A test access port (TAP) management method and system

Abstract: The Test Access Port (TAP) functions of a plurality of components arranged on a single chip (10) are managed by selectively driving the TAP function (20, 30, 40) of each of the components with respective clocks (TCK, DCK), whilst the further signals for driving the TAP function (TDI, TDO, TMS, NTRST) are used in shared mode among the various components. Preferably, associated to the aforesaid clocks is a pull-down function for selectively blanking the respective clocks in conditions of non-use. In a preferred way, the aforesaid dedicated clocks (DCK, TCK) are generated on board the chip (10).

Description: Field of the invention The present invention relates to management of Test Access Port (TAP) functions and has been developed with particular attention paid to the possible application to systems on a single chip (systems on a chip - SOCs), in particular in compliance with the JTAG/IEEE 1149.1 Standard. Description of the prior art The test standard JTAG/IEEE 1149.1 has been developed and has met with considerable success as regards the need to overcome the problems that are likely to arise, for example, in surface-mounted packages, in boards with components mounted on both faces (the so-called "double-sided boards), in multichip modules and, in general, in all the situations in which problems of gaining access to given signals may arise. In particular, the standard in question makes it possible to carry out testing on circuits which otherwise could not be tested, a fact that implies the possibility of cutting down considerably on times and costs for developing and commercializing a product. The standard in question enables a TAP configuration to be obtained based upon an architecture currently known as boundary scan. This solution is based upon the possibility of providing, in the context of an integrated circuit, circuit elements that are able to perform functions of testing, maintenance and support of the circuit, even after the latter has been mounted, for instance, on a board. The circuitry in question usually comprises a standard interface through which the test instructions and data are communicated. A set of testing characteristics are defined, including a so-called "boundary-scan register", in such a way that the component may be able to respond to a minimum set of instructions defined so as to enable testing of the circuit. In particular, the IEEE 1149.1 Standard defines the architecture of the TAP and of the corresponding circuits (which are essentially based upon a shift register) comprised in a device of the boundary-scan type. Each signal pin is connected to a cell of the shift register. The cells in question are connected in a path which may be likened to a shift register set along the periphery or boundary (hence, the term "boundary scan") of the device, so as to supply a virtual access to the signal pins. Using the virtual access provided by the boundary-scan architecture, it is possible to provide a testing function, even in the framework of rather complex circuits, in conditions where physical access is not feasible. The main advantage of the boundary-scan technique lies in the possibility of observing data on the inputs of the device, as well as control data on the device outputs, all of which is obtained regardless of the logic of the system implemented on the chip. In addition, the boundary-scan technique provides a better diagnostic function than those offered by traditional testing techniques. Usually, a testing function of a traditional type envisages, in fact, application of given signal configurations to the input pins, and then observation of the response given by the circuit on the output pins. If there is a defect present on one of the input pins, traditional testing techniques are able to detect the existence of this defect, but in general call for a rather complex investigation to identify which of the input pins is the defective one. Instead, the cells of a boundary-scan scheme observe the response of the circuit by monitoring the input pins, so that a test carried out using the boundary-scan technique makes it possible to determine very easily, for example, which input pin is not making contact with the circuit. Furthermore, this can be achieved even without the need for making a physical contact with the pin in question. At least in principle, by making a set based totally on components operating according to a boundary-scan scheme, the shift-register paths of all the devices may be connected together to form a single path. In this way, it is possible to check and monitor the behaviour of all the pins and of all the interconnections of the device simply from an edge connector. Object and summary of the present invention More specifically, the present invention relates to a process and a system according to the preambles of claims 1 and 5, which were patterned after WHETSEL L: "AN IEEE 1149.1 BASED TEST ACCESS ARCHITECTURE FOR ICS WITH EMBEDDED CORES" INTERNATIONAL TEST CONFERENCE,US,NEW YORK, NY: IEEE, vol. CONF. 28, 1 November 199, pages 69-78, ISBN: 0-7803-4210-0. Traditionally, in the case of a SOC system, the TAP function is used for two main purposes. In the first place, it is used to handle all the signals and the control and testing functions according to the JTAG/IEEE 1149.1 Standard. In the second place, it is used as external connection for the debugging system present on the chip. In the case of SOC systems in which a number of chips are integrated, there may arise the problem deriving from the fact that the corresponding TAP functions, one for testing and the other for debugging, must be made to operate on the same chip, using the same JTAG interface and hence preventing proliferation of the corresponding pins. The purpose of the present invention is to overcome the problem just referred to. According to the present invention, the above purpose is achieved thanks to a process and a system having the further characteristics specifically called for in the claims 1 and 5, respectively. Basically, the solution according to the invention envisages that, in the context of the various components comprised on the same chip and provided with respective TAPs, each of the TAP functions will present a dedicated clock signal (TCK, DCK, ..., NCK), whilst the other signals (TDI, TDO, TMS, NTRST) are shared. The need to drive the corresponding input without infringing the IEEE Standard referred to above, and at the same time the need to use one TAP function for testing and the other for debugging without any mutual interference is preferably solved by adding a pull-down module to the various dedicated clock inputs TCK, DCK, etc., so as to eliminate the problem of driving the input. Brief description of the annexed drawings The invention will now be described, purely by way of non-limiting example, with reference to the attached drawings, in which: Figure 1 illustrates, in the form of a block diagram, the architecture of a system operating according to the invention; and Figure 2 illustrates, again in the form of a block diagram, some constructional details of the system architecture of Figure 1. Detailed description of an exemplary embodiment of the invention The block diagram of Figure 1 is a general illustration of the organization of the TAP function (TAP controller) in a SOC system comprising in general a plurality of individual chips, each of which is in turn provided with a respective TAP function with respective debugging connections. Just to clarify the concept (but without any implication that this is to be interpreted in any way as a limit to the extent of the invention), one of the possible applications of the idea made by the present applicant envisages the association, within a single SOC system, designated as a whole by 10, of two chips consisting of the chip sold by the present applicant under the commercial name ST20 and of the chip sold by the company Hitachi under the commercial name SuperH40. The respective TAP modules are designated by the reference numbers 20 and 30, these designations being limited to the debugging connections. The reference number 40, instead, designates the set of JTAG pins. It will moreover be appreciated that the solution according to the invention is of general application in a configuration comprising any number of chips. Basically, the solution according to the invention envisages, for controlling operation of each TAP function, the use of a dedicated clock (TCK, DCK, ..., NCK), whilst the other signals (TDI, TDO, TMI, NTRST) are shared. The table below shows a possible arrangement. SIGNAL USED FOR: TDI TAP + UDI TDO TAP + UDI TCK Only for TAP NTRST TAP + UDI TMS TAP + UDI DCK Only for UDI In particular, the table shows that the signals TDI, TDO, NTRST, and TMS (persons skilled in the sector are familiar with these acronyms and what they stand for, and hence no detailed explanation is here required) may be used for both of the TAP functions, i.e., both for the function of the chip ST20, designated simply as TAP, and for the homologous function of the chip SuperH40, generally designated as UDI. The clock signal TCK is instead used only for the TAP function of the chip ST20, whilst the signal DCK is used only for the UDI function of the chip SuperH40. The various signals TDI, TDO, NTRST, and TMS may then be shared, in particular as regards the signals that are to be forwarded, through lines designated as a whole by 55, to the individual debugging connections. The need for selective application of the clock signal TCK or DCK is instead met by connecting the respective generator blocks (designated, as are the clock signals themselves, by DCK and TCK in the diagram of Figure 2) to a common line 50 which distributes the clock signals to the various modules 20, 30, etc., associating to each clock a corresponding pull-down function. The logic signal present on the line PD1, PD2 acts, via a respective inverter 60, which also has the function of impedance separator, so as to isolate or apply the corresponding clock on the line 50. In situations like the one dealt with in the case illustrated, the clock signals involved are basically two, namely, the signal TCK and the signal DCK. It will moreover be appreciated that the diagram of Figure 1 applies in general to any number n of clock signals, and therefore bears the reference nDCK. Thus if, for example, the TAP function 20 of the chip ST20 is to be used, the debug clock (DCK) is not used, which in fact assumes the value 0. The TAP function of the Hitachi chip (UDI) is stuck in the Test Logic Reset state since the transition of the state takes place on the rising edge of the clock. The solution according to the invention proves easy to implement and does not require any change at the level of IP cells. Furthermore, no circuitry complexities arise at the pad level, nor are there any appreciable differences between, for instance, pre-production specimens and full-production specimens. From the diagram of Figure 1 it will moreover be appreciated that, even in the case where a certain number of different chips is present, no pull-up or pull-down modules are required at a board level. The solution according to the invention is compatible with low-cost emulation schemes, in particular with emulation schemes implemented through clock control with the use of a JTAG controller in a scanning environment. Furthermore, the solution according to the invention is altogether compatible with internal clock generators, and hence generators on board the circuit, and is moreover transparent in regard to the specific solutions adopted for the debugging function, possibly in combination with pipeline-type architectures. Of course, without prejudice to the principle of the invention, the details of construction and the embodiments may vary widely with respect to what is described and illustrated herein, without thereby departing from the scope of the present invention as defined in the annexed claims.

Claims: A process for management of the Test Access Ports, TAP, in a plurality of components arranged on a single chip (10), each of said components being provided with a respective TAP function (20, 30, 40) adapted to be driven by a respective clock signal (TCK, DCK) and by at least one further signal (TDI, TDO, TMS, NTRST), the process comprising the operations of: using said at least one further signal (TDI, TDO, TMS, NTRST) in a shared way between the TAP functions of the components of said plurality; and selectively driving the TAP functions of the components of said plurality with respective clock signals (TCK, DCK); characterized in that it comprises the operations of: generating respective clock signals (DCK, TCK) for the TAP functions of the components of said plurality; providing a common line (50) for application of said respective clock signals (TCK, DCK) to the TAP functions (20, 30, 40) of the components of said plurality; and applying said respective clock signals (DCK, TCK) to said common line (50) via pull-down modules isolating or applying the corresponding clock signal on said common line (50). The process according to Claim 1, characterized in that it comprises the operation of generating said respective clock signals (TCK, DCK) on board said single chip (10). A system for management of the Test Access Ports, TAP, in a plurality of components arranged on a single chip (10), each of said components being provided with a respective TAP function (20, 30, 40) adapted to be driven by a respective clock signal (TCK, DCK) and by at least one further signal (TDI, TDO, TMS, NTRST), the system comprising at least one line (55) for use of said at least one further signal (TDI, TDO, TMS, NTRST) in a shared way between the TAP functions of the components of said plurality; characterized in that it comprises: generators of respective clock signals (TCK, DCK) for selectively driving the TAP functions of said plurality of components; a common line (50) for the application of said respective clock signals (TCK, DCK) to the TAP functions (20, 30, 40) of the components of said plurality; and pull-down modules (PD1, PD2, 60) to isolate or apply a respective clock signals (DCK, TCK) on said common line (50). The system according to Claim 3, characterized in that said clock signal (TCK, DCK) generators are arranged on said single chip (10).

Inventors: LA SCALA AMEDEO, LA SCALA, AMEDEO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2001
Main classification: G01R31
IPC codes: G01R31