Techniques to Improve Quality of Memory Interface Tests

in SoCs Using Synopsys TetraMAXs RAM Sequential ATPG

Sanjay Krishna H V

Srivaths Ravi

Texas Instruments

Bangalore, India

www.ti.com

At-speed testing of memory interfaces in 45nm and below technologies has become critical for

defect screening as well as for ascertaining the correct operational frequency of a device (fmax).

In this paper, we describe our experience of using TetraMAXs RAM sequential ATPG for a production 45nm SoC to generate high quality memory interface tests. We discuss the

advantages and challenges involved in using the native tool features, suggest DFT hooks that

assist the tool in ATPG, and propose a flow for using the native sequential pattern generation

engine in an optimum manner to achieve high coverage of the targeted faults with low pattern

counts. Results on our production SoC show that the achieved coverage is 3.5 times more than

that what is achieve-able conventionally. Further, the pattern count reduced by nearly 75%, with

an order of magnitude runtime improvement. We also share results from silicon that clearly show

the impact of using these tests on the fmax of a device (silicon data collection underway).

Table of Contents 1 Introduction and Motivation ................................................................................................ 3

2 Details of Production SoC .................................................................................................. 5

3 Conventional RAM Sequential ATPG................................................................................. 5

4 Challenges with Conventional RAM Sequential ATPG ...................................................... 7

5 Techniques to improve the performance of RAM Sequential ATPG .................................. 7

5.1 Memory interface fault list segregation and pruning .................................................... 8

Table 2: Effect of fault pruning ...................................................................................... 8

5.2 Exploit the BIST resources for ATPG .......................................................................... 8

5.2.1 Capture off BIST (COB) ........................................................................................ 8

5.2.2 Launch off BIST (LOB)....................................................................................... 10

5.2.3 DFT Careabouts ................................................................................................... 11

6 Results ................................................................................................................................ 12

7 Conclusions ........................................................................................................................ 13

8 References .......................................................................................................................... 13

Table of Figures

Figure 1. Differences in paths from/to a typical memory in various modes of operation .............. 3

Figure 2. Setup slack histogram of BIST versus functional paths to different memories of Chip A

......................................................................................................................................................... 4

Figure 3. Memory setup slack difference distribution view for the data in Figure 2 ..................... 4

Figure 4. A memory tested using conventional RAM sequential ATPG ....................................... 6

Figure 5. Sequence of Events in Capture off BIST (COB) based RAM sequential ATPG ............ 9

Figure 6. Constraints, switches and sequential capture procedure used in Capture off BIST

(COB) based RAM sequential ATPG ........................................................................................... 10

Figure 7. Sequence of Events in Launch off BIST (LOB) based RAM sequential ATPG ........... 10

Figure 8. Constraints, switches and sequential capture procedure used in Launch off BIST (LOB)

based RAM sequential ATPG ....................................................................................................... 11

Figure 9. Pipelining Bistmode control of the memory ................................................................. 12

Figure 10. Projected fmax comparisons between LOC TFT and RAM sequential ATPG patterns

....................................................................................................................................................... 13

Table of Tables

Table 1. Memory details of CHIP A ............................................................................................... 5

Table 2. Performance of Conventional RAM sequential ATPG on CHIP A ................................ 7

Table 3. CHIP A: Comparison of proposed and conventional RAM sequential ATPG ............... 12

1 Introduction and Motivation

Conventional techniques to address at-speed testing of memory interfaces rely on functional tests

that are too expensive to design (hence not available) or structural tests that are inadequate in

their defect screening (mismatch between functional and test paths). Since structural tests such as

memory BIST are primarily targeted to identify defects within the memory, ATPG patterns must

be used additionally to target the memory interface logic. But bypassing the memory array and

using only the collar flops during ATPG typically results in false coverage since the paths

to/from test logic are likely to be associated with relaxed timing compared to true functional

path.

This observation can be understood using Figure 1, where an example memory array is

shown with address and data inputs D_mem and ADR_mem, and output Q_mem respectively.

The memory collar added for structural testing (BIST/ATPG) includes multiplexers and

observe/control flops to provide inputs/observe outputs in functional and test modes of operation.

The modified memory IP includes both the memory array and the memory collar.

D

TD

Memory

Array

Q

D_mem

Q_mem

ADR

TADR

ADR_mem

functional

functional

functionalbist

bist

bist

LEGEND

Functional use mode

Pbist/BIST use mode

ATPG use mode

LEGEND

Functional use mode

Pbist/BIST use mode

ATPG use mode

TM

Figure 1. Differences in paths from/to a typical memory in various modes of operation

The different paths in the functional and test modes of operation in Figure 1 are associated

with different timing characteristics. To empirically understand this problem, Figure 2 (next

page) shows data from our 45nm production SoC (labeled Chip A henceforth). For different

memories in the design, Figure 2 shows the worst-case setup slack for timing paths to the

memory via the BIST and functional interfaces. The data clearly shows that for these memories,

the BIST interface timing paths have relaxed timing compared to the functional timing paths in

37 out of 39 memories shown. Figure 3 summarizes this data as a setup slack difference

distribution and shows that most of the memories have a setup slack difference higher than1.5ns

(upto 2.3ns).

Setup Slack Histogram of BIST versus Functional Paths to

Different Memories of Chip A

0

1000

2000

3000

4000

5000

6000

7000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

Memory id

Se

tup

sla

ck

(p

s)

BIST Functional

Figure 2. Setup slack histogram of BIST versus functional paths to different memories of Chip A

Memory Setup Slack Difference Distribution

0

2

4

6

8

10

12

14

16

18

-500 0 500 1000 1500 2000 2500 3000

Setup slack difference (ps)

Nu

mb

er

of

me

mo

rie

s

Figure 3. Memory setup slack difference distribution view for the data in Figure 2

The above data clearly indicates that test coverage of memory interface paths using only

test paths will lead to a test escape or DPPM problem since the ignored functional paths are

significantly more speed limiting. ATPG tools have begun to take cognizance of this issue and

Synopsys TetraMAXs RAM sequential ATPG is a critical feature that addresses this gap it allows the DFT engineer to generate tests that target memory interface faults using complete or

partial functional paths in a scan environment, using its native sequential ATPG engine.

As mentioned in the abstract, this paper describes our experience of using TetraMAXs RAM sequential ATPG for a production 45nm SoC to generate high quality memory interface

tests. We propose a flow for using the native sequential pattern generation engine in an optimum

manner to achieve high coverage of the targeted faults with low pattern counts. We will also

suggest DFT hooks that ease the process of RAM sequential ATPG. Results on our production

SoC show that the achieved coverage is 3.5 times more than that what is achieve-able

conventionally. Further, the pattern count reduced by nearly 75%, with an order of magnitude

runtime improvement. We also share results from silicon that clearly show the impact of using

these tests on the fmax of a device

2 Details of Production SoC

CHIP A, the 45nm SoC used in this work has approximately 400k flip-flops as a part of the top-

level scan architecture. The design includes both synchronous and asynchronous memories

operating in different clock domains. Table 1 indicates the number of each kind, along with the

number of associated clock domains. Both the synchronous single and multi port memories are

associated with one cycle read and write latencies.

Table 1. Memory details of CHIP A

Synchronous single

port memories

Synchronous multi

port memories

Asynchronous

memories

Number of

top-level memories

41 16 37

Number of

clock domains

7 3 5

As seen later from the results presented in the paper, even for a medium-sized SoC with this

many memories, the ATPG tool finds it challenging to perform RAM Sequential ATPG in an

effective manner.

3 Conventional RAM Sequential ATPG

In conventional RAM sequential ATPG, pattern generation on memories with BIST collars is

performed as depicted in Figure 2. A typical memory would have a signal (labeled Bistmode in Figure 2) to enable BIST interface of the memory and a memory array bypass signal (labeled

ATPGmode in Figure 2) used in ATPG mode to bypass the memory array. The memory array is associated with a memory enable signal that enables memory array operations as well as

read/write signal.

ADR

bist

D

TD

Memory

Array

Q

D_mem

Q_mem

TADR

ADR_mem

functionalbist

bist

Bistmode = 0

L CLLL L CLLL

SHIFT Phase

Capture Phase

L = Launch

C = Capture

Me

mo

ry w

rite

bist

Me

mo

ry w

rite

Me

mo

ry R

ea

d

WE

Write enable/

Read dsable

TWE

CLK CLK

Path

sensitized in a

conventional

ram

sequential

ATPGbist

EN

TEN

Memory Enable

TADR

EN

ADR

D

WE

Q

functional

Me

mo

ry R

ea

d

Ca

ptu

re D

2

ATPGmode = 0

Figure 4. A memory tested using conventional RAM sequential ATPG

In order to use the functional paths to/from memory, ATPG would be done with

Bistmode = 0, ATPGmode =0, Memory enable = 1. The tool would then follow a sequence in

capture phase of the pattern as explained below to achieve coverage on the memory input and

output interface pins.

Memory input interface fault testing: In order to sensitize and observe the input interface faults

of memory, below is a typical sequence done at speed during capture phase of ATPG.

Step 1: Write D1 into address say A1. (To initialize data and address lines of the targeted

memory)

Step 2: Write D2 into another address A2. (To create transition on the data and address

lines)

Step 3: Read D2 from address A2. (Observe the transition on the input data and address

lines by reading the data on the memory output)

Step 4: Capture. (Capture the response on flops interfacing the memory output)

Memory output interface fault testing: In order to detect the output interface faults of memory,

below are the typical steps followed.

Step 1: Write D1 into address say A1. (To initialize memory location A1)

Step 2: Write D2 into another address A2. (To initialize memory location A2)

Step 3: Read D1 from address A1. (To initialize output pins of the targeted memory)

Step 4: Read D2 from address A2. (To create transition on the faults on output pins of the

targeted memory)

Step 5: Capture. (Capture the response on flops interfacing the memory output)

4 Challenges with Conventional RAM Sequential ATPG

When conventional RAM sequential ATPG was run on CHIP A (section 2) using Synopsys

TetraMAX to target memory interface faults using the transition delay fault model, there were

clear challenges from coverage, the associated pattern count and runtime. Though usage of some

Tetramax switches to sensitize the memory faults as indicated below improved the overall

pattern count the required coverage goals were not achieved.

set_simulation -store_memory_contents -last

The data is as indicated in Table 2.

Table 2. Performance of Conventional RAM sequential ATPG on CHIP A

RAM Sequential ATPG

Methodology

Coverage #Patterns Run-time

Conventional 25.3% 3028 12087 s

It is intuitive to surmise that the biggest challenge to the ATPG tool is dealing with the

high sequential controllability and observability on memory interface pins when a sequence of

write and read operations are done using functional paths only (although the flops are

initializable using scan). Each of the memory input signals is typically associated with complex

combinational decoding logic in the functional cloud shown in Figure 4. Due to this, the ATPG

tool has a challenging task of computing the values required for multiple flops across several

timeframes that will cause the transitions on memory address/data/output lines as mentioned in

Section 3.

For example, from the waveform in Figure 4, it can be seen that to gain the coverage on

Q pins of the memory, complementary data has to be written to two different locations for which

WE (write enable) needs to be constant for at least 2 cycles and EN (Memory enable) have to be

static for at least 4 cycles. But when multiple flops of same clock domain have to be controlled

to achieve these requirements on the memory pins in specific cycles, the limitations of sequential

ATPG play a role and hence, pattern coverage and runtime get impacted.

5 Techniques to improve the performance of RAM Sequential ATPG

Techniques to address the challenges that ATPG faces for RAM sequential ATPG can be

developed from two directions.

Memory interface fault list segregation and pruning: Targeting full list of memory interface faults in one pattern generation run leads to unnecessary overheads. Segregating

into input and output faults, and further using information from timing analysis to present

a reduced fault list to the ATPG tool, ensures that the ATPG tool focuses efficient and

maximal effort on critical faults.

Exploit the DFT (BIST) resources in the design: In a typical SoC, memory BIST logic has controllability/observability paths to/from memory test interface. For example, since

the memory BIST controller is scanned, memory test pins are easily controllable. Further,

with the presence of pipeline flops (which are also scanned) on memory BIST data,

address, write enable, memory enable, etc., we gain better controllability.

5.1 Memory interface fault list segregation and pruning

Fault segregation and filtering was done to create two lists of critical memory output and input

interface faults.

Memory_op_int.flts: To obtain the functionally critical memory output interface paths, static timing analysis was done using Synopsys PrimeTime (PT) in the TFT capture

mode, with ATPGMODE pin of all memories set to 0 through case analysis. All the worst case timing paths through Q pins of the memories were reported. All paths with

flop endpoints that were non-BIST were chosen.

Memory_ip_int.flts: To cover functionally critical memory input interface paths, the PT session was used to report all the worst case timing paths to the memories. All paths with

Start-points from Non-BIST Flop paths were then chosen.

After using the above method of fault filtering, we could reduce the number of faults to be

targeted for pattern generation to ~20.1% of the full list of memory interface faults. These faults

were roughly associated with 39 of the total number of memories in the design (Table 2).

Table 2: Effect of fault pruning

CHIP A Synchronous Single

port memories

Synchronous Multi

port memories

Asynchronous

memories

Technology 45nm 45nm 45nm

#Total Top-Level

Memories RAMs

41 16 37

# Memory Clock

Domains (Clock

Groups)

3 3 1

#Top-Level Critical

Memories RAMs

18 16 5

5.2 Exploit the BIST resources for ATPG

Once the two sets of faults are created, we adopt a two phased pattern generation approach using

BIST resources described in [2,3] that fetches higher quality coverage on the functional interface

of the memory. In this section, we discuss suitable constraints, spf changes and switches to be

used in TetraMAX for implementing these techniques, along with ATPG friendly DFT

careabouts.

5.2.1 Capture off BIST (COB)

In order to gain coverage on functional input pins of the memory, the ATPG tool can be guided

to generate patterns such that the that the memory inputs can be controlled through functional

interface pins, but the data read from the memory can be captured through the BIST interface of

the memory. This technique is therefore called Capture off BIST (COB) and is shown in Figure

5. Clearly, it helps reduce the overall pattern generation effort to observe the faults.

The fault list used for ATPG is Memory_ip_int.flts as described in Section 5.1. An example

illustration of the cell constraints and pin constraints required to perform COB are provided in

Figure 6.

X000 X000

D

TD

Memory

Array

Q

D_mem

Q_mem

ADR

TADR

ADR_mem

functional

functional

functionalbist

bist

Bistmode

CLLL CLLL

Bist

mode

SHIFT Phase

Capture Phase

L = Launch

C = Capture

bist

Write

da

ta D

1 @

A1

with

fun

ctio

na

l inte

rface

Write

da

taD

2 @

A2

with

fun

ctio

nal in

terfa

ce

Rea

d d

ata

D2

@ A

2 w

ith fu

nctio

na

l inte

rface

Cap

ture

da

ta o

n b

ist

inte

rface

Figure 5. Sequence of Events in Capture off BIST (COB) based RAM sequential ATPG

// Constraints

set_drc clock seq_capture

set_delay nocommon

add_cell_constraints 0 Bistmode

add_pi_constraints 0 ATPGmode.

// Fault model used

set_fault model transition

set_atpg noclk_constraint_fault_pruning

set_atpg full_seq_atpg

set_atpg capture 4

--------------------------------------------------------------------

//Capture Procedure for Capture on BIST in SPF file.

"sequential_capture" { W "_default_WFT_";

F { ;"lcd_pclk"=P;"lcd_vsync"=0; }

V { "_pi"=\j \r196 #; }

V { "DIV10_125"=P; } //Launch: Write D1(A1)

V { "DIV10_125"=P; } //Launch: Write D2(A2)

V { "DIV10_125"=P; } //Launch: Read D2(A2)

V { "DIV10_125"=P; } //Capture

V { "_po"=\j \r196 #; }}}

Figure 6. Constraints, switches and sequential capture procedure used in Capture off BIST (COB) based

RAM sequential ATPG

In Figure 6, we can see that a four cycle sequential capture procedure associated with a design

internal clock DIV10_125 (pseudo-primary input) is defined in the spf. In order to detect the

output interface faults of memory, the tool will use 3 Launch cycles and 1 capture cycle in every

pattern generated. The signal Bistmode is controllable using a looped back test register on

scan. Therefore, it is associated with a cell constraint of 0 so that the functional path selected on

the input side. The signal ATPGmode, in this case, is a pseudo-primary input which is

constrained to 0 using add_pi_constraint.

5.2.2 Launch off BIST (LOB)

In order to gain coverage on output pins of the memory, the ATPG tool can be guided to

generate patterns such that the memory inputs can be controlled through the BIST interface of the

memory, but the data read from the memory can be captured through the BIST interface of the

memory. This technique is therefore called Launch off BIST (LOB) and is shown in Figure 7.

Clearly, it helps reduce the overall pattern generation effort from a controllability perspective.

1 1 1 1 X1 1 1 1 X

D

TD

Memory

Array

Q

D_mem

Q_mem

ADR

TADR

ADR_mem

functional

functional

functionalbist

bist

Bistmode

Bist

mode

SHIFT Phase

Capture Phase

L = Launch

C = Capture

bist

Write

da

ta D

1 @

A1

with

bis

t inte

rface

Write

da

ta D

2 @

A2

with

bis

t inte

rface

Rea

d d

ata

D1

@ A

1 w

ith b

ist in

terfa

ce

Cap

ture

da

ta o

n fu

nctio

nal in

terfa

ce

Rea

d d

ata

D2

@ A

2 w

ith b

ist in

terfa

ce

L L L L CL L L L C

Figure 7. Sequence of Events in Launch off BIST (LOB) based RAM sequential ATPG

The fault list used for ATPG is Memory_op_int.flts as described in Section 5.1. An example

illustration of the cell constraints and pin constraints required to perform LOB are provided in

Figure 8. In this case, we can see that a five cycle sequential capture procedure associated with a

design internal clock DIV10_125 (pseudo-primary input) is defined in the spf. In order to detect

the output interface faults of memory, the tool will use 4 Launch cycles and 1 capture cycle in

every pattern generated. The signal Bistmode is now associated with a cell constraint of 1 so

that the test path is selected on the input slide. The signal ATPGmode is constrained to 0 using

add_pi_constraint. Further, to ensure that coverage credit is provided for capture using functional

flops only, the BIST flops in transitive fanout of memory Q outputs are masked during capture.

Figure 8. Constraints, switches and sequential capture procedure used in Launch off BIST (LOB) based

RAM sequential ATPG

5.2.3 DFT Careabouts

Design hooks for controlling Bistmode and ATPGmode signals to use the above two methods

more efficiently, are listed below.

Bistmode control: In the COB scheme, only the transition that sensitizes the fault

should be from the functional path, while other initialization and propagation can use the

BIST interface. For example, in the COB scheme, the functional path is mandatory only

for the second capture cycle (i.e write @ A2) that launches a transition at the memory input. Neither the first capture cycle (that initializes the memory input), nor the third

capture cycle (that observes the transition), nor the last capture cycle need to be along the

functional path as illustrated in Figure.9. Pipelining Bistmode signal to support

functional and BIST modes access to memory dynamically during launch/capture cycles

provides flexibility for the tool in pattern generation. In this way, we can improve the

overall coverage and generation time.

// Constraints

set_drc clock seq_capture

set_delay nocommon

add_cell_constraints 1 Bistmode

add_pi_constraints 0 ATPGmode

// Fault model used

set_fault model transition

set_atpg noclk_constraint_fault_pruning

set_atpg full_seq_atpg

set_atpg capture 5

-----------------------------------------------------------------

//Capture Procedure for Launch on BIST in SPF file.

"sequential_capture" { W "_default_WFT_";

F { ;"lcd_pclk"=P;"lcd_vsync"=0; }

V { "_pi"=\j \r196 #; }

V { "DIV10_125"=P; }//Launch: Write D1(A1)

V { "DIV10_125"=P; }//Launch: Write D2(A2)

V { "DIV10_125"=P; }//Launch: Read D1(A1)

V { "DIV10_125"=P; }//Launch: Read D2(A2)

V { "DIV10_125"=P; }//Capture

V { "_po"=\j \r196 #; }}}

X101 X101

D

TD

Memory

Array

Q

D_mem

Q_mem

ADR

TADR

ADR_mem

functional

functional

functionalbist

bist

Bistmode

CLLL CLLL

Bist

mode

SHIFT Phase

Capture Phase

bist

Write

@ A

1 w

ith b

ist

inte

rface

Write

@ A

2 w

ith fu

nctio

nal in

terfa

ce

Read

@ A

1 w

ith b

ist

inte

rface

Write

@ A

2 w

ith b

ist

inte

rface

Bistmode requires to be

toggled atspeed in to

achieve better coverage

Bistmode requires

to be pipelined

L = Launch

C = Capture

A1 = some Address location

A2 = another Address location

Figure 9. Pipelining Bistmode control of the memory

ATPGmode control: During RAM sequential pattern generation, when multi load option is disabled, the output (Q pins) of the memory is not initialized to a known state at the

end of shift. In LOB, for example, the flops capturing the Q pins of the memory would be

in unknown state for the 4 Launch cycles of the capture phase. This in turn leads to X

propagation, leading to high pattern inflation. This can be taken care in scan insertion by

having ATPGmode pin of each memory to be controlled by scan flops during capture,

such that

ATPGMODE = 1 during shift

1/0 during capture

6 Results

The techniques described in Section 5 (fault segregation and filtering technique, LOB and

COB based RAM sequential ATPG as well as the associated DFT changes) were used in the

45nm production SoC CHIP A (taped out in 2011). The performance of Synopsys Tetramax

ATPG (tool version : 2011.09) using the conventional and proposed techniques is listed in Table

3.

Table 3. CHIP A: Comparison of proposed and conventional RAM sequential ATPG

RAM Sequential ATPG Coverage #Patterns Run-time

Conventional 25.3% 3028 12087 s

Proposed 84% 736 1046 s

From Table 3, we can see that the test coverage has improved by 3.5X, while the pattern

generation time has reduced by 75%, while pattern generation time has improved by 10X.

Using timing analysis from PT, we projected the fmax for 4 clock domains by looking at the

worst-case setup slack for the timing paths exercised in conventional TFT as well as RAM

sequential ATPG. The fmax comparisons are shown in Figure 10. The results show that the

RAM sequential is likely to bin the parts at a lower fmax compared to conventional TFT. Silicon

data collection is currently underway to validate the STA data.

0

100

200

300

400

500

600

1 2 3 4

Clock domains

Fre

qu

en

cy

(MH

z)

LOC TFT

RAM sequential

Figure 10. Projected fmax comparisons between LOC TFT and RAM sequential ATPG patterns

7 Conclusions

In this paper, we saw how Synopsys TetraMAXs sequential ATPG engine can be leveraged to perform effective at-speed testing of the memory interface faults. We provide data on a

production SoC to show the challenges in a nave approach to use TetraMAX to perform RAM

sequential ATPG. We further show that through a combination of fault list segregation and

pruning, usage of a two-phased ATPG approach to exploit the DFT resources in a design along

with necessary DFT hooks, the performance of TetraMAX can be significantly improved. This

would be a critical enabler to make RAM sequential ATPG patterns part of a production test

program.

8 References

[1] Synopsys, Tetramax User Guide (tool version : 2011.09)

[2] Devanathan, V.R.; Hales, A.; Kale, S.; Sonkar, D.; , "Towards effective and compression-friendly test of memory interface logic," Test Conference (ITC), 2010 IEEE International , vol., no., pp.1-10, 2-4

Nov. 2010

[3] Devanathan, V.R.; Vooka, S.; , "Techniques to improve memory interface test quality for complex SoCs," Test Conference (ITC), 2011 IEEE International , vol., no., pp.1-10, 20-22 Sept. 2011