# Patent

## Title
Two-latch clocked-LSSD flip-flop

## Abstract

A clocked level-sensitive scan design may have flip-flops designed to have data, scan-in, and output ports and to utilize two clock signals. Such a clocked level-sensitive scan flip-flop may be built utilizing two latches.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. application Ser. No. 10/909,382, filed Aug. 3, 2004, now U.S. Pat. No. 7,262,648, which is hereby incorporated by reference.


BACKGROUND OF THE INVENTION
It is possible to build test hardware into integrated circuit devices, to ease automated testing. Such test hardware may support scan testing and may include, for example, a scan chain and/or a level-sensitive scan design (LSSD). In an LSSD, at least a portion of the test hardware may also be used in normal integrated circuit operation, as well as in testing. Many LSSDs, however, have had the drawback of taking up large die areas. Many types of LSSDs, and in particular clocked LSSDs, may also require several clock signals for operation.



BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the invention will now be described in connection with associated drawings, in which:
FIG. 1A depicts a circuit diagram of and FIGS. 1B and 1C depict a timing diagram associated with a first embodiment of the invention;
FIG. 2 depicts a circuit diagram of a second embodiment of the invention;
FIG. 3 depicts a block diagram of a system according to a further embodiment of the invention; and
FIG. 4 depicts a circuit diagram of a third embodiment of the invention.



DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and/or techniques have not been shown in detail in order not to obscure an understanding of this description.
References to “one embodiment”, “an embodiment”, “example embodiment”, “various embodiments”, etc., indicate that the embodiment(s) of the invention so described may include a particular feature, structure, or characteristic, but not every embodiment necessarily includes the particular feature, structure, or characteristic. Further, repeated use of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.
In the following description and claims, the terms “coupled” and “connected,”along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other.
FIG. 1A depicts a circuit diagram showing a clocked LSSD flip-flop according to an embodiment of the invention. The embodiment of FIG. 1A may only require two clock signals, CK (“clock”) and TCK (“test clock”), may have two inputs, D (“data”) and SI (“scan in”), and may only require a single output O, which may be used as the output for both data mode and scan mode, whichever mode in which the circuit happens to be being used. Note that inverters (not shown) may also be provided to generate the complements, CK and TCK, of the signals, CK and TCK, respectively.
The circuit of FIG. 1A may contain an input portion, which may consist of inverters 11 and 12. Signal SI may be provided as an input to inverter 11, and signal D may be provided as an input to inverter 12. Inverters 11 and 12 may be equipped with enable inputs. TCK may be used as an enable input for inverter 12, and TCK may be used as an enable input for inverter 11. The effect of this is that the test clock, TCK, may act as a selection signal to select which input inverter, 11 or 12, may be enabled at any given time. If TCK is high, then inverter 12 may be selected, and D may be input. If TCK is low, then inverter 11 may be selected, and SI may be input (for scan mode).
Whichever signal is selected as input, D or SI, may then be fed to a pass gate 13. Pass gate 13 may have as control inputs CK and CK. Such a pass gate 13 may be constructed out of a PMOS transistor and an NMOS transistor, as shown, where the PMOS transistor may permit a signal to pass through when its gate signal (here, CK) is low, and the NMOS transistor may permit a signal to pass through when its gate signal (here, CK) is high. Therefore, when CK is high, no signal may pass through pass gate 13, and when CK is low, a signal may pass through pass gate 13.
A signal that passes through pass gate 13 may then be fed to a first, or “master,”latch 14. Latch 14, in the embodiment of FIG. 1A, may comprise two inverters 14A and 14B, which may be connected in a feedback configuration, as shown. One having ordinary skill in the art would understand, however, that latch 14 is not limited to the embodiment shown in FIG. 1A and that alternative implementations may be possible. In the implementation of latch 14 shown in FIG. 1A, inverter 14A may be equipped with an enable input, to which CK may be fed as an enable signal. As a result, in the implementation shown in FIG. 1A, a value is latched into latch 14 only when CK transitions high.
A value stored in latch 14 may then be fed through an inverter 15, and from inverter 15 through pass gates 16 and 17, which form a pass gate assembly. Pass gates 16 and 17 may be constructed in a fashion similar to pass gate 13. Pass gates 16 and 17 may be arranged, as shown in FIG. 1A, such that a signal may be permitted to pass through both gates only when both CK and TCK are high (noting the above discussion in connection with pass gate 13).
The output of pass gate 17 may then be fed into a second (“slave”) latch 18. Latch 18 may be constructed out of inverters 18A and 18B, connected in a feedback fashion, as shown in FIG. 1A, or in other ways that may be known to one of ordinary skill in the art. Latch 18 may hold whatever value appears at the output of pass gate 17. This value may also be fed through an inverter 19, whose output, O, may provide an output of the shown embodiment of the invention.
FIGS. 1B and 1C together show a timing diagram that shows how clock signals CK and TCK may be used to implement regular and scan modes in the embodiment of FIG. 1A. In data mode, TCK may be maintained in a high state. This may permit D to be selected as an input signal (to inverter 12), and it may further ensure that pass gate 16 always passes its input signal to pass gate 17. CK may run at a regular rate of oscillation between high and low states. When CK is low, pass gate 13 may pass the signal received from inverter 12 to latch 14, which may latch in the value of the signal when CK transitions to a high state. The value stored in latch 14 may then pass through inverter 15, whose output may, in turn, pass through pass gate 16 (which, as discussed above in connection with TCK, may always pass its input signal) to pass gate 17. Pass gate 17 may permit the value from pass gate 16 to pass through it when CK is high. The value that passes through to the output of pass gate 17 may then be stored in latch 18 and may be output through inverter 19 as output signal O.
In scan mode, TCK may oscillate between high and low states, as shown in FIG. 1C, while CK may also oscillate between high and low states, as shown in FIG. 1B. CK and TCK may be chosen such that they are non-overlapping and are not limited to the clock signals shown in FIGS. 1B and 1C, as would be apparent to one of ordinary skill in the art.
Using the exemplary timing diagram of FIGS. 1B and 1C, operation of the circuit of FIG. 1A may be described as follows. When TCK goes to a low state, scan input SI may be chosen by enabling inverter 11. The resulting data value may then be permitted to pass through pass gate 13 and to be loaded into latch 14 when CK goes to a high state. The value stored in latch 14 may then pass through inverter 15 to the input of pass gate 16. As discussed above, the input to pass gate 16 may only pass through to the output of pass gate 17 and may be loaded into latch 18 when both TCK and CK are in a high state. Hence, in the example of FIGS. 1B and 1C, this may occur when TCK transitions to a high state, as noted by the arrow labeled “Load slave.” As a result of these operations, the output signal O may correspond to a scan output. The circuit may reenter the data mode when TCK is again held high.
A problem may arise if the latch 18 is not being fed a value. This may result in a spurious output value that may depend upon what value results, if any, when pass gate 17 is in a quiescent state. A solution to this problem found in one embodiment of the invention may be to design latch 18 with so-called “weak” feedback, so that it holds its value static unless it is actually driven by a (new) input signal value.
A second solution to the above-described problem may be demonstrated by the embodiment shown in FIG. 2. In FIG. 2, latch 18 may be constructed such that it may be provided with an enable signal (e.g., to inverter 18A). The enable signal provided to latch 18 may be used to ensure that it loads a new value only at desired times, i.e., when a new value is passed to the output of pass gate 17. Such an enable signal may be generated based on CK and TCK, using control logic 20. One of ordinary skill in the art would recognize that there may be many ways to design control logic 20 so as to obtain a desired enable signal. For example, control logic 20 may be designed to provide an enable signal to latch 18 that is complementary to an enable signal provided to pass gates 16 and 17.
It is noted that the various components of FIGS. 1A and 2 may be designed in alternative ways that would be apparent to one of ordinary skill in the art. For example, the various components may be selected such that one or more of the pass gates pass signals when the clock polarities are the opposite of those described above.
Furthermore, the principles of the invention may be applicable to other flip-flop designs to convert them to clocked-LSSD flip-flops by adding an additional joint clock/control signal (similar to TCK in the embodiments described above) and any necessary associated control logic to such other flip-flop designs. In general, embodiments of the invention may use a single signal as a clock/control signal that selects input data versus a scan value and to delay operation of, for example, a slave latch or other downstream storage component of the flip-flop (e.g., in a master-slave-type design having first and second storage portions (which may be latches)).
FIG. 4 shows an example of such an alternative embodiment. The embodiment of FIG. 4 may, once again, have master 41 and slave 42 latch portions. This embodiment may use a differential pull-down approach to loading data. However, as in the above embodiments, TCK may be used to select whether D or SI is used as input, and there may, once again, be a single output O, for both data and scan modes. In general, the embodiment of FIG. 4 may operate in a functionally similar fashion to the embodiment shown in FIG. 1A and as described above.
The invention may also be embodied as part of a larger device and/or system, as shown in FIG. 3. The inventive clocked LSSD flip-flop, for example, as described in connection with FIGS. 1A-1C and/or FIG. 2 and/or FIG. 4, may be incorporated into LSSD 33 of FIG. 3. LSSD 33 may interface with additional logic 32 found on a common chip or within a common system 34. Such additional logic 32 may include, but is not limited to, memory, combinational logic, and/or a processor. Logic 32 may provide LSSD 33 with data (D), for which a corresponding output (O) may be generated at the output of LSSD 33 (and which may be fed back to logic 32, if desired). LSSD 33 may also interface with a testing system 31. Testing system 31 may reside at least partially on a common chip or within a common system 34, or it may be totally external to the chip or system 34. Testing system 31 may be used to provide a scan input (SI) to LSSD 33, which may then provide an associated scan output (O) back to testing system 31. In this fashion, testing system 31 may perform diagnostic testing of the chip or system 34. The embodiment of FIG. 3 may further include a memory 35, which may be coupled to testing system 31, LSSD 33, and/or logic 32.
The invention has been described in detail with respect to various embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. The invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications as fall within the true spirit of the invention.

## Claims

1. A circuit, comprising:
a data input configured to receive a data signal;
a scan input configured to receive a scan signal;
a control input configured to receive a control signal;
a clock input configured to receive a clock signal;
an input portion configured to select the data signal when the control signal is constant or select the scan signal when the control signal is periodic, and to provide a selected signal that is substantially the same as the data signal or the scan signal;
a first storage portion in communication with the input portion and configured to store the selected signal in response to the clock signal;
a second storage portion in communication with the first storage portion and configured to store the selected signal in response to the control signal and the clock signal; and
an output in communication with the second storage portion and configured to provide an output signal.

2. The circuit of claim 1, wherein the clock signal comprises a first frequency when the control signal is constant and a second frequency when the control signal is periodic.

3. The circuit of claim 1, where the first storage portion comprises a first pass gate and a first latch, the first pass gate in communication with the input portion and configured to pass the selected signal in response to the clock signal, and the first latch in communication with the first pass gate and configured to store the selected signal in response to the clock signal.

4. The circuit of claim 3, where the first latch comprises a first plurality of inverters in a feedback configuration, where at least one of the first plurality of inverters is enabled by the clock signal.

5. The circuit of claim 1, where the second storage portion comprises a second pass gate and a second latch, the second pass gate in communication with the first storage portion and configured to pass the selected signal in response to the control signal and the clock signal, and the second latch in communication with the second pass gate and the output and configured to store the selected signal.

6. The circuit of claim 5, where the second latch comprises a second plurality of inverters in a feedback configuration.

7. The circuit of claim 6, where the second storage portion further comprises control logic to generate a latch signal in response to the control signal and the clock signal, where at least one of the second plurality of inverters is enabled by the latch signal.

8. The circuit of claim 1, where the input portion and first storage portion comprise a differential pull-down circuit.

9. The circuit of claim 1, further comprising:
a logic circuit in communication with the data input and configured to provide the data signal and receive the output signal;
a testing system in communication with the scan input and configured to provide the scan signal and receive the output signal; and
a memory in communication with the logic circuit and the testing system.

10. A circuit, comprising:
data input means for receiving a data signal;
scan input means for receiving a scan signal;
control input means for receiving a control signal;
clock input means for receiving a clock signal;
selection means for selecting the data signal when the control signal is constant or the scan signal when the control signal is periodic, and for providing a selected signal that is substantially the same as the data signal or the scan signal;
first storage means in communication with the selection means for storing the selected signal in response to the clock signal;
second storage means in communication with the first storage means for storing the selected signal in response to the control signal and the clock signal; and
output means in communication with the second storage means for providing an output signal.

11. The circuit of claim 10, wherein the clock signal comprises a first frequency when the control signal is constant and a second frequency when the control signal is periodic.

12. The circuit of claim 10, where the first storage means comprises first pass gate means and first latch means, the first pass gate means in communication with the selection means and configured to pass the selected signal in response to the clock signal, and the first latch means in communication with the first pass gate means and configured to store the selected signal in response to the clock signal.

13. The circuit of claim 12, where the first latch means comprises a first plurality of inverters in a feedback configuration, where at least one of the first plurality of inverters is enabled by the clock signal.

14. The circuit of claim 10, where the second storage means comprises second pass gate means and second latch means, the second pass gate means in communication with the first storage means and configured to pass the selected signal in response to the control signal and the clock signal, and the second latch means in communication with the second pass gate and the output and configured to store the selected signal.

15. The circuit of claim 14, where the second latch means comprises a second plurality of inverters in a feedback configuration.

16. The circuit of claim 15, where the second storage portion means further comprises control means to generate a latch signal in response to the control signal and the clock signal, where at least one of the second plurality of inverters is enabled by the latch signal.

17. The circuit of claim 10, where the selection means and the first storage means comprise a differential pull-down circuit.

18. The circuit of claim 10, further comprising:
logic means in communication with the data input means for providing the data signal and receiving the output signal;
testing means in communication with the scan input means for providing the scan signal and receiving the output signal; and
storage means in communication with the logic means and the testing means.

19. A method, comprising:
selecting a data input when a control signal is constant;
selecting a scan input when the control signal is periodic;
providing a selected signal that is substantially the same as the data input or the scan input;
storing the selected signal in response to a clock;
storing the selected signal in response to the control signal and the clock; and
providing an output signal that is substantially the same as the selected signal.

20. The method of claim 19, wherein the clock comprises a first frequency when the control signal is constant and a second frequency when the control signal is periodic.

21. The method of claim 19, where storing the selected signal in response to the clock comprises:
passing the selected signal in response to the clock; and
latching the selected signal in response to the clock.

22. The method of claim 21, where latching the selected signal comprises using a first plurality of inverters in a feedback configuration, where at least one of the first plurality of inverters is enabled by the clock.

23. The method of claim 19, where storing the selected signal in response to the control signal and the clock comprises:
passing the selected signal in response to the control signal and the clock; and
latching the selected signal.

24. The method of claim 23, where latching the selected signal comprises using a second plurality of inverters in a feedback configuration.

25. The method of claim 24, where the storing the selected signal in response to the control signal and the clock further comprises:
generating a latch signal in response to the control signal and the clock; and
enabling at least one of the second plurality of inverters with the latch signal.