EE 587SoC Design & Test Partha Pande School of EECS Washington State University email@example.com
IP Blocks in SoCs • SoC jargon • Core • Intellectual Property (IP) • Virtual Component • Macro • ……………
VIPER – A Multiprocessor SoC • Highly integrated SoC for digital TV • Digital Set-Top box • Viper receives, optionally decrypts, decodes, converts, and displays multiple media streams having different data formats. • Besides MPEG-2 transport streams, the chip typically handles live video, audio, and various other stream types, in compressed or uncompressed formats.
VIPER Architecture (cont’d) • Two processor cores • standard 32-bit MIPS RISC core (PR3940) and a 32-bit very long instruction word (VLIW) TriMedia core (TM32) • The VLIW core handles Viper’s real-time multimedia processing tasks. • PR3940 core is the second on-chip CPU to run the operating system and handle various control processing tasks.
General Peripherals • Enhanced IEEE 1394 link layer • Expansion bus interface • Drawing engines • Transport stream processor • Audio interfaces • Video-processing blocks
Complexity • 35 million transistors in 0.18 micron process • Synthesized in eight hours running on multiple CPUs • The design was partitioned into chiplets of 200k cells, with total nine chiplets • The chip was designed to be fully scan testable
INTEL IXP2400 • 8 integrated programmable microengines • Integrated Intel XScale core • One DRAM and two SRAM interfaces • JTAG support • Additional Integrated hardware
Intel’s 80-core Chip • Intel’s 80-core chip • In 65-nm technology with 80 single-precision, floating point cores delivers performance in excess of a teraflops while consuming less than 100 w. • A 2D on-die mesh interconnection network operating at 5 GHz provides the high-performance communication fabric to connect the cores.
Challenges in Testing a SoC • One of the most critical challenges of this emerging discipline is manufacturing test. • Core provider only delivers a description of the core design to the SOC designer. • That is the IP of the vendor, who may provide only limited details of the design to the user. • The user considers the core as a black-box. • Accessibility to component peripherals
Conceptual Architecture of Core Test • Three separate elements in the embedded core test infrastructure. • Test pattern source and sink • Test Access Mechanism • Core test wrapper
Test Pattern Source & Sink • The source generates the test stimuli and the sink compares the response to the expected response. • Source and sink of a core can be implemented either off-chip, on-chip or as a combination of both • The choice for a certain type of source or sink is determined by • The type of circuitry in the core • The type of predefined tests that come with the core • Quality and cost considerations.
Test Access Mechanism (TAM) • It takes care of on-chip test pattern transport. • The key components of any TAM are its width and length. • The width refers to the TAM’s transport capacity. • The length of a TAM is the physical distance it has to bridge between the source and core or core and sink.
Implementation of TAM • When implementing a TAM, we have the following options • A TAM can either reuse existing functionality to transport test patterns or be formed by dedicated test access hardware. • A TAM can either go through other modules on the IC or pass around those other modules. • One can either have an independent access mechanism per core, or share an access mechanism with multiple cores. • A test access mechanism can either be a plain signal transport medium, or may contain certain intelligent test control functions.
Core Test Wrapper • Interface between the embedded core and its system chip environment. • It connects the core terminals both to the rest of the IC, as well as to the TAM. • It is implemented on chip.
ARM’s AMBA System • An example of reusing existing functionality as TAM. • In this approach, the 32-bit system bus transfers test stimuli from IC pin to the core under test and test responses vice-versa. • Advantages of this approach • The low additional area cost • The relative simple test expansion • A disadvantage of the approach is that the fixed 32-bit bus does not allow to make trade-offs between area cost and test time
Connection of System Pins to Core Terminals • Connect additional wires to the core’s terminals and multiplex those onto existing IC pins • testing of memories • The advantage of this approach is : the embedded core can be tested as if it were a stand alone device, the translation of core-level tests into IC-level tests is simple and straightforward. • The disadvantage is : it is not scalable.
Testability Measures • Need approximate measure of: • Difficulty of setting internal circuit lines to 0 or 1 by setting primary circuit inputs • Difficulty of observing internal circuit lines by observing primary outputs • Uses: • Analysis of difficulty of testing internal circuit parts – redesign or add special test hardware • Guidance for algorithms computing test patterns – avoid using hard-to-control lines • Estimation of fault coverage • Estimation of test vector length
Observability • The observability of a particular circuit node is the degree to which we can observe that node at the output of an integrated circuit • Measure the output of a gate within a larger circuit to check whether it operates correctly • Limited number of nodes can be directly observed
Controllability • The controllability of an internal circuit node within a chip is a measure of the ease of setting the node to a 1 or 0 metric • Degree of difficulty of testing a particular signal within a circuit • An easily controllable node would be directly settable via an input pad
Scan Design • In test mode, all flip-flops functionally form one or more shift registers • The inputs and outputs of these shift registers are made into PI/Pos • Using the test mode, all flip-flops can be set to any desired states • The states of the flip-flops are observed by shifting the contents of the scan register out
Scan Design • Circuit is designed using pre-specified design rules. • Test structure (hardware) is added to the verified design: • Add a test control (TC) primary input. • Replace flip-flops by scan flip-flops (SFF) and connect to form one or more shift registers in the test mode. • Make input/output of each scan shift register controllable/observable from PI/PO. • Use combinational ATPG to obtain tests for all testable faults in the combinational logic. • Add shift register tests and convert ATPG tests into scan sequences for use in manufacturing test.
Scan Design Rules • Use only clocked D-type of flip-flops for all state variables. • At least one PI pin must be available for test; more pins, if available, can be used. • All clocks must be controlled from PIs. • Clocks must not feed data inputs of flip-flops.
Scan Flip-Flop (SFF) Master latch Slave latch D TC Q Logic overhead MUX Q SD CK D flip-flop Master open Slave open CK t Normal mode, D selected Scan mode, SD selected TC t
Need for Standard • Bed-of-nails printed circuit board tester • We put components on both sides of PCB & replaced DIPs with flat packs to reduce inductance • Nails would hit components • Reduced spacing between PCB wires • Nails would short the wires • PCB Tester must be replaced with built-in test delivery system -- JTAG does that • Need standard System Test Port and Bus • Integrate components from different vendors • Test bus identical for various components • One chip has test hardware for other chips