  and  506 . Therefore, SEL 2  is propagated through synchronization logic  506 . Synchronization logic  506  is clocked by the positive edge of CLOCK_ 1 . This results in EN 1  synchronously enabling the CLOCK_ 2  output on CLOCK_OUT by going low after a rising edge, but prior to a falling edge, of CLOCK_ 2 . This synchronous enabling of CLOCK_ 2  on CLOCK_OUT prevents a glitch output. In addition, EN 2 , going high cause power control logic  510  to prevent the clocking of synchronization logic  504  and  506 . 
     As previously described, operation of selection circuit  500  also depends upon the inputs nReset, StopCK and StopClockout. The signal nReset places selection circuit  500  in a reset state During initialization of the logic, nReset is low and Select is high. This forces the output of AND gate  514  to be low and the output of OR gate  520  to be high. Also the output of OR gate  512  is low. The output of OR gate  520  sets flip-flops synchronization logic  506 , while the output of OR gate  512  forces CLOCK_ 1  to clock synchronization logic  504 , which will be initialized after a few clock edges(i.e., SEL 1  will propagate through the flip-flops). 
     When StopClockout goes high, CLOCK_OUT is masked high, effectively preventing the output of either CLOCK_ 1  or CLOCK_ 2  on CLOCK_OUT, as shown in FIG. 5 c . StopClockout is typically used by the processor clocked by selection circuit  500  to enter a power down mode in which it is not clocked. When a processor enters a power down mode, however, there must be a manner to wake up the processor. Therefore, secondary circuitry which still receives a clock signal and which can wake up the processor is used. So that the secondary circuitry can still receive a clock signal, preferably, a clock signal, IO_CK, is still available from selection circuit  500  while StopClockout is high. As such, when StopClockout is high, CLOCK_OUT remains high, but IO_CK continues to function as a clock signal. 
     StopCK completely stops the output of selection circuit  500 , including IO_CK. As can be seen with reference to FIG. 5 e , when CLOCK_ 2  is output on CLOCK_OUT, EN 1  is high and EN 2  is low. When StopCK goes high EN 1  goes high. This results in CLOCK_OUT and IO_CK remaining high. Likewise, as can be seen with reference to FIG. 5 f , when CLOCK_ 1  is output on CLOCK_OUT, EN 2  is high and EN 1  is low. When StopCK goes high, EN 2  goes high. This also results in CLOCK_OUT and IO_CK remaining high. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.