Abstract:
An apparatus includes a circuit and a signal source to supply a trigger signal to the circuit. The signal source is adapted to supply the trigger signal such that a reflection of the trigger signal delays the time at which the circuit is triggered.

Description:
BACKGROUND  
         [0001]    1. Field  
           [0002]    The invention relates to the field of electrical signals and more particularly, to the reflection of electrical signals along a circuit path.  
           [0003]    2. Background Information  
           [0004]    Electrical circuits often have their operation driven by a signal which is known as a “clock” signal (which may also be called a “trigger” signal). The trigger signal typically takes the form of a pulse which rises from a first predetermined voltage level (typically called “low”) to a second predetermined voltage level (typically called “high”). Of course, the designation of which voltage level constitutes a “low” or “high” is merely a matter of convention. Circuits which receive a trigger signal typically have their operation triggered when the trigger signal crosses a “trigger” level. The trigger level is a voltage level between the first predetermined level and the second predetermined level. As the voltage of the trigger signal rises between these levels, it crosses the trigger level with the result that the circuit receiving the trigger signal may perform an operation. For example, the well known latching circuit may read in and store a signal on a data input terminal where the trigger signal crosses the trigger level. When this occurs, the latch circuit is said to have been “triggered” or “clocked”. Of course, a circuit&#39;s operation may also be triggered by the transition of the trigger signal from the higher predetermined voltage level to the lower predetermined voltage level. The transition of a trigger signal from low to high voltage levels may be referred to as a “rising” edge of a trigger signal. Likewise, the transition from high to low voltage levels of a trigger signal may be referred to as the “falling” edge.  
           [0005]    Some circuits are capable of performing multiple operations, with some operations triggered on a rising edge and others triggered on the falling edge of a trigger signal. For example, a memory circuit may write (e.g. store) signals on its data input terminals and may read (e.g. output) signals stored in the memory to its data output terminals. The memory write operation may be triggered on the rising edge of a trigger signal and the memory read operation may be triggered on the falling edge of the trigger signal. Some memory circuits may be capable of performing a write operation and a read operation each triggered by the rising and falling edges of the same trigger signal.  
           [0006]    In some situations it may be desirable to substantially delay the triggering of the operation on the rising edge, without causing substantial delay to the triggering of the operation on the falling edge, or vice versa. For example, it may be desirable to delay the triggering of a memory write operation on the rising edge of a clock pulse, without delaying the triggering of a memory read operation on the falling edge of the same trigger signal. This may be desirable when the signals on the data input terminals are not available at the point in time when the rising edge of the trigger signal triggers a memory write operation. The circuits which read data signals from the data output terminals of the memory may be configured to receive the data signals shortly after the same trigger signal triggers a memory read operation. Thus it may not be acceptable to simply delay the entire trigger signal to delay the rising edge, because by delaying the entire trigger signal, both the rising and falling edges are delayed, which interferes with the memory read operation. The circuit reading data signals from the memory would be forced to incur delays to accommodate the delays in the memory write operation.  
           [0007]    One solution to this problem is to narrow the trigger signal so that the falling edge occurs sooner after the rising edge. By narrowing the trigger signal, the time at which the rising edge occurs may be delayed without altering the time in which the falling edge occurs. This approach may not be feasible in applications where the trigger signal is distributed to multiple circuits, some of which are adapted to expect the rising edge to occur at a predetermined point in time and at least one circuit adapted to expect the rising edge to be delayed. In this situation, simply adapting the trigger signal generator to produce a narrower trigger signal may be undesirable because the operation of some of the circuits receiving the trigger signal may be adversely affected. Those skilled in the art will recognize that the same situation could arise in situations where the timing of the rising edge is to be left unchanged, but where the falling edge needs to occur sooner in time.  
           [0008]    Thus, there exists a continuing need for a mechanism by which the timing of one edge of a signal received by circuit may be adjusted without substantially changing the timing of the other edge of the signal, and without altering the timing of the signal edges to other circuits which receive the signal.  
         SUMMARY  
         [0009]    An apparatus includes a circuit and a signal source to supply a trigger signal to the circuit. The signal source is adapted to supply the trigger signal such that a reflection of the trigger signal delays the time at which the circuit is triggered. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, may be further understood by reference to the following detailed description read with reference to the accompanying drawings.  
         [0011]    [0011]FIG. 1 shows a system in accordance with one embodiment of the present invention.  
         [0012]    [0012]FIG. 2 shows an embodiment of stud path in accordance with the present invention.  
         [0013]    [0013]FIG. 3 illustrates an embodiment of an incident trigger signal in accordance with the present invention.  
         [0014]    [0014]FIG. 4 is an illustration showing the trigger signal embodiment of FIG. 3 as it travels over a distance D.  
         [0015]    [0015]FIG. 5 shows an embodiment of a composite signal produced in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    In one embodiment of the present invention an incident trigger signal and a reflected trigger signal are superimposed to form a composite trigger signal. Relative to the incident trigger signal, the rising edge of the composite trigger signal is delayed without creating substantial delay in the falling edge of the composite trigger signal. The following description and drawings describe the present invention in terms of specific embodiments and examples, however, the scope of the present invention is defined only by the amended claims.  
         [0017]    [0017]FIG. 1 shows a system in accordance with one embodiment  100  of the present invention. The system comprises a processor  118 , a memory controller  102 , and a memory  104 . The processor  118  and memory controller  102  are coupled by way of processor bus  120 . Memory controller  102  and memory  104  are coupled by way of memory bus  1   10 . Processor  118  may write data to memory by placing data signals on processor bus  120 . Memory controller  102  may transfer these data signals to memory bus  110 , from which they may be received into memory  104 , e.g. written to memory  104 . Processor  118  may read data signals from memory  104  by indicating to memory controller  102  an address in the memory  104  from which to read data signals. Memory controller  102  may signal memory  104  to place data signals from this address on memory bus  110 . Memory controller  102  may transfer the data signals from memory bus  110  to processor bus  120 .  
         [0018]    Embodiment  100  further comprises trigger signal generator  106  to generate synchronized trigger signals to memory controller  102  and memory  104 . Trigger signals serve to synchronize the operation of memory controller  102  and memory  104 . This is commonly referred to as a common clock circuit configuration. Trigger signals propagate from signal generator  106  to memory controller  102  over signal path  114 . Trigger signals propagate from signal generator  106  to memory  104  over signal path  116 . The trigger signal generated by signal generator  106  is referred to as the incident trigger signal. In accordance with one embodiment of the present invention, a junction  1   12  is formed on signal path  116  and signal path  108  is joined thereto. Signal path  108  will henceforth be referred to as stub path.  
         [0019]    [0019]FIG. 2 shows an embodiment  200  of stud path  108  in accordance with the present invention. In this embodiment junction  112  is a simple “T” connection of stub path  108  and signal path  116 . Stud path is unterminated. That is, no resistive, capacitive, inductive, or other electrical load is coupled between stud path  108  and an electrical ground. Stud path  108  floats electrically and is may be a strip of conductive material of length D′ which is unterminated. Length D′ may be chosen to be approximately the same as the length D of signal path  116  as is present between junction  112  and memory  104 . Stud path length D′ may not be exactly equal to the length D, and may in fact fall within some percentage of length D. For example, length D′ may “of an order” of the length D. In some embodiments, stud path length D′ may vary between approximately 5% and 50% of the length of signal path  116  between junction  112  and memory  104 . Determination of stub length D is described further below.  
         [0020]    [0020]FIG. 3 illustrates an embodiment  300  of an incident trigger signal in accordance with the present invention. Trigger signal  300  is illustrated in accordance with complimentary metal oxide semi-conductor technology (CMOS), which comprises a well known predetermined low voltage level of Vss and a predetermined high voltage level of Vdd (source and drain voltages respectively for CMOS transistors). Of course, other semiconductor technologies are equally applicable to the present invention. Trigger signal  300  is illustrated in terms of its voltage level over time. Trigger signal  300  comprises a rising edge  302 , a plateau  306 , and a falling edge  304 . Clock pulse  300  takes a certain period of time Tr to rise from low voltage level Vss to the high voltage level Vdd. This period of time may be referred to as the rise time of the leading edge  302  of trigger signal  300 .  
         [0021]    [0021]FIG. 4 is an illustration showing the trigger signal embodiment of FIG. 3 as it travels over a distance D. Two distinct points in time are illustrated. At a first time t, the trigger signal  300  begins to rise from the low voltage Vss. At a later time t+Tr, the trigger signal  300  has reached plateau level  306 . During the time it took trigger signal  300  to rise from the low voltage level to the high voltage level, e.g. the rise time Tr, the trigger signal may propagate a distance D down signal path  116 . For example, a trigger signal with a ins (one nanosecond) rise time may propagate approximately five inches down the signal path  116  during the rise time. This distance may be calculated by multiplying ins by the speed of electrical signal propagation, which may vary according to the electrical properties of signal path  116  but which may, in some embodiments, approximate the well-known value of the speed of light. As previously described, the length D′ of stub path  108  need only be “of an order” or D and not precisely equal to D.  
         [0022]    [0022]FIG. 5 shows an embodiment of a composite signal produced in accordance with the present invention. Stud path  108  may reflect an incident trigger signal  508  to produce a reflection signal  510  on signal path  116 . The length of stud path  108  is appropriately chosen is described previously. The rising and falling edges of reflection signal  510  may be offset from the rising and falling edges of the incident signal  508 .  
         [0023]    Incident signal  508  and reflected signal  510  may superimpose over time to form a composite trigger signal  506 . Composite signal  506  may have several advantageous properties. A plateau  502  may be formed in rising edge of composite signal  510 . Plateau  502  serves to delay the attainment of voltage levels above the plateau level  502 . A plateau  504  may also be formed on falling edge of composite signal  506 , however, plateau  504  of falling edge may occur at a voltage level substantially below plateau  502  of rising edge. A circuit whose operation is driven by composite trigger signal  506  is adapted to be triggered at a voltage level above plateau level  502 . Triggering of the circuit&#39;s operation may thus be delayed, due to the rising edge delay in reaching voltage levels above the plateau level  502 .  042390 .P 7235   
         [0024]    For example, consider a memory circuit with a memory write operation triggered by the rising edge of incident signal  508  at a trigger level of 0.5 volts. According to the signal timings illustrated in FIG. 5, said circuit may be triggered for write operation at approximately Ins and 1 Ins. Now consider a memory circuit with a write operation triggered at a  1  IV trigger level by composite signal  506 . The write operation of such a circuit will be triggered at approximately 2.5ns and 12.5ns.  
         [0025]    Now consider a memory circuit with a memory read operation triggered by the falling edge of incident signal  508  at a trigger level of 0.5 volts. According to the signal timings illustrated in FIG. 5, said circuit may be triggered for read operation at approximately 6ns and 16ns. Now consider a memory circuit adapted to trigger a read operation at trigger level of  1 .IV by falling edge of composite signal  506 . The read operation of such a circuit will again be triggered at approximately 6ns and 16ns. In other words, the read operation of the two memory circuits is triggered at approximately the same time. In other words, by applying composite trigger signal  506  to a circuit with appropriately adapted trigger levels, the trigger time of an operation on the rising edge of composite trigger signal  506  may be substantially delayed without affecting the trigger time of an operation triggered on the falling edge of composite trigger signal  506 .  
         [0026]    The invention is in no way limited to the use of stub paths to produce the composite signal  506 . Any mechanism for producing a trigger signal with the properties of composite signal  506  may also be employed. One embodiment employs a stub path  108  to produce a reflection signal  510  to combine with an incident signal  508  produced by a signal generator  106 . However, other embodiments could produce a signal with properties similar to those of composite signal  506  using an arrangement of transistors or other circuit components. Such embodiments could potentially employ reflection signals, but would not necessarily do so. From the perspective of the circuit being triggered by the composite signal  506 , the source (e.g. the specific circuit arrangements and adaptations) which produce composite signal  506  is less important than the properties of composite signal  506  itself. Thus, the invention is not limited to a particular circuit arrangement acting as the source of the composite signal  506 .  
         [0027]    Returning to FIG. 1, a memory write operation triggered on the rising edge of a trigger signal could be substantially delayed by applying the present invention to signal path  116  and memory  104 . This may provide memory controller  102  with substantial additional time to establish data signals on memory bus  1   1   0  before the write operation is triggered. Trigger signals on other signal paths, for example path  114 , would not be affected. Furthermore, memory read operations triggered by the falling edge of the trigger signal would not be substantially delayed as a result of delaying the memory write operations. This may be advantageous in applications where more time is needed to set up the data on memory bus  110  for a write operation, without affecting the performance of a read operations, and without affecting the timing of trigger signals to other circuits supplied by signal generator  106 .  
         [0028]    Those skilled in the art will of course recognize that the present invention may also be applied to delay circuit operations triggered on the falling edge of a signal. In such a case, the trigger level of the circuit for both rising and falling edge operations would be adjusted below the level of the plateau on the falling edge of composite signal  506 . Thus, operations triggered on the rising edge would not be substantially delayed, because they are triggered at levels less than the level of the rising edge plateau. Operations triggered on the falling edge might be substantially delayed because they are triggered at levels less than the level of the falling edge plateau.  
         [0029]    While the invention has been described in terms of specific embodiments and examples, those skilled in the art will appreciate numerous modifications are possible which fall within the scope of the invention. The specific examples and embodiments described herein are presented for purposes of illustration only, and the scope of the present invention should be construed only in light of the claims which follow.