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EE434 ASIC & Digital Systems. Jacob Murray School of EECS Washington State University [email protected] Test Methodologies. Adopted from “Essentials of Electronic Testing” by Bushnell and Agrawal. Lecture 26. Design for Testability (DFT): Partial-Scan & Scan Variations. Definition

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ee434 asic digital systems
EE434ASIC & Digital Systems

Jacob Murray

School of EECS

Washington State University

[email protected]

test methodologies
Test Methodologies

Adopted from “Essentials of Electronic Testing” by Bushnell and Agrawal

Lecture 26

design for testability dft partial scan scan variations
Design for Testability (DFT): Partial-Scan & Scan Variations
  • Definition
  • Partial-scan architecture
  • Random-access scan (RAS)
  • Scan-hold flip-flop (SHFF)
  • Boundary scan
  • Summary
partial scan definition
Partial-Scan Definition
  • A subset of flip-flops is scanned.
  • Objectives:
    • Minimize area overhead and scan sequence length, yet achieve required fault coverage
    • Exclude selected flip-flops from scan:
      • Improve performance
      • Allow limited scan design rule violations
    • Allow automation:
      • In scan flip-flop selection
      • In test generation
    • Shorter scan sequences
partial scan architecture
Partial-Scan Architecture

PI

PO

Combinational

circuit

CK1

FF

FF

CK2

SCANOUT

SFF

TC

SFF

SCANIN

flip flop for partial scan
Flip-flop for Partial Scan
  • Normal scan flip-flop (SFF) with multiplexer of the LSSD flip-flop is used.
  • Scan flip-flops require a separate clock control:
      • Either use a separate clock pin
      • Or use an alternative design for a single clock pin

D

Master

latch

Slave

latch

MUX

Q

SD

TC

SFF

(Scan flip-flop)

CK

TC

CK

Normal mode

Scan mode

scan hold flip flop shff
Scan-Hold Flip-Flop (SHFF)

To SD of

next SHFF

D

  • The control input HOLD keeps the output steady at previous state of flip-flop.
  • In the normal mode, TC=HOLD=1. In the scan mode, TC=1 and HOLD=0. This isolates the combinational logic from the scan register activity.
  • The state inputs of combinational logic driven by the hold latch remain frozen at their pre-scan value
  • Power Savings

Q

SD

SFF

TC

Q

CK

HOLD

random access scan ras
Random-Access Scan (RAS)

PI

PO

Combinational

logic

RAM

nff

bits

CK

TC

SCANOUT

SCANIN

SEL

Address decoder

Address scan

register

log2 nff bits

ADDRESS

ACK

slide11
RAS
  • All flip-flops form a RAM in the scan mode
  • In the normal mode, all flip-flops receive data from the combinational logic under the control of the clock CK
  • Flip-flop outputs directly feed into the combinational logic
  • Number of bits in flip-flop address = log nff
  • The address is loaded into an address scan register (ASR) using an address clock ACK
  • The address decoder now produces the select signal SEL=1 for the addressed flip-flop
  • For scanning data into a flip-flop, the scan mode is used
ras flip flop ram cell
RAS Flip-Flop (RAM Cell)

D

Q

To comb.

logic

From comb. logic

SD

Scan flip-flop

(SFF)

SCANIN

CK

TC

SCANOUT

SEL

ras applications
RAS Applications
  • Logic test:
    • Reduced test length
    • Reduced scan power
  • Advantage: RAS may be suitable for certain architecture, e.g., where memory is implemented as a RAM block.
  • Disadvantages:
      • Not suitable for random logic architecture
      • High overhead – gates added to SFF, address decoder, address register, extra pins and routing
boundary scan bs ieee 1149 1 standard
Boundary Scan (BS)IEEE 1149.1 Standard
  • Developed for testing chips on a printed circuit board (PCB).
  • A chip with BS can be accessed for test from the edge connector of PCB.
  • BS hardware added to chip:
    • Test Access port (TAP) added
      • Four test pins
      • A test controller FSM
    • A scan flip-flop added to each I/O pin.
  • Standard is also known as JTAG (Joint Test Action Group) standard.
system configuration
System Configuration
  • Each pin of the chip has a register at that position
  • Serial connection of this register – boundary register
  • TDI – Serial input
  • TDO – Serial output
  • A number of registers can be connected between TDI and TDO
tap controller signals
Tap Controller Signals
  • Test Access Port (TAP) includes these signals:
    • Test Clock Input(TCK) -- Clock for test logic
      • Can run at different rate from system clock
    • Test Mode Select(TMS) -- Switches system from functional to test mode
    • Test Data Input(TDI) -- Accepts serial test data and instructions -- used to shift in vectors or one of many test instructions
    • Test Data Output(TDO) -- Serially shifts out test results captured in boundary scan chain (or device ID or other internal registers)
    • Test Reset(TRST) -- Optional asynchronous TAP controller reset
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