Operational analysis device of the scan path type having a single scanning clock and a single output phase for an integrated circuit

Abstract: The device comprises a first chain of scanning cells (2) located at the stimulation input of each respective functional block (1) of the integrated circuit and a second chain of scanning cells (3) located at the assessment output of each respective functional block (1) of the integrated circuit. Each cell (2, 3) comprises a master part (M), a slave part (S) and switching means (8, 50) to alternately enable said master (M) and slave parts (S) under the control of respective master clock and slave clock signals coincident with opposite phases of a scanning clock signal (SCK) having a substantially square wave. With each pair of chains of scanning cells (2, 3) there are associated clock generation means (25, 26) to locally obtain said master and slave clocks from said scanning clock (SCK).

Description: The present invention relates to an operational analysis device of the scan path type having a single scanning clock and a single output phase for an integrated circuit. The so-called scan path is a type of analysis the use of which is well-known for checking whether one or more functional blocks of an integrated circuit, such as, say, a microprocessor, are operating correctly. It is based on the use of serial-type scanning signals which operate on chains of scanning cells (latch) located at the input and output of individual functional blocks with a function of stimulation and assessment, respectively, of the operational state of the same functional blocks. In this way it is possible to progressively update the operational state of the functional block under examination and to correspondingly obtain at output a signal indicative of the various operational states assumed by the same functional block with each inquiry of the serial signal at input. In this respect it should be noted that a device such as a microprocessor is constituted to a large extent by a clocked sequential logic, that is, operating at a fixed rate set by a clock signal. It is thus easy to identify functional blocks whose input and output signals are clocked, that is, are controlled by a clock signal. It appears evident that, if the clocking phase of the inputs of said functional blocks is lengthened to allow by means of serial scanning the alteration of some or all of the same inputs, nothing is damaged. This hypothesis is naturally valid as long as the logic is static. In the case of a generic functional block made with a static logic it is possible to define the "stimulation phase" the phase wherein the inputs are memorized and the "assessment phase" the phase wherein the outputs of the functional block are stored in outputs cells or latches. It is also possible to say in general that the alteration requested by a scan path analysis with respect to the original logic is represented by the addition of scanning or latch cells at each input and output, connecting the inserted one to the pre-existing one so as to maintain the old function, that is memorizing in parallel the input signal coming from the normal functional path or receiving and memorizing the serial inputs coming from an external path. Each scanning cell (latch) consists of a master part and of a slave part, which, on the occasion of opposite phases of a scanning clock that controls the scanning signal, memorize the new data presented at input and present the previously stored data at output, respectively. In known devices, for example as disclosed in GB-A-2 193 330, the two phases of the scanning clock are generated at the input of the integrated circuit and made to propagate through it until they reach the cells concerned. This involves the risk of a superimposition of the two phases, especially in the case of very long paths, with the consequent possible charging of the master part with the slave part still open and the equally consequent possible immediate loss of the new content introduced into the master part. US-A-4 855 669 discloses a system for scan testing of logic circuit networks in which master and slave clocks are derived from a single clock by local means. US-A-4 799 004 discloses a transfer circuit for operation test of LSI systems. The main object of the present invention is that of providing an improved operational analysis device of the scan path type for an integrated circuit. According to the invention such objects are attained with a device as defined in claim 1. In this way the scanning clock signal can propagate itself through the integrated circuit in a single phase while the other phase is generated only locally, avoiding problems of partial superimposition of the two phases such as would occur in the case of the propagation of both phases over long paths. Since, on the other hand, we are dealing with structures locally concentrated round the individual functional blocks, the local generation of the second phase clearly presents no difficulty of any kind. As far as the problem of the reciprocal insulation of the integrated circuit's different functional blocks is concerned, this is solved according to the invention by hooking the scanning clocks of all the stimulation and assessment cells to respective phases of a machine clock and by disabling the operational clock of a given functional block under analysis for the phases required for updating the stimulation cells and the assessment cells of that functional block, during the stimulation phase and during the assessment phases, respectively, of the same block. On the other hand, while still operating in two different phases of the machine clock, the device according to the invention is capable of having a single output phase. The features of the present invention shall be better understood by means of the following detailed description of an embodiment illustrated as a non-limiting example in the enclosed drawings, wherein: Fig. 1 shows the general diagram of a functional block of an integrated circuit provided with chains of stimulation and assessment scanning cells for the execution of operational analyses of the scan path type according to the present invention; Fig. 2 shows the detailed diagram of a scanning cell included in the device according to the invention; Fig. 3 shows the block diagram of an output circuit suitable for combining in a single phase the two output phases of the scan path analysis executed according to the invention; Fig. 4 shows the logic diagram of clock generation means to obtain locally from the scanning clock the second phase necessary for the master clocks of the individual scanning cells; Fig. 5 shows the logic diagram of clock generation means to obtain an operational clock for an individual functional block from a machine clock available inside the integrated circuit of Fig. 1; Fig. 6 shows in superimposed graph forms the waveforms of the signals related to the analysis of the pre-charging circuit of Fig. 1; Fig. 7 shows in superimposed graph forms the waveforms of the signals related to the analysis of the assessment cells of the circuit of Fig. 1. There is indicated in Fig. 1 with 1 a functional block of an integrated circuit, such as a PLA (Programmable logic Array), a ROM memory (Read-Only Memory) or more in general a static logic block. With the functional block 1 there are associated a chain of input scanning cells 2 and a chain of output scanning cells 3, which under normal operating conditions receive logic signals at input I to be inserted as data Di in the functional block 1 in the stimulation phase, and deliver output signals U corresponding to the logic data Du read in the same functional block in the assessment phase, respectively, all under the control of high and low phases CKPH and CKPL, respectively, of an operational clock CKP obtained from a machine clock CK in turn obtained from an external system clock XTALIN, more of which later. But during the scan path analysis the operational clock is clamped as described below and the scanning cell 2 and 3 receive respective alternated phases SINH and SINL of a serial scanning signal SIN under the control of a scanning clock SCK and of respective phases SCKH and SCKL alternating with those of the scanning clock. The state of the scanning cell chains 2 and 3, indicative of the operational situation of the functional block 1, is provided at output from the above mentioned chains in the form of the signals SOUTH and SOUTL, respectively. Each scanning cell 2 or 3 is preferably made as illustrated in Fig. 2, that is, it comprises essentially a master part M and a slave part S which have the task of storing the new data at arrival and making available at output the data previously stored. The master part M comprises an inverter 4 and two resistive transistors 5 and 6 of the CMOS type, with a P channel and an N channel, respectively, having the drain-source junctions placed in series between a positive power supply and the inverter's input and between the latter and ground, respectively, and the gates connected in common to the output of the inverter 4. According as to whether it is a cell 2 or a cell 3, there is applied to its input during normal operation the signal I or a data Du, that is controlled by a transistor 7 driven by the operational clock CKP obtained from the machine clock CK and determines at output, a corresponding data Di or the output signal U, respectively. During the scan path, on the other hand, said master parts M receive under the control of a transistor 8 driven by the respective alternating phases SCKH and SCKL of the scanning clock SCK respective scanning signals SINH and SINL which determine their switching to a different operational state, representative of the received signal. The slave part S comprises in turn an inverter 9 with input connected to the output of the inverter 4 of the master part M through a transistor 50 driven by the scanning clock SCK and two resistive transistors 10 and 11 of the CMOS type, with a P channel and an N channel, respectively, having the drain-source junctions placed in series between a positive power supply and the inverter's input and between the latter and ground, respectively, and the gates connected in common to the output of the inverter 9. The slave part S operates only during the scan path and, under the control of the scanning clock SCK, operates so as to acquire the operational state of the corresponding master part M, which is thus put into the position of receiving and memorizing a new operational state determined by the subsequent scanning signal SINH or SINL. The operational state of the slave part S is provided at output as a signal SOUTH or SOUTL, for the cells 2 and 3, respectively. The output signals SOUTH and SOUTL of the two cell chains 2 and 3 are received by a single serial output circuit illustrated in Fig. 3 and comprising two slave cells S2 and S3 coincident with the slave parts of the last scanning cell of the two chains 2 and 3 and a further additional slave cell S4. The slave cell S2 receives the high-phase output signal SOUTH through the series of two transistors 12 and 13 respectively controlled by the signals SCK and CK, while the slave cell S3 receives the low-phase output signal SOUTL through the series of two transistors 14 and 15 controlled by the signals SCK and CK, respectively. Both the cells S2 and S3 are also subjected to the control of respective reset transistors 16 and 17. The slave cell S4 receives the output signal of the cell S2 through a transistor 18 controlled by the signal SCKH and its output is fed back to the input of the cell S3 through the series of an inverter 19 and of a transistor 20 controlled by the signal CK, in the same way that the output of the cell S3 is fed back to the input of the cell S3 through a transistor 21 controlled by the signal CK. The outputs of the cells S4 and S3 are lastly applied at an input of respective logic gates 22 and 23 of the NAND type, to the other input of which the signal CK is respectively present. The outputs of the logic gates 22 and 23 are lastly applied at respective inputs of a logic gate 24 of the NAND type, which gives a serial signal STOUT at output on a single phase of the machine clock CK. Fig. 4 shows the logic diagram of the clock generation means that are used to obtain locally from the scanning clock SCK the second phase SCKH, SCKL necessary for the master clocks of the individual scanning cells. Such clock generation means comprise two logic gates 25 and 26 of the NOR type, at an input of which the signal SCK is applied while at the other input a signal DISLO or DISHO, respectively, is applied, indicative of the low-phase or high-phase situation, respectively, of the machine clock signal CK. The outputs of the logic gates 25 and 26 provide the signals SCKL and SCKH, respectively. Fig. 5 shows in turn the means used to generate and possibly to disable the operational clock CKP starting from the machine clock signal. Such means comprise logic gates 27 and 28 of the NAND type, to an input of which, through an inverter 29 and in a direct manner, respectively, the machine clock signal CK is applied, while at the other input a signal DISLO or DISHO, respectively, is applied. The outputs of the logic gates 27 and 28 provide the operational clock signals CKPL and CKPH through respective inverters 30 and 31. The manner of operation of the device according to the invention can be deduced through the examination of the graphs of Fig.s 6 and 7, that refer to the high and low phases, respectively, of the machine clock CK. More accurately, it can be deduced from Fig. 6 that in the presence of a system clock signal XTALIN the start of a scan path operation on a high phase determines the rise of a signal ENSH1 which clamps the machine clock CK on its high phase. A suitable circuit not shown and known in itself enabled by the signal ENSH generates the scanning clock signal SCK, whose first leading edge determines the activation of the signal DISHO, which through the logic circuits of Fig.s 4 and 5 activates the scanning clock phase SCKH and clamps the operational clock CKP so as to allow the access of the serial scanning signal SINH to the various cells 2 of the high phase; more in general it disables the parallel access from the normal functional path so as not to have any conflict between the parallel data and the serial data. SCKH alternates with SCK to introduce into the master part M of the high-phase cells new serial data after SCK has caused the transfer of the previous data to the slave part S of the same cells, making them available at the corresponding outputs SOUTH. The output SOUTH of each individual cell of the chain 2 constitutes the input signal for the subsequent cell of the same chain, which is thus scanned in succession by the scanning signal SIN at the rate imparted by SCK and SCKH, while the machine clock remains clamped in the high phase. It is, on the other hand, deduced from Fig. 7 that the scanning of the chain of cells of assessment or low phase 3 is also executed in the same manner, with the only difference that the signal ENSH this time clamps the machine clock CK in the low phase and SCK's first leading edge determines the activation of DISLO, which through the logic circuits of Fig.s 4 and 5 activates the opposite scanning clock phase SCKL and clamps the operational clock CKP so as to allow the access of the serial scanning signal SINL to the various cells 3 of the low or assessment phase. SCKL alternates with SCK to introduce into the master part M of the high-phase cells new serial data after SCK has caused the transfer of the previous data in the slave part S of the same cells, making them available at the corresponding outputs SOUTL. The output SOUTH of each individual cell of the chain 2 constitutes the input signal for the subsequent cell of the same chain, which is thus scanned in succession by the scanning signal SIN at the rate imparted by SCK and SCKH, while the machine clock remains clamped in the low phase. Thus globally for each functional block that is to be identified and stimulated through a direct and external access care shall have to be taken to connect serially all the output signals SOUTH related to the scanning cells that operate on the high clock and all the output signals SOUTL related to the scanning cells that operate on the low clock. More in general there can be groups of scanning cells with an output SOUTH connected as input or output cells of a functional block, and vice versa for SOUTL. It can thus occur that a serial string SOUTH or SOUTL refers partly to inputs and partly to outputs. As already said earlier, the output signals SOUTH and SOUTL related to the high-phase and low-phase scanning cells are collected by the output circuit of Fig. 3, which through the intervention of the slave cell S4 transfers the high-phase outputs to the low phase and thus allows an output signal STOUT to be collected on a single phase CK and thus on a single output pin of the integrated circuit.

Claims: Operational analysis device of the scan path type for an integrated circuit, comprising a first chain of stimulation scanning cells (2) located at a stimulation input of each respective functional block (1) of the integrated circuit and a second chain of assessment scanning cells (3) located at an assessment output of each respective functional block (1) of the integrated circuit, said first and second chains of scanning cells (2, 3) being controlled by a scanning clock signal (SCK) having a substantially square wave to provide respective output signals (SOUTH, SOUTL) on opposite phases of said scanning clock signal (SCK), each cell (2, 3) comprising a master part (M), a slave part (S) and switching means (8, 50) to alternately enable said master (M) and slave (S) parts under the control of respective master and slave clock signals coincident with opposite phases of said scanning clock signal (SCK), characterized in that it comprises clock generation means (25, 26) associated with said first and second chains of scanning cells (2, 3) to locally derive said master and slave clock signals from said scanning clock signal (SCK), and phase shifting means (S2-S4, 12-24) operating on the output signals (SOUTH, SOUTL) of said first and second chains of scanning cells (2, 3) to shift the phase of one of said output signals to coincide with the phase of the other output signal to obtain a single-phase output signal (STOUT). Device according to claim 1, characterized in that said scanning clock signal (SCK) is hooked to a phase of a machine clock (CK) from which an operation clock signal (CKP) for analysis of each functional block (1) is derived, there being provided clock disabling means (27-31) for disabling the operation clock signal (CKP) of a given functional block (1) during stimulation and assessment phases, respectively, of said block (1).

Inventors: GAIBOTTI MAURIZIO, SCARRA FLAVIO, GAIBOTTI, MAURIZIO, SCARRA', FLAVIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1991
Main classification: G06F11
IPC codes: G06F15, H01L21, G01R31, G06F11, H01L27