Abstract:
An integrated circuit which allows for ease of testing thereof, and a method of such testing. The integrated circuit includes a linear shift register without feedback, which converts signals on a bus into a serial bitstream. The integrated circuit has an output pin connected to the linear shift register to receive the serial bitstream thereon. The method of testing the chip includes supplying test inputs to the chip, reading the serial bitstream data during the test from an output pin of the integrated circuit, and comparing the output data in a test device with an expected bitstream. The signals on the bus under investigation can be monitored continually throughout the test. In the event that a fault is discovered, the test can be terminated much more quickly than when using a device which requires the test to be completed before any data can be read out.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a method of testing an integrated circuit, in particular an integrated circuit having an internal bus which is not observable on the pins of the circuit. An example of this is a chip with a Harvard architecture. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of ASICs (Application-Specific Integrated Circuits), it is particularly advantageous to be able to test the circuits before they are mounted in a product, because of the extra costs which arise if a circuit is found at that late stage to be faulty. 
     A particular problem in testing ASICs is that it is often desirable to monitor signals on an internal bus as part of the test. However, there may be no way of monitoring signals which are carried on this bus. This is of particular concern when the system has an internal processor, and it is desired to monitor signals passed between the processor and memories or other circuit blocks. For example, in the case of a Harvard architecture chip, in which the central processor has an address bus and a data bus for the program, and an address bus and a data bus for data, the program buses are typically connected to an external memory, and used for reading instructions to the processor. Since this bus goes to an external device, it is possible to monitor signals which are passed on the bus. However, the data buses typically stay on-chip, which means that there will be no way to monitor the activity on these buses, unless special measures are taken. 
     JP-63-133234 discloses a microcomputer, having a data bus and an address bus, provided with an on-chip device for carrying out a test. 
     In particular, this device includes a CRC arithmetic register, which may typically be in the form of a linear feedback shift register, which is set into an initial operational state, before the test is carried out. During the test, data on the bus which is studied are used to calculate a checksum, which after the test, can be read out. 
     The use of a CRC-register in the form of a shift resister with feedback can cause problems in the event that an “undefined” signal enters the shift register. The feedback means that, eventually, undefined bits can fill the register. 
     Moreover, the process of storing the data during the complete test, and subsequently reading the data, means that no error can be detected until the test is complete. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, an integrated circuit includes a linear shift register without feedback, which converts signals on a bus into a serial bitstream. The integrated circuit has an output pin connected to the linear shift register to receive the serial bitstream thereon. 
     This has the advantage that the signals on the bus under investigation can be monitored continually throughout the test. In the event that a fault is discovered, the test can be terminated much more quickly than when using a device which requires the test to be completed before any data can be read out. 
     Moreover, the absence of feedback means that the presence of an undefined bit in the register will persist only for a limited number of clock cycles. 
     In accordance with a second aspect of the invention, there is disclosed a method of testing an integrated circuit, in which signals on a bus are converted into a bitstream, and read during the test from an output pin of the integrated circuit, and are then compared in a test device with an expected bitstream. 
     Again, this has the advantage that, as soon as a device is determined to be faulty, the test can be terminated without wasting further time. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block schematic diagram of the relevant parts of an integrated circuit in accordance with the invention. 
     FIG. 2 is a block diagram of a shift register forming part of the integrated circuit of FIG.  1 . 
     FIG. 3 shows an alternative form of the shift register shown in the integrated circuit of FIG.  1 . 
     FIG. 4 shows a further alternative form of the shift register forming part of the integrated circuit shown in FIG.  1 . 
     FIG. 5 is a block schematic diagram of the relevant parts of an integrated circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a block schematic representation of an ASIC  2 , having an embedded processor. It will be appreciated that the circuit may have a relatively complex design; however, for ease of explanation, only the relevant parts of the circuit are shown in FIG.  1 . In particular, FIG. 1 shows the important elements of a Harvard architecture circuit, having a processor, or CPU,  4 , which has two pairs of address and data buses, namely a pair of program buses and a pair of data buses. Each pair of buses includes an address bus and a data bus. 
     In this case, the program bus  6  is connected to input/output pins of the integrated-circuit  2 , for connection to an external memory device (not shown), for reading instructions to the processor  4 . 
     The data bus  8  is connected to other core logic functions  10  of the circuit, such as memories or other circuit blocks. 
     However, the bus is “embedded”, that is, not connected to external pins of the integrated circuit. In the absence of special measures, signals on the data bus  8  cannot be monitored, and this is an important aspect of any testing procedure. 
     In accordance with the preferred embodiment of the invention, therefore, the data bus  8  is provided with a series of connections to a shift register  12 . As will be described in more detail below, with reference to FIG. 2, each line of the data bus  8  is provided with a separate connection to a respective flip-flop of the shift register. As is well known, the shift register can convert the parallel data, in which bits on each line of the bus  8  are presented simultaneously, to serial data in the form of a bitstream. The output data are supplied to an output pin  14  of the integrated circuit  2 , where they can be monitored by an external test device. 
     FIG. 2 shows in more detail the structure of the shift register  12 . The shift register  12  has a plurality (N) of input lines  16 . 0 ,  16 . 1 , . . . ,  16 .N- 1 , each connected to a respective line of the data bus  8 , and receiving at any time a respective data bit d 0 , d 1 , . . . , dN- 1  thereon. Each input line  16 . 0 ,  16 . 1 , . . . ,  16 .N- 1  is associated with a respective flip-flop  18 . 0 ,  18 . 1 , . . . ,  18 .N- 1 . Each input data line  16 . 0 ,  16 . 1 , . . . ,  16 .N- 1  is connected to one input of a respective single bit adder, which may be in the form of a XOR gate,  20 . 0 ,  20 . 1 , . . . ,  20 .N- 1 . The second input of the first adder  20 . 0  receives an arbitrary known bit, and the second inputs of the other adders  20 . 1 , . . . ,  20 .N- 1  come from the outputs of the respectively immediately preceding flip-flops  18 . 0 , . . . ,  18 .N- 2 . Each flip-flop  18 . 0 ,  18 . 1 , . . .  18 .N- 1  receives the output of the respective adder  20 . 0 ,  20 . 1 , . . . ,  20 .N- 1  as its input. 
     The output from the final flip-flop  18 .N- 1  is connected to the output pin  14  of the ASIC. The clock inputs of the flip-flops are clocked simultaneously at the data rate on the data bus  8 . 
     In operation of the circuit  2  in a test process, a known test pattern of data is supplied to the device, and the signals appearing on the data bus  8  are monitored via the bitstream appearing at the output pin  14 . This bitstream is then compared with a reference bitstream, which is what is expected to be obtained from a correctly functioning device. 
     Moreover, the test will still work if some bits on the data bus are unknown. 
     FIG. 3 shows an alternative form of the shift register, in which new data is added only when the input is valid on the bus. This is achieved by multiplexing the flip-flops, or using mulitplexers in front of the flip-flops. 
     The shift register  32  has a plurality (N) of input lines  36 . 0 ,  36 . 1 , . . . ,  36 .N- 1 , each connected to a respective line of the data bus  8 , and receiving at any time a respective data bit d 0 , d 1 , . . . , dN- 1  thereon. Each input line  36 . 0 ,  36 . 1 , . . . ,  36 .N- 1  is associated with a respective flip-flop  38 . 0 ,  38 . 1 , . . . ,  38 .N- 1 . Each input data line  36 . 0 ,  36 . 1 , . . . ,  36 .N- 1  is connected to one input of a respective single bit adder, which may be in the form of a XOR gate,  40 . 0 ,  40 . 1 , . . . ,  40 .N- 1 . The second input of the first adder  40 . 0  receives an arbitrary known bit, and the, second inputs of the other adders  40 . 1 , . . . ,  40 .N- 1  come from the outputs of the respectively immediately preceding flip-flops  38 . 0 , . . . ,  38 .N- 2 . The first flip-flop  38 . 0  receives the same arbitrary bit as its A input. 
     Each subsequent flip-flop  38 . 1 , . . . ,  38 .N- 1  receives the output of the respectively immediately preceding flip-flop as its A input. Each flip-flop  38 . 0 ,  38 . 1 , . . . ,  38 .N- 1  receives the output of the respective adder  40 . 0 ,  40 . 1 , . . . ,  40 .N- 1  as its B input. 
     The output from the final flip-flop  18 .N- 1  is connected to the output pin  14  of the ASIC. The clock inputs of the flip-flops are clocked simultaneously at the data rate on the data bus  8 . An enable signal on line  42  is used to select whether or not information on the bus should be added into the shift register. 
     FIG. 4 shows a further alternative form of the shift register  12 , in which new data is added only when the input is valid on the bus, and this is achieved by gating the input with a logic gate. 
     The shift register  52  has a plurality (N) of input lines  56 . 0 ,  56 . 1 , . . . ,  56 .N- 1 , each connected to a respective line of the data bus  8 , and receiving at any time a respective data bit d 0 , d 1 , . . . , dN- 1  thereon. Each input line  56 . 0 ,  56 . 1 , . . . ,  56 .N- 1  is associated with a respective flip-flop  58 . 0 ,  58 . 1 , . . . ,  58 .N- 1 . Each input data line  56 . 0 ,  56 . 1 , . . . ,  56 .N- 1  is connected to a first input of a respective logic gate  64 . 0 ,  64 . 1 , . . . ,  64 .N- 1 , the outputs of which are respectively connected to one input of a respective single bit adder, which may be in the form of a XOR gate,  60 . 0 ,  60 . 1 , . . . ,  60 .N- 1 . The second input of the first adder  60 . 0  receives an arbitrary bit, and the second inputs of the other adders  60 . 1 , . . . ,  60 .N- 1  come from the outputs of the respectively immediately preceding flip-flops  58 . 0 , . . . ,  58 .N- 2 . 
     The output from the final flip-flop  18 .N- 1  is connected to the output pin  14  of the ASIC. The clock inputs of the flip-flops are clocked simultaneously at the data rate on the data bus  8 . The second inputs of the logic gates  64 . 0 ,  64 . 1 , . . . ,  64 .N- 1  receive an enable signal on a line  62 , which determines whether data bits are added into the shift register. The logic gates may be AND gates, which are activated by a high signal on line  62 , or OR gates, which are controlled by a low signal on line  62 . 
     Thus, the embodiments of FIGS. 3 and 4 ensure that data, from the data bus, are only added to the shift registers when they are expected to be of interest. Although the invention has been described above with reference to a Harvard architecture device, it should be noted that a von Neumann architecture processor  74 , in which a common bus  76  is used for both program  80  and data  82 , having internal buses  78  which are not observable on the circuit pins, can also be tested in the same way, by providing the same on-chip shift register  12 . 
     It will therefore be appreciated that, in the preferred embodiment, the use of a linear shift register, without feedback, means that the output test data can be examined immediately, with the result that, if any error is detected, the test can be regarded as terminated, which means that faulty chips can be detected more efficiently.