# Patent

## Title
pBIST engine with distributed data logging

## Abstract

A programmable Built In Self Test (pBIST) system used to test embedded memories where the memories under test are incorporated in a plurality of sub chips not integrated with the pBIST module. A distributed Data Logger is incorporated into each sub chip, communicating with the pBIST over serial and a compressed parallel data paths.

## Background
TECHNICAL FIELD OF THE INVENTION
The technical field of this invention is high speed memory testing, and more particularly a built-in self-test (BIST) system for embedded memories.
BACKGROUND OF THE INVENTION
Testing fabricated integrated circuits to determine proper operation has always been a challenging task, particularly with regard to on-board memory functions. There are two major types of device malfunctions caused by design defects. A design defect arises when the integrated circuit was manufactured to a design specification that did not provide proper function for the intended use purpose. Such a defect affects any manufactured integrated circuit until the design defect is corrected. The integrated circuit manufacturer must detect and correct such defects before shipping large number of devices to customers to avoid a costly recall. In contrast to a design defect, a manufacturing defect involves some fault in the manufacture of the integrated circuit. A manufacturing defect will generally affect less than all parts manufactured. Such defects are corrected by identification and correction of the manufacturing fault.
Most integrated circuit manufacturers test integrated circuits for proper operation before shipment to customers. Increasing integrated circuit complexity makes this testing increasingly difficult. Rather than rely on increasingly expensive external testing devices, many manufacturers test integrated circuits using a built-in self-test (BIST). BIST uses circuits on the integrated circuit designed solely to test the integrated circuit. When triggered either automatically in circuit operation or by an external test device, the BIST circuits produce a set of test conditions run on the ordinary circuit hardware. Comparison of the state of the integrated circuit following test to an expected state indicates whether the integrated circuit passed. An example of such a test is writing to a read/write memory and recalling the data written. A match between the data written and the data read passes the test. BIST typically involves other more complex tests.
A subset of BIST is programmable built-in self test (pBIST) that uses a general purpose test engine programmed by a set of instructions. This set of test instructions is typically stored on the integrated circuit in a read only memory (ROM) and includes instructions particularly developed for that integrated circuit. pBIST enables re-use of hardware and test instructions to cover a family of similar but not identical integrated circuits.
U.S. Pat. No. 7,324,392 entitled ROM-Based Memory Testing includes a description of an exemplary set of instructions for use in a pBIST. This patent is incorporated by reference in its entirety.
In conventional VLSI systems memory testing is done in three steps. In the first step hardwired logic (often available through third-party vendors, examples are memBIST (MBIST) use algorithms developed before the device is committed to tape-out. Determining the detailed make-up of hardwired logic is not feasible at this time. It is impossible to predict the appropriate hardware circuits because the necessary information comes from process model drivers during the process qualification window. Secondly, conventional memory testing attempts to close testing gaps using CPU based techniques. These techniques have a number of limitations. A major limitation is the CPU interface with largely inaccessible memory functions. The inability to do back-to-back accesses to all memories is another severe limitation. Thirdly, during memory testing while the device is in wafer form direct memory access (DMA) external memory accesses cannot be accomplished at full processor speed. This may result in a significant number of failures not being observable.
SUMMARY OF THE INVENTION
An SOC (System On Chip) usually contains a plurality of sub chips performing embedded memory system testing and data logging functions.
This invention describes an embedded memory test system wherein a single pBIST engine is employed that is capable of asynchronously interfacing to a plurality of sub chips with a Distributed Data Logger (DDL) incorporated in each sub chip.
Memory test data is collected by each DDL and failures are detected locally by each DDL. Actual and expected memory data is compared and in case of a failure a failure signature is generated and communicated to the controlling pBIST.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of this invention are illustrated in the drawings, in which:
FIG. 1 is a block diagram of a programmable BIST (pBist) unit built into a CPU/memory function of the prior art);
FIG. 2 is a detailed block diagram of a pBist controller of the prior art;
FIG. 3 is a diagram illustrating a prior art example two-block memory addressed by three address components: column address; row address; and block address.
FIG. 4 shows the pBIST architecture with Distributed Data Logging;
FIG. 5 shows an example of the data compression;
FIG. 6 shows the serial bus timing diagram;
FIG. 7 illustrates the block diagram of the Distributed Data Logger.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
SRAM/memory structures of different devices vary by technology, design and implementation. To test memories effectively the address access pattern sequence of the memory testing algorithm should follow a particular pattern that sensitizes and tests the electrical structure within the memory.
In a simple memory structure the physical addresses and logical addresses are contiguous and are matched. Effective testing can be preformed with simple algorithms that linearly increment or decrement addresses. In these memories any possible address scrambling automatically matches the input to the output. Namely bit <0> of the input goes to bit <0> of the output and so on.
FIG. 1 illustrates a representative prior art integrated circuit (IC), a system-on-chip (SOC) device 100 that includes programmable built-in self-test (pBIST) 130.
The SOC device 100 includes multiple modules that can be highly complex to test. SOC 100 includes central processing unit (CPU) 110 and memories 111 and 112 through 119 coupled by bus 120. Other SOC devices may include multiple processors, complex assemblages of memory and cache subsystems, peripheral devices and interfaces, various types of memory storage such as random access memory (RAM), read only memory (ROM) and possibly various types of alterable memory or flash ROM.
The programmable built-in self-test unit pBIST 130 includes a pBIST controller 129, pBIST ROM 131, an ID Value interface 132, ID Compare unit 128 and external interface 133. pBIST controller 129 controls SOC tests in much the same fashion as CPU 110 controls the normal operation of integrated circuit 100. pBIST unit 130 is controlled by test instructions stored in pBIST ROM 131. pBIST unit 130 may couple to circuits outside integrated circuit 100 via external interface 133. Addresses enter and exit pBist unit 130 via Address I/O 134.
pBIST controller 129 selects a particular pBIST controller within a group of pBIST controllers by using a pBIST ID (identification) value interface 132. The pBIST ID value is typically a five-bit value that allows selection of up to thirty-one pBIST controllers.
FIG. 2 is a block diagram of functional units included in prior art pBist 130. pBIST unit 130 includes pBIST controller 129, registers 221 through 228, dual address register 230, match unit 232 and multiple input signature register (MISR) unit 233. Addr[15:0] I/O 134 allows for input or output of pBist addresses.
Configuration registers 221 through 228 are memory mapped within the address space of CPU 110. Thus CPU 110 can read from or write to any register 221 through 228 by a memory operation to the corresponding address. Configuration registers 221 through 228 control the configuration and the operational mode of pBIST unit 130. Data registers 222 store test data recalled from pBIST ROM 131. Program registers 223 store test program instructions recalled from pBIST ROM 131. Other registers 224 include miscellaneous general-purpose registers. Configuration registers 221 includes four additional registers algorithm register 225, upper RAM information (RINFOL) register 226, lower RAM information (RINFOU) register 227 and pBIST ID register 228 which will be more fully discussed below.
Algorithm register 225 is actually an algorithm mask register. Bit [0] of this register indicates whether the first algorithm stored in pBIST ROM 131 would be executed. Bit [1] indicates whether the second algorithm is executed and so on. A total of 32 algorithms stored in pBist ROM 131 can be controlled the 32-bit word width of algorithm register 225. For an algorithm to be executed, both the corresponding bit of algorithm register 225 and a valid bit in the previous algorithm header must be set.
RINFOL register 226 and RINFOU register 227 are group mask registers similar to algorithm register 225. RINFOL register 226 and RINFOU register 227 indicate whether a particular RAM group is tested. This capability is provided because not all algorithms can be run on all memories. For a particular RAM group to be tested the corresponding bit in RINFOL register 226 or RINFOU register 227 and the valid bit in the previous RAM group header must both be set. RINFOL register 226 indicates the validity of RAM groups 0 to 31 and RINFOU register 227 indicates the validity of RAM groups 32 to 63.
pBIST ID register 228 is a memory mapped register that is loaded with a pBIST ID at the beginning of a programming sequence to specify which of a multiple of pBIST controllers 129 is being programmed by an external tester or by the local CPU 110. Upon being reset, pBIST register assumes a value of 0x0000. Each pBIST controller 129 is assigned a unique ID value input via ID value interface 132 when the SOC integrated circuit is designed. This may be embodied by simply tying off the five-bit field to either a high or to a low reference voltage to form a five-bit ID value. Dual Address registers 230 are used in accessing memory, such as memories 111, 112, through 119.
FIG. 3 illustrates memory read portion of a prior art device included here as an example of how address scrambling may be used. In the example FIG. 3, the memory has N blocks, each block has M columns and each column has R rows. Output data from two memory blocks (block—0 300 and block—1 301_ are selected by multiplexers 302, 303 and 304. Each of the thirty-two vertical units of block—0 300 and block—1 301 contain thirty-two rows of data, each row containing four eight-bit bytes labeled bytes 0 through 3.
Row address <R-1:0> supplies the row address input of block—0 300 and block—1 301 and selects one row out of rows 2R−1 to 0 in each block. Data from column 2M−1 to column 0 are output from block—0 300 to multiplexer 302. Column address <M-1:0> supplied to the control input of multiplexer 302 selects the data for the corresponding column. Similarly, data from column 2M−1 to column 0 are output from block—1 301 to multiplexer 303. Column address <M-1:0> supplied to the control input of multiplexer 303 selects the data for the corresponding column. The outputs of multiplexers 302 and 303 are supplied as inputs to multiplexer 304. Block address <N-1:0> supplied to the control input of multiplexer 304 selects data from the corresponding block for output as data 231.
In the memory of FIG. 3: a column address <(M-1):0> bits wide selects between M columns; a block address <(N-1):0> bits wide selects between N blocks of memory banks; and a row address <(R-1):0> bits wide selects between R rows of logical addresses inside each bank of memory.
FIG. 3 illustrates a partitioning of the example memory blocks. The example memory of FIG. 3 requires that the SRAM addresses have two-bit column addresses <0> and <1>, two-bit row A addresses <2> and <3>, a single-bit block address <4> and a three-bit row B address <5>, <6> and <7>. The address supplied to the memory is divided into these three sections. The positions of the above regions may vary from design to design.
The prior pBIST 130 illustrated in FIGS. 1 and 2 is designed for straightforward linear addressing. Row addresses are the address least significant bits (LSBs) <0> through <4>. Column addresses are bits <5> and <6>. The block address is bit <7>. Incrementing through these addresses would fetch data from row 0 through row 31 in sequential order in block—0 300 and then data from row 32 through 63 in sequential order in block—1 301.
The first pass of this linear addressing would address block—0 300 and proceed through all row addresses sequentially fetching all column 0 data first, and all column 1 data next, followed by column 2 data and finally column 3 data. The second pass of this linear addressing would address block—1 301 and proceed through all row addresses sequentially fetching all column 0 data first, and all column 1 data next, followed by column 2 data and finally column 3 data.
The prior art shown in FIGS. 1-3 employs an architecture where the pBIST and the data logger are integrated. The architecture demonstrated in FIG. 4 consists of a single pBIST, with a Distributed Data Logger system. Each sub chip incorporates a data logger that communicates with the controlling pBIST.
Data is input from the tester (VLCT) to combiner 401 to format the data, then to pBIST block 402, with pBIST memory 403. pBIST 402 communicates with the applicable sub chips via the compressed data bus, and receives fail and log information from each sub chip. The log information is presented in a serial manner to reduce the number of connections.
The sub chips may contain an asynchronous bridge 405 if they operate in a different voltage and/or clock domain from the pBIST. Asynchronous bridge 405 connects to distributed data logger 406 which communicates to control block 407. Block 407 expands the compressed data before writing the test pattern to memory 408, and then reads the result from 408. The comparison of the expected and actual memory data is performed in DDL 406. Since the comparison is done locally in each sub chip, there is no need to return the read data to the pBIST thus reducing the number of connections.
FIG. 5 shows an example of the expander operation. Expander 501 receives 4 bit data 502 and configuration data 504, generating 32 bit output 503. the expansion is done as follows:

wdata[7:4]= ̃(wdata[3:0]) when (ctl[0]=1) else wdata[3:0]

wdata[15:8]= ̃(wdata[7:0]) when (ctl[1]=1) else wdata[7:0]

wdata[31:16]= ̃(wdata[=15:0]) when (ctl[2]=1) else wdata[15:0]

The timing diagram of the serial data bus is shown on FIG. 6 where 601 is the pBIST clock, 602 control scan enable, 603 is the scan data, 604 pBIST run signal and 605 is the pBIST done signal. 606 shows the scanned data. the following control signals are communicated through the serial interface:
















CSR, RGS
MDP control signals



WCTL
expander control signals



PLAT
pipeline latency



MISR, CMISR,
testing mode control



DW
data width










FIG. 7 shows an implementation of the Distributed Data Logger 700. Serial control signals are received from pBIST 402 by CTL_SCAN_IN register 709. Expander and control unit 701 receives it's input from pBIST 402. This input comprises of the following signals:
















addr/16
16 address lines



wdata/4
4 data lines



wren/4
4 write enable lines



mems
memory select



readi
read ignore signal



iddq
enable IDDQ test mode










701 generates outputs to memory data path 407 comprising of the following signals:
















addr/16
16 address lines



wdata/32
32 expanded data lines



wren/4
4 write enable lines



mems
memory select










701 further outputs the 4 wdata signals to register block 702. 702 is organized as a pipeline, with the number of pipeline stages defined by the PLAT signal communicated by the pBIST. The purpose of block 702 is to synchronize the data that will be compared to the actual memory read data during test. This block is needed as the read return data from memory is pipelined and may take several cycles to return to the DDL. The output of pipeline register block 702 is connected to expander 705, where the 4 bit wide wdata is expanded to 32 bits wide. The 32 bit output of block 705 is connected to one input of comparator 707.
The 32 bit wide memory read data is returned from memory data path block 407, and is connected to signature generator 704 for MISR and CMISR testing modes, and to return data register 706. The output of 706 is connected to the second input of comparator 707.
Comparator 707 communicates the comparison results to first input of multiplexer 708, and the output of signature generator 704 is connected to second input of multiplexer 708. The output of multiplexer 708 is connected to serial out shift register 711, and to fail state machine 710. In case of a comparison failure detected by 707, block 710 inserts a time stamp generated by 703 into the comparison results and communicates the data to shift register 711. Block 710 also generates FAIL and a STALL signal. The FAIL and STALL signals, together with the output of shift register 711 are communicated to pBIST 402. Upon receipt of a STALL signal, pBIST 402 will suspend outputting new data to allow the DDL to consume data already in the pipeline registers.

## Claims

1. An embedded memory test system comprising of:
a programmable Built In Self Test (pBIST) engine;
a plurality of sub chips;
a plurality of distributed data loggers incorporated into said sub chips, each distributed data logger including a pipeline register file operable to synchronize the actual memory data with the expected data.

2. The embedded memory test system of claim 1 wherein:
the distributed data loggers receive test data and configuration information from said pBIST.

3. The embedded memory test system of claim 1 wherein:
the pipeline register file stores compressed data prior to expansion.

4. The embedded memory test system of claim 2 wherein:
said distributed data loggers write a test pattern into selected locations in an applicable memory under test;
read the data present in the memory locations the pattern was written into;
compare the actual memory content to an expected memory content;
generate an fail error condition if the memory contents do not match the expected pattern;
generate an error signature;
incorporate a time stamp into said error signature;
generate a STALL signal, communicated to the pBIST after multiple fail conditions;
communicate the error conditions to the pBIST through a serial link.