Circuit for the Generation of a Scanning Clock in an Operational Anaylsis Device of the Serial Type for an Integrated Circuit

1. Technical Field

The present invention relates to a circuit for the generation of a scanning clock in an operational analysis device of the serial type for an integrated circuit, otherwise known as a scan path.

2. Background of the Invention

As described in the Italian patent application No. 21820 A/90 dated 22 Oct. 1990 filed in the name of the same applicant, the so-called scan path is a type of analysis that is widely used in checking whether one or more functional blocks of an integrated circuit, such as, say, a microprocessor, operate correctly.

It is based on the use of scanning signals of the serial type which act on chains of scanning cells (latches) located at the input and at the output of individual functional blocks with a function of stimulation and assessment, respectively, of the operational state of the blocks themselves.

In this way it is possible to progressively update the operational state of the functional block under examination and to obtain correspondingly at output a signal that is indicative of the different states assumed by the same functional block at each inquiry of the serial signal at input.

The most natural way to obtain these scanning signals is that of supplying them from the outside on additional pins of the integrated circuit to be analyzed.

This requires an increase in the normal number of pins of the integrated circuit with consequent greater structural complexity, increase in cost and departure from the concept of product standardization.

The object of the present invention is to accomplish a circuit that allows such scanning signals to be obtained without adding further pins beyond those already normally provided for the operation of the integrated circuit.

These and other objects are accomplished according to the invention by a circuit that comprises first switching means which receive at input a system clock normally provided for the operation of the integrated circuit and produce at output a machine clock normally coincident with said system clock, means for clamping said first switching means which after a firing signal of the serial analysis determines the clamping of the state of the machine clock and second switching means which receives at an input said system clock and is fired by said firing signal to produce a scanning clock which repeats the system clock in an inverted or non-inverted manner according to the state in which the machine clock has been clamped.

In this way no additional pin is required and the scanning clock can simply be obtained from the system clock already in the integrated circuit.

According to a possible embodiment of the present invention, said first switching means comprises a first and a second amplifier stage of the system clock at input, each followed by a respective inverting stage, and said clamping means is constituted by a switching stage operated by the trailing edge of said firing signal of the serial analysis to evolve from a closed position suitable for allowing the passage through it of the system clock for the generation of the machine clock and an open position in which said clamping of the state of the machine clock takes place.

In addition, between the output of said switching stage and the input of said second amplifier stage there is a feedback stage suitable for preventing the production of spurious signals during the switching operation of said switching stage to the open position.

Lastly, between the output of the second amplifier stage and the input of said switching stage there is a latch circuit which during the switching operation of said switching stage to the open position prevents the second amplifier stage from going into oscillation giving rise to energy losses.

The features of the present invention shall be made more evident by an embodiment illustrated as a nonlimiting example in the enclosed drawings.

FIG. 1 is an example of an embodiment of the circuit according to the invention;

FIGS. 2 and 3 illustrate different waveforms of the signals through the circuit.

With reference to FIG. 1, the circuit comprises first switching means 11, which comprises in turn a first amplifier stage A1 constituted by a series of a p-channel transistor M1 and an n-channel transistor M2. The gates of the transistors M1 and M2 are connected together and supplied with an input signal XTALIN which is the system clock already normally provided for the operation of the integrated circuit. The source of the transistor M1 is at a voltage Vcc and the drain is connected to the drain of the transistor M2. The source of the transistor M2 is grounded.

The input of an inverter I1 is connected between the drain of the transistor M1 and the drain of the transistor M2. The output of the inverter I1 is connected to the gates of two p-channel and n-channel transistors M3, M4, respectively, forming part of a second amplifier stage A2 which, in series with them, comprises a p-channel transistor M5 and an n-channel transistor M6. The source of the transistor M3 is at a voltage Vcc and the drain is connected to the source of the transistor M5. The drain of the transistor M5 is connected to the drain of the transistor M6 and the source of the transistor M6 is connected to the drain of the transistor M4; in addition, the source of the transistor M4 is grounded. The gates of the transistors M5 and M6 are connected to the output of a feedback stage 12 whose objects shall be explained later.

Between the drain of the transistor M5 and the drain of the transistor M6 there is connected the input of a second inverter I2 whose output represents the input of a third inverter I3. The output of the third inverter I3 represents the input of a switching stage 13, which operates as a clamping means for the first switching means 11 and comprises an n-channel transistor M7 and a p-channel transistor M8 arranged in parallel to one another. The gate of the transistor M7 is controlled by a firing signal of the serial analysis, or scan path analysis, ENSH, while the gate of the transistor M8 is controlled by a signal ENSH' which is complementary to the signal ENSH.

The output of the switching stage M7, M8 represents the input of a third amplifier stage A3 at whose output the machine clock CK is taken. Said third amplifier stage A3 comprises the series of two n-channel and p-channel resistive transistors M9, M10, respectively, an inverter I4 and another series of two n-channel and p-channel resistive transistors M11, M12, respectively, connected in parallel with the transistors M9 and M10. The source of the transistor M10 is at a voltage Vcc and the drain of the same is connected to the drain of the transistor M9, whose source is grounded. The input of the inverter I4 is connected between the drain of the transistor M10 and the drain of the transistor M9. The output of the inverter I4 is connected to the interconnected gates of the transistors M10, M9, M12, M11. The source of the transistor M12 is at a voltage Vcc and the drain is connected to the drain of the transistor M11, whose source is grounded. An output U' of the first switching means 11, on which a machine clock CK is made available, is connected between the drain of the transistor M12 and the drain of the transistor M11.

To the output of the inverter I3 and to the terminal ENSH' there are connected as many inputs of a logic gate NAND N4 included in the feedback stage 12. The latter also comprises an inverter I5, whose input is connected to the output of the amplifier stage A3 and whose output is connected to an input of a logic gate NAND N1, whose output is connected to an input of a logic gate NAND N2, at whose other input there is connected the output of a series of inverters I6, I7 supplied at input with the machine clock CK. The other input of the logic gate N1 is connected to the output of the logic gate N2. The output of the logic gate NAND N2 represents one of the two inputs of a logic gate NAND N3 whose other input is supplied with the signal ENSH. The output of the logic gate N3 and the output of the logic gate N4 represent the inputs of a logic gate NAND N5. The output of the logic gate N5, which represents the output of the feedback stage 12, is connected to the gates of the transistors M5 and M6 present in the second amplifier stage A2.

A latch circuit L1, biased with a voltage Vcc, is connected to the input of the inverter I2.

The output of the inverter I1 is connected through two inverters I8, I9 to the input of a logic gate NAND N6. The other input of the logic gate N6 is supplied with a serial test setting signal TEST1.

The output of the logic gate N6 represents the input of second switching means 14, which comprise transistors M13, M14. The drains of the transistors M13, M14 are connected to the output of the logic gate N6 directly and through an inverter I10, respectively, while the sources are connected at a common node F to the input of an inverter I11, across whose output U" the scanning signal SCK is taken.

The gates of the transistors M13, M14 are operated by the outputs of logic NOR gates, NO1, NO2. The inputs of the logic gate NO1 are supplied with the machine clock CK and with the firing signal ENSH, respectively, while the inputs of the logic gate NO2 are supplied with the signal ENSH and, through an inverter I12, with the machine clock CK.

The drain of a pull-up transistor M15 is connected to the source of a transistor M14. The gate of the transistor M15 is operated by the signal ENSH and the source is connected to the voltage Vcc.

The drain of the resistive transistor M16 is connected to the input of the inverter I11. The source of the transistor M16 is connected to the voltage Vcc and the gate to the output U".

As is better illustrated in detail in FIGS. 2 and 3, with reference to the circuit arrangement described in detail above, as long as the firing signal of the serial analysis ENSH is at a logic level 1 the switching stage M7, M8 is in the closed position.

The input signal XTALIN, after passing through the first amplifier stage A1 and the subsequent inverter I1, the second amplifier stage A2 and the subsequent inverters I1, I3, can pass through the switching stage M7, M8 and then be sent through the amplifier stage A3 to the output U' of the first switching means 11 to be taken as a machine clock CK, which repeats the oscillations of the system clock XTALIN.

Let us now suppose that a serial test setting signal TEST1, suitable for preparing the circuit for the scan path operation, is produced across the corresponding input of the logic gate N6, so that at the output from the latter, up to now clamped at the logic level 1, there is an inverted signal XTALIN.

Let us also suppose that under this condition the firing signal of the serial analysis ENSH, normally at the logic level 1, is taken to the logic level 0, and consequently ENSH' is taken to the logic level 1. This causes the opening of the switching stage 13, which clamps the oscillation of the signal of the machine clock CK.

If, at that instant, the machine clock is in the condition illustrated in FIG. 2, that is, in the high position, or in the condition illustrated in FIG. 3, that is, in the low position, it shall be clamped in its respective state. Under this condition the latch circuit L1 prevents the production of oscillations of the amplifier A2 with consequent energy losses.

Simultaneously, through the feedback stage 12 the transistors M5 and M6 are switched off with the object of saving energy and of avoiding spurious signals.

With the firing signal ENSH of the serial analysis at a low level and the machine clock CK clamped either at a high level (FIG. 2) or at a low level (FIG. 3) the NOR gates, NO1 and NO2 of the second switching means 14 cause the transistors M13 and M14 to transfer to the output U", in an inverted or in a direct manner, the signal XTALIN at the output of the gate NAND N6.

In this way a scanning clock SCK shall be available at the output U" of the circuit, which, as highlighted by the corresponding curves of FIGS. 2 and 3, repeats with a given delay, the system clock XTALIN in a non-inverted or in an inverted manner, respectively.

It can be seen from FIGS. 2 and 3 that the scanning clock SCK is suitable for locking onto the first edge of the system clock XTALIN so that under all conditions the first edge is a leading edge. This depends on the fact that SCK, being destined to be used for the direct control of the passage of the data from the master part to the slave part of the scanning cells, as described in the previous Italian patent application No. 21820 A/90, during normal operation of the system is stably at the logic level 0.