Method and apparatus for testing pipelined dynamic logic

A method and apparatus for testing pipelined dynamic logic makes it possible to set and retrieve values from dynamic logic pipelines that have no internal latches. A modification to the pipeline circuits and clocking circuitry enable scanning logic to set and retrieve values from the pipelined circuits.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to boundary scan testing, and more particularly, to boundary scan testing in integrated circuits incorporating pipelined dynamic logic.

2. Description of the Related Art

Functional tests of integrated circuits and printed circuit boards are necessary to assure defect-free products. Using a low pin-count serial interface, the Joint Test Action Group (JTAG) interface provides a mechanism for testing both the internal features of an integrated circuit and the connections between two integrated circuits mounted on a printed circuit board or other substrate. The details of the JTAG interface are defined by IEEE (Institute of Electrical and Electronics Engineers) standard 1149.1 IEEE Standard Test Access Port and Boundary Scan Architecture. Latches within an integrated circuit permit a manufacturing tester to load test values and latch test results by manipulating the signals in the JTAG interface.

The market for integrated circuits requires increasing speed and functional density as requirements for computing power and storage continue to rise. A technology that has been applied in recent years to an increasing number of microprocessor and other integrated circuits, is dynamic logic, sometimes referred to as Domino Logic Circuits . Domino logic circuits are very efficient from a device per gate standpoint. But, due to the dynamic nature of the logic (logic signals exist as pulses propagating through a dynamic logic pipeline, rather than clocked through static stages), testing of integrated circuits containing domino logic typically involves only the input and output of the domino logic blocks. Since the logic is dynamic, there is no mechanism for loading values within the domino logic circuits so that the entire logic state of an integrated circuit can be set. In a typical dynamic logic pipeline, level-sensitive scan design (LSSD) latches are inserted between groups of several levels of dynamic gates. These latches provide the ability to store data within the logic for scan testing, but at a penalty of reduced performance and increased circuit area.

In a wave pipeline, rather than relying on latches to store state, the logic state is maintained within the combinatorial logic circuits. The values that would otherwise be stored in latches are maintained on capacitive nodes within the logic circuits. Because there are no explicit static latches within the wave pipeline interior, the interior of the pipeline cannot be observed or controlled for testing purposes. Static latches could be introduced in the middle of a pipeline, but this defeats the advantages of the wave pipeline constructed solely of cascaded dynamic stages. In light of the foregoing, it would be desirable to provide a method and apparatus for testing dynamic logic pipelines that have no internal latches.

SUMMARY OF THE INVENTION

The objective of testing dynamic logic pipelines having no internal latches is accomplished by initially loading a value from a scan chain into a latch, determining whether or not the integrated circuit is transitioning from scan mode to functional mode and discharging an evaluation node within the dynamic logic. The evaluation node is discharged in conformity with the loaded value in response to determining that the integrated circuit is transitioning from scan mode to functional mode.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

With reference now to the figures, and in particular with reference to FIG. 1 , there is depicted a schematic diagram of a prior art integrated circuit 10 incorporating scan latches and dynamic logic. A tester 11 is coupled to a Joint Test Action Group (JTAG) interface 12 within integrated circuit 10 , which contains boundary scan circuits. JTAG interface 12 transfers data and commands between tester 11 and boundary scan blocks within integrated circuit 10 , permitting values to be loaded or scanned in to state-storing elements within integrated circuit 10 . JTAG interface 12 also enables resulting states to be read from or scanned out from the state-storing elements within integrated circuit 10 , after clocks from tester 11 have operated integrated circuit 10 through a cycle of operation. The boundary scan blocks comprise Level-sensitive scan design (LSSD) master/slave latches 14 connected serially in LSSD scan chains 13 to load test data into combinatorial logic 15 or read data from combinatorial logic 15 that has been latched by LSSD latch 16 .

LSSD scan chain 13 provides the capability to read and write the states of logic circuits within integrated circuit 10 , except states that are propagating through dynamic combinatorial logic 15 . Access to all of the static state-holding elements within integrated circuit 10 allows both design verification and production line testing to be performed prior to use in a computer system. LSSD scan chain 13 architectures that may be used with the techniques of the present invention are described in U.S. Pat. No. 5,920,575 entitled VLSI Test Circuit Apparatus and Method, which is incorporated herein by reference, but other boundary scan and test port circuits and topologies may be used without departing from the spirit and scope of the invention.

The present invention provides an improved test method and apparatus that permit insertion of data within sub-networks of a dynamic pipeline.

Referring now to FIG. 2 , there is depicted a schematic diagram of an integrated circuit 20 incorporating scan latches and dynamic logic in accordance with a preferred embodiment of the invention. A JTAG interface 22 is coupled to tester 21 , so that the internal states of integrated circuit 20 can be set and read during manufacturing test. An LSSD latch 24 A provides data to a dynamic buffer circuit 25 A, which produces a pulse input to a first pipeline stage in dynamic combinatorial logic 26 . Dynamic combinatorial logic 26 contains a pipeline constructed from cascaded dynamic logic gates 27 B and modified dynamic gate 27 A. The input scan value for the first stage in the pipeline is latched in LSSD latch 24 A by an Aclk signal. A Scan Bclk signal is coupled to one-shot 23 A to provide a clock pulse to launch the scan value into the pipeline. Because one shot 23 A output is coupled to both the foot device and the precharge device within dynamic buffer 25 A, the output of dynamic buffer 25 A will be a pulse, just as the input of dynamic combinatorial logic 26 would be pulsed in normal operation. The Scan Bclk signal is the clock used to load values into the scan latches during scan mode to functional mode transition. Dynamic buffer 25 A provides the data pulse to dynamic combinatorial logic 26 .

A second LSSD latch 24 B, latches input scan values for intermediate stages in the pipeline. Modified dynamic logic gate 27 A is the mechanism whereby intermediate values are introduced. The Scan Bclk signal is used to load the scan value from LSSD latch 24 B into modified dynamic logic gate 27 A. The end of the dynamic logic pipeline is sampled by dynamic buffer 25 B, which is clocked by a pulse generated by one-shot 23 B from the Cclk signal, which is the clock used to load values into the scan latches at the end of a test clock cycle. The resultant value from dynamic buffer 25 B is latched into the scan chain by LSSD latch 24 C, which forms part of the scan chain coupled to JTAG interface 22 . The resulting state at LSSD latch 24 C can then be read out of the scan chain by serial shifting to JTAG interface 22 .

Referring now to FIG. 3 , a schematic diagram of modified dynamic gate 27 A from FIG. 2 is depicted. Logic input ladders 41 provide the functional operation of modified dynamic gate 27 A. For example, a logic input ladder 41 as depicted implements the logic function: (A and B) or (C and D). The actual effect on the evaluation node is the inverse of this equation, but inverter 44 inverts the value of the evaluation node to generate the output value from the logic gate. Since the evaluation node will be discharged if either of signal pairs (A,B) or (C,D) are (1,1) (logic high states), the evaluation node represents a logical NAND of the input values when a foot transistor N 43 is activated after the Bclk signal has precharged the evaluation node.

Scan ladder 45 is used to introduce an input scan data value to modified dynamic logic gate 27 A. A transistor N 41 is enabled to discharge the evaluation node if the Scan Bclk signal is in a logic high state. Since the Scan Bclk signal is only active during scan mode, scan ladder 45 does not affect the normal functional operation of integrated circuit 20 . Logic input ladders 41 are prevented from affecting the scan value loaded during test by foot transistor N 43 . An AND gate 46 or other means for gating the Bclk signal provides a bclk signal that is valid only during run mode. Run mode is an operational mode as opposed to tester scan mode. This prevents interference with the loading of values via scan ladder 45 while integrated circuit 20 is in scan mode. AND gate 43 prevents the Bclk signal from turning on foot transistor N 43 , effectively removing logic input ladders 41 from dynamic logic gate 27 A during scan mode.

Preset transistor P 41 is clocked by the Bclk signal so that the evaluation node is preset for both scan mode and run mode. The preset voltage will be selectively discharged by logic input ladders 41 , or scan ladder 45 , depending on the mode of integrated circuit 20 as set by tester 21 . The output value is presented to the next stage in the dynamic pipeline, which may or may not have a scan ladder coupled to its evaluation node, depending on testability requirements for integrated circuit 20 .

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims.