Testing and Diagnosis of Interconnect Faults in Cluster-Based FPGA

ArchitecturesDavid Mohabir

University of ArizonaMarch 19th, 2012

Testing and diagnosis of interconnect faults in cluster-based FPGA architectures

Section 1

Motivation Quickly identify faulty components Design new, efficient testing methodologies

to offset the complexity of FPGA testing as compared to ASIC testing› Defect location information is an important

modern strategy as FPGAs can be reconfigured to avoid faults

› Increased test generation complexity› Increased test application time› Multiple configurations to test assortment of

switch settings

Limitations High complexity for test generation Increased test application time Need for external controllability and

observability Multiple configurations to test

assortment of switch settings, compared to a single configuration for an ASIC› As FPGAs have more programmable switch

points, this becomes a bigger issue

Previous and related work FPGA testing has been divided into interconnect

testing and FPGA logic testing Reduction in the need for I/O pads for testing

› Several configurations are required to ensure all FPGA logic is tested in some configuration

Unutilized FPGA logic and routing are being used to implement modular redundancy

Faults can be targeted for the entire FPGA structure, or those that are application-specific

Related work (con’t) Need for external controllability and

observability has also been reduced using iterative logic array (ILA) test architecture› one-dimensional configuration with one direction

for signal propagation› A complete array of m x m LUT/RAM modules

requires 4 test configurations independent of size of array and of modules [11]

Problems of defining a set of test configurations for cluster-based architectures and diagnosis

Related work (con’t) The use of LUTs with logic checkers to

implement testing schemes in interconnects Using LUTs to form shift registers to easily

check the output of the test pattern Built-in Self Test (BIST) architecture to locate

any single and most multiple fault PLBs› This is FPGA logic

Cluster-based FPGA test methodologies› Does not cover specific fault extra-cluster

Geometric Scaling Increased defect rates Increased device variation Increased change in device parameters Increased single die capacity Increased susceptibility to transient

upsets

Defect Tolerance If device failure renders a bitop or an

interconnect unusable, the device should be reconfigured to avoid these failing areas› Substitute good resources for bad ones› As defect rates increase, spare resources

should be strategically reserved

Interchangeable LUTs

Interchangeability Not all unused units will be

substitutable, as location strongly affects interconnections to other logic blocks

Preferable to have fewer large pools of mostly interchangeable resources

Cluster-based architectures Primitive logic components are grouped into

coarse-grained clusters Richness of internal connectivity means large

range of potential interconnect patterns External access to internal test points becomes

increasingly difficult as device sizes scale Cluster I/O are the input and output pins of the

cluster Tile I/O pins include the endpoint of wire

segments which can connect to a neighboring tile via programmable interconnect points

Structure

Built-in Self Test BIST overhead not an issue

› Easily inserted and removed by reconfiguration

Test logic inside the FPGA enables test access to internal components

Each BISTER is composed of› Test pattern generator› Output response analyzer› Two blocks under test

BISTER test structure

BISTER

BIST strategy To guarantee testing of all tiles, the FPGA is

reconfigured to shift the BISTERs across the entire array› All tiles will be tested by acting as a BUT

Perimeter tiles are tested by using the I/O pads to access the periphery

Total test application time is related to the area of the TPG/ORA logic

Decomposes the problem into many identical problems of a size which is determined by the test requirements for a single tile

Interconnect Fault Detection

High density of internal cluster interconnect makes test access difficult

Must test intra-cluster interconnect and extra-cluster interconnect

Four classes of faults› Permanent connection

PIP off› Permanent disconnection

PIP on› Stuck-at 0› Stuck-at 1

Detection and Diagnosis Defines testability and diagnosis

requirements of each fault and fault pair

Some test pattern must exist to detect each fault and differentiate each fault pair

All LUTs are configured as 4 input XOR gates

The detectability of each fault can be expressed as a function of the tile I/O

Fault Detection Conditions Faulty line segment s1 must be both

controllable by at least one tile input and observable by at least one tile output

Fault Detection Conditions (con’t)

A faulty pair of segments must be both controllable, separately controllable, and both observable

The PIP between the two segments must be switched off

Fault Detection Conditions (con’t)

If s2 is the floating segment, then the non-floating segment must be controllable and the floating segment must be observable

PIP between the two segments must be switched on

Interconnect Fault Equivalence

Equivalent faults cannot be differentiated Fault equivalence is determined by the FPGA configuration

› Faults that are equivalent in one configuration may not be equivalent in another

Maximum diagnostic resolution is achieved when every pair of faults is non-equivalent in at least one configuration

Two faults are equivalent if their corresponding faulty machines produce the same output with all possible test patterns, at all outputs of the circuit

Two segments are test equivalent in a configuration if the segments have identical control sets and identical observe sets

Interconnect Fault Equivalence (con’t)

Two segments are test equivalent when they are controlled by the same set of tile inputs and observed by the same set of tile outputs

Interconnect Fault Equivalence (con’t)

Each segment in a faulty segment pair must be test equivalent to a segment in the other faulty segment pair

Interconnect Fault Equivalence (con’t)

Pair of faults may be equivalent if a segment which is not driven by a signal floats to a ‘v’ value

The two faults are equivalent if the floating segment is test equivalent to the segment associated with the stuck-at ‘v’ fault

The segment with the stuck-at fault and the floating segment must be controlled by the same set of tile inputs and observed by the same set of tile outputs

Interconnect Fault Equivalence (con’t)

The pair of segments involved in one fault are test equivalent to the pair of segments involved in the other fault

Each segment in a faulty segment pair must be test equivalent to a segment in the other faulty segment pair

Test Configurations Identifies a set of configurations for the

tiles acting as BUTs in a BISTER Size of configuration should be

minimized to reduce test application time

Intra-cluster configurations are defined separately from extra-cluster configurations

Intra-Cluster Configurations Fault effect on a cluster input must

propagate to at least one cluster output

Cluster outputs must be separately controllable

BLE configurations Observability of cluster inputs and BLE output branches

must be achieved by propagating fault effects Controllability of the BLE outputs must be achieved

through the BLEs Each BLE is composed of a LUT and a multiplexer

› Both must be configured› Each LUT acts as a 4-input XOR gate› Good controllability because output value can be determined by

controlling any single input› Good observability because a fault effect on any input will

propagate to output Majority of test configurations bypass the flip-flop

› A single configuration will test the interconnect associated with the flip-flops

BLE input multiplexer configurations

Input muxes determine controllability of BLE outputs by determining the function which defines the output of each BLE ‘n’

BLE output function:› All inputs XORed together

Multiplexers are not configured to create loops All BLE outputs are separately controllable from each

other, and from all cluster inputs Each input multiplexer is configured to select data

from each of its inputs in at least one configuration There is a sensitized path from each cluster input

stem to a cluster output in every configuration

Algorithm 1

Input Multiplexer configurations

Extra-Cluster Configurations Defines current flow paths through the

extra-cluster interconnect Modeled as a flow graph Create flow paths between tile I/O nodes

which allow the detection criteria of each fault to be satisfied in at least one configuration

Flow paths are created from tile I/Os to every cluster input, and from every cluster output to tile I/Os

Transparent Extra-Cluster Configuration

Algorithm 2

Algorithm 3

Results Assumptions

› Cluster inputs and outputs are equally distributed around the sides of the cluster› Each cluster I/O on the north face may connect to all horizontal tracks via a set of

PIPs› West face I/O connects to all vertical tracks› Cluster I/O for east and south faces connect directly to tracks in neighboring tiles

Results› Intra-cluster configuration, and two sets of extra-cluster configuration

Extra-Cluster (specific) is for when the fault independent algorithm has reached its coverage limit

› By using the fault specific extra-cluster configuration algorithm, 100% fault coverage can be guaranteed At a cost of increased number of configurations

› Fault Coverage Achieved› Percent of fault pairs which are differentiated across all configurations

A small set of test configurations can detect and diagnose nearly all targeted interconnect faults

Results

Summary Approach is encompassing, can guarantee

100% fault detection› Does require good deal of computation time for

extra-cluster Does a good job of describing fault classes

› I personally believe they could have described it using less mathematical jargon, so that it would make more sense to a digital logic engineer

Algorithms are described neatly in pseudocode

All details are covered

Discussion topicsSection 2

Discussion #1 Let’s discuss the logical ways to test

circuitry for the various faults› Permanent open› Permanent closed› Stuck-at 0› Stuck-at 1

How could you design test patterns without access to all internal signals?

Discussion #2 Algorithms

› Intra-cluster› Extra-cluster

Discussion #3 Defect mapping Annealing placers

› Marks physical location of defective units as Costly Invalid

Routers› Marks wires and switches that are defective as

In use High cost

Avoids these defective components of the FPGA

Discussion #4 Parity