Abstract:
A stop and release circuit of a sync signal, to temporarily suspend or interrupt the sync signal, the input sync signal having a plurality of leading edges and a plurality of trailing edges, the circuit including a first divider that receives the input sync signal and supplies a first signal made up of the sync signal divided by two starting from a leading edge, a second divider that receives the inverse input sync signal and supplies a second signal made up of the sync signal divided by two starting from a trailing edge, an exclusive OR circuit that receives the first signal and the second signal and that supplies an output sync signal, a stop circuit for the first divider and the second divider, and an asynchronous command signal generated by the stop circuit for the temporary interruption of the output sync signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to a stop and release circuit for a sync signal and, more particularly, to a circuit capable of temporarily suspending or interrupting the sync signal. 
   2. Description of the Related Art 
   Sync signals are used in every digital circuit for coordinating the events. However it is often necessary to stop the sync signal, for example in the case of inactivity of a circuit, and to make it start up again as soon as it is needed. 
   However the action of temporarily interrupting a sync signal, in particular asynchronously, can cause glitches, that is pulses with a shorter duration than that of a half-wave of the sync signal. These pulses can create considerable disturbance to the circuits to which they are applied and also to those nearby. 
   BRIEF SUMMARY OF THE INVENTION 
   In view of the state of the technique described, the disclosed embodiments of the present invention provide a circuit for the temporary interruption of a sync signal that eliminates the presence of glitches. 
   In accordance with one embodiment of the present invention, a circuit for the temporary interruption of an input sync signal is provided that has a plurality of leading edges and a plurality of trailing edges, including a first divider that receives the input sync signal and supplies a first signal made up of the sync signal divided by two starting from a leading edge; a second divider that receives the inverse input sync signal and supplies a second signal made up of the sync signal divided by two starting from a trailing edge; an exclusive OR circuit that receives the first signal and the second signal and supplies an output sync signal; a stopping circuit for the first divider and the second divider; an asynchronous command signal of the stopping circuit for the temporary interruption of the output sync signal. 
   In accordance with another embodiment of the invention, a circuit for the temporary interruption of an input sync signal is provided that includes a first flip flop having a first input for receiving the input sync signal and an output; a second flip flop having a first input for receiving the input sync signal and an output; an exclusive OR circuit having first and second inputs coupled to the outputs of the first and second flip flops, respectively and generating an output sync signal; and a control circuit having first and second inputs coupled to the outputs of the first and second flip flops, respectively, third and fourth inputs coupled to second outputs of the first and second flip flops, respectively, a control input for receiving a stop signal, and first and second outputs coupled to inputs of the first and second flip flops. 
   In accordance with another embodiment of the invention, a circuit for the temporary interruption of an input sync signal is provided that includes a first flip flop having a first input for receiving a control signal and a second input for receiving the input sync signal, and a first output; a second flip flop having a first input for receiving a second control signal and a second input for receiving an inverse of the input sync signal, and a first output; an exclusive OR circuit having a first input coupled to the first output of the first flip flop and a second input coupled to the first output of the second flip flop; and a control circuit having a first input coupled to the first output of the first flip flop, a second input coupled to an inverting output of the second flip flop, a third input coupled to the first output of the second flip flop, and a fourth input configured to receive a stop signal. 
   In accordance with another embodiment of the invention, a method for the temporary interruption of an input sync signal is provided that includes supplying an input sync signal to a first flip flop and an inverse of the input sync signal to a second flip flop; generating an output from the first flip flop that is made up of the input sync signal divided by two starting from a leading edge, and generating from the second flip flop an output signal that is made up of the input sync signal inverted divided by two starting from a trailing edge; and supplying a stop signal to a control circuit that in turn sends stop signals to the first and second flip flops that stops their outputs at the state in which they were previously. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The characteristics and advantages of the present invention will be evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the enclosed drawings, in which: 
       FIG. 1A  shows a circuit for the temporary interruption of a sync signal in accordance with the known art, and  FIG. 1B  shows the characteristic digital signals of the circuit of  FIG. 1A ; 
       FIG. 2A  shows another circuit for the temporary interruption of a sync signal in accordance with the known art, and  FIG. 2B  shows the characteristic digital signals of the circuit of  FIG. 2A ; 
       FIG. 3A  shows yet another circuit for the temporary interruption of a sync signal in accordance with the known art, and  FIG. 3B  shows the characteristic digital signals of the circuit of  FIG. 3A ; 
       FIG. 4A  shows a circuit for the temporary interruption of a sync signal in accordance with the present invention, and  FIG. 4B  shows the characteristic digital signals of the circuit of  FIG. 4A ; 
       FIG. 5A  shows another circuit for the temporary interruption of a sync signal in accordance with the present invention, and  FIG. 5B  shows the characteristic digital signals of the circuit of  FIG. 5A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A through 3B  will now be described as illustrations of the known art, and the disadvantages thereof discussed in the context of these figures. 
     FIG. 1A  shows an AND circuit with an input sync signal CKIN applied to an input, and at the other input the inverse signal CKSTOP from the inverting circuit NOT, and which supplies at the output the output sync signal CKOUT.  FIG. 1B  shows the characteristic digital signals CKIN, CKSTOP, and CKOUT of the circuit of  FIG. 1A . 
     FIG. 2A  shows an AND circuit with an input sync signal CKIN applied to an input, and at the other input the inverse output signal QN of a flip flop FF of the D type, and which supplies at the output the output sync signal CKOUT. At the input D of the flip flop FF, the signal CKSTOP is applied; and at the synchronism input CP, the inverse input sync signal CKIN from the inverting circuit NOT is applied.  FIG. 2B  shows the characteristic digital signals CKIN, CKSTOP, and CKOUT of the circuit of  FIG. 2A . 
     FIG. 3A  shows a flip flop circuit FF of the D type with an input sync signal CKIN applied to its input D, and which supplies to its non-inverse output Q the output sync signal CKOUT. At the sync input CP of the flip flop circuit FF, the inverse input sync signal CKIN is applied by the inverting circuit NOT.  FIG. 3B  shows the characteristic digital signals CKIN, CKSTOP, and CKOUT of the circuit of  FIG. 3A . 
   The action of temporarily interrupting a sync signal, in particular in asynchronous manner, can be carried out as done in the previously described circuits. In the case of  FIG. 1A , if the signal is asynchronous, narrow pulses can be generated at the output of the circuit AND, that is pulses of shorter duration than a half-wave of the input sync signal, both before and after the stop signal CKSTOP. 
   In the case of  FIG. 2A , to avoid the narrow pulses mentioned above, the stop signal CKSTOP has been synchronized with the same sync signal CKIN, but this implies that it is necessary to wait for an edge before having the sync signal CKIN stopped, and in addition, if the sync signal CKIN is absent, there is never the certainty of having it in a stable state. 
   In the case of  FIG. 3A  a latch (flip flop FF) is used instead of the circuit AND to stop the sync signal CKIN. In this case the sync signal CKIN is stopped by the asynchronous control signal CKSTOP without narrow pulses, but when the latch FF becomes transparent in a non-synchronous manner with the clock, narrow pulses can be generated. 
     FIG. 4A  shows a circuit for the temporary interruption of a sync signal in accordance with the present invention. The circuit  10  comprises a flip flop FF 1  of the D type to which the input sync signal CKIN is applied at its sync input CP, the output signal of a multiplexer MUX 1  is applied at its input D, its non-inverse output Q is applied to an input of a circuit EXOR and to the input  1  of the multiplexer MUX 1 , and its inverse output QN is applied to the input  0  of the multiplexer MUX 1 . 
   In addition, the circuit  10  comprises a flip flop FF 2  of the D type at its sync input CP, of which the input sync signal CKIN inverted by the inverting circuit NOT is applied, while at its input D the output signal of a multiplexer MUX 2  is applied, its non-inverted output Q is applied to another input of the circuit EXOR and to the input  1  of the multiplexer MUX 2 , and its inverse output QN is applied to the input  0  of the multiplexer MUX 2 . 
   The stop signal CKSTOP is applied to the control inputs of the multiplexers MUX 1  and MUX 2 . 
   The output of the circuit EXOR supplies the output sync signal CKOUT. The non-inverse output Q of the flip flop FF 1  supplies the signal CKTOGGLE, and the non-inverse output Q of the flip flop FF 2  supplies the signal CKNTOGGLE. 
   A signal RESET is applied to the input R of the flip flop FF 2  and to the input S of the flip flop FF 1 . 
     FIG. 4B  shows the characteristic digital signals CKIN, CKSTOP, and CKOUT, CKTOGGLE, and CKNTOGGLE of the circuit of  FIG. 4A . 
     FIG. 5A  shows another circuit  20  for the temporary interruption of a sync signal in accordance with the present invention. The circuit  20  includes a flip flop FF 1  of the D type to which at its sync input CP the input sync signal CKIN is applied. At its input D, the output signal of a multiplexer MUX 1  is applied, and its non-inverse output Q is applied to an input of a circuit EXOR, to the input  1  of the multiplexer MUX 1 , and to the input  0  of a multiplexer MUX 2 . 
   In addition, the circuit  20  includes a flip flop FF 2  of the D type. At its sync input CP is applied the input sync signal CKIN inverted by the inverting circuit NOT. To its input D is applied the output signal of a multiplexer MUX 2 , its non-inverse output Q is applied to another input of the circuit EXOR and to the input  1  of the multiplexer MUX 2 , and its inverse output QN is applied to the input  0  of the multiplexer MUX 1 . 
   The stop signal CKSTOP is applied to the control inputs of the multiplexers MUX 1  and MUX 2 . 
   The output of the circuit EXOR supplies the output sync signal CKOUT. The non-inverse output Q of the flip flop FF 1  supplies the signal CKTOGGLE, and the non-inverse output Q of the flip flop FF 2  supplies the signal CKNTOGGLE. 
   A signal RESET is applied to the input R of the flip flop FF 2  and to the input S of the flip flop FF 1 . 
     FIG. 5B  shows the characteristic digital signals CKIN, CKSTOP, and CKOUT, CKTOGGLE, and CKNTOGGLE of the circuit of  FIG. 5A . 
   In the case of  FIG. 4A , when the signal CKSTOP is activated on the low phase of CKIN and deactivates on the high phase (or vice versa) the phase of the output sync signal CKOUT is inverted in relation to the input sync signal CKIN. This occurs because the signals CKTOGGLE and CKNTOGGLE change their phase relation. When instead the signal CKSTOP is activated and deactivated on equal phases, there is no inversion of the signal CKOUT. 
   While in the case of  FIG. 5A , the signals CKTOGGLE and CKNTOGGLE never change their phase relation and therefore there cannot be inversion of the signal CKOUT. When the signal CKSTOP is low, the flip-flop FF 1  changes state at every leading edge of the CKIN, generating the signal CKTOGGLE. The commutation has effect because, as it can be seen from  FIG. 5B , when the leading edge of CKIN arrives, the data in input to FF 1  is of an opposite value to its output. Similarly, the signal CKNTOGGLE is generated by FF 2 , which commutates on the trailing edge of CKIN. It can be easily verified from  FIG. 5B  that at the output of the exclusive OR, a signal CKOUT equal to CKIN is obtained. 
   When the control signal CKSTOP becomes high, the two flip flops FF 1  and FF 2  no longer commutate because the data at their input is equal to their output. Consequently, the signal CKOUT remains in a stable state. 
   When the control signal returns low, the two flip flops FF 1  and FF 2  return to commutate again, generating CKOUT similar to CKIN. 
   In view of the method by which CKOUT is generated, there cannot be glitches. 
   Both in the case of  FIG. 4A  and in the case of  FIG. 5A  the flip flops FF 1  and FF 2  the behavior is like that of the dividers for two of the input sync signals CKIN. In particular, the flip flop FF 1  supplies the signal CKTOGGLE which is made up of the input sync signal CKIN divided by two starting from a leading edge, and the flip flop FF 2  supplies the signal CKNTOGGLE which is made up of the input sync signal CKIN divided by two starting from a trailing edge. 
   The stop circuit of the sync signal is made up of the multiplexers MUX 1  and MUX 2  commanded by the stop signal CKSTOP. Following the change of state of the signal CKSTOP, the multiplexers MUX 1  and MUX 2  send suitable signals to the flip flops FF 1  and FF 2 , stopping their outputs at the state in which they were previously, and therefore stopping the signal CKOUT at the output of the circuit EXOR. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims and the equivalents thereof.