Kobayashi Lab.

Gunma University

SAR TDC Architecture

for One-Shot Timing Measurement

with Full Digital Implementation

Y. Ozawa, T. Ida, R. Jiang, S. Sakurai, R.Takahashi,

R. Shiota, H. Kobayashi

Gunma University, Socionext Inc.,

Nov. 8 WP-L2 14:30-15:50

2/40

Objective

Development of

highly-linear, fine time-resolution TDC

for high-speed digital I/O interface timing measurement

3/40

Innovation

Trigger Circuit

One-shot timing measurement

&

Good for low frequency repetitive timing

Ring Oscillator

One-shot timing measurement

&

Good for high & low frequency repetitive timing

4/40

Approach

Full digital FPGA implementation

SAR TDC

SAR TDC + Ring Oscillator

SAR TDC + Trigger CircuitMeasure one-shot timing

Include analog circuit

Measure one-shot timing

Without analog circuit

5/40

Problems in Operation of SAR

SAR TDCInput signal Output signal

Analog Digital

During measurement

The necessity to always input certain time difference

Circuit approach to problem

Multiple steps required

6/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

7/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

8/40

Time to Digital Converter （TDC）

T

Start

Stop

Start

StopDoutTDC

1996 1998 2000 2002 2004 2006 20080

10

20

30

40

50

Year

LS

B [

ps]

Higher resolution with CMOS scaling

● Time interval → Measurement → Digital value

● Key component of Time-

domain analog circuit

● Higher resolution can be

obtained with scaled CMOS

8

9/40

Background

Voltage domain

Fine CMOS

Time domain

time

Fine CMOStime

A Time-to-Digital Converter (TDC) provides a digital outputproportional to time between two clock transitions.

（e.g. I/O interface, Sensor Interfaces, All-Digital PLLs, ADCs, .. ）

Supplyvoltage

The TDC is a key component in time-domain analog circuits,

Advanced CMOS VLSI● Low power-supply voltages● Fast switching speeds

10/40

Time Domain Analog Circuit Features

● Voltage domain:

Signal range : Up to power supply voltage

Time domain:

Signal range : Time continues indefinitely

Large dynamic range

● Time domain analog circuit：

Binary amplitude（Vss, Vdd)

Can consist of digital circuit

11/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

12/40

Flash-type TDC

12

D1 D2 D3

・・・DQ

DQ

DQ

Stop

Start τ τ τ τ

DQ

D4

a b c d

13/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

14/40

SAR ADC Block

Sample Hold

DAC

SAR

Logic

Analog

input

Digital

output

ComparatorCLK

SAR ADC is digital centric.

→ Suitable for fine CMOS implementation.

15/40

D-FF can be greatly reduced

by using MUX

Circuit operation loop can be made

with Successive Approximation

Flash type TDC

SAR TDC

SAR TDC Architecture

SAR : Successive Approximation Register

Multiplexer

16/40

SAR ADC : Comparator DAC

SAR TDC : D-FF Delay Line

SAR-ADC

SAR-TDC

SAR-ADC VS SAR-TDC

17/40

STEP１

Example ΔT = 4.3 τ

t t t t t t t

CLK1

Multiplexerr

D QCLK2SAR Logic

Dout

3

3

select100

1

0

SAR TDC Operation

ΔT ΔT ΔT

CLK1

CLK2

ΔT

18/40

t t t t t t t

CLK1

Multiplexerr

D QCLK2SAR Logic

Dout

3

3

select

10

0

110

STEP2

ΔT ΔT ΔT

CLK1

CLK2

ΔT

Example ΔT = 4.3 τ

SAR TDC Operation

19/40

CLK1

t t t t t t t

Multiplexerr

D QCLK2SAR Logic

Dout

3

3

select

0

101

100

STEP3

ΔT ΔT ΔT

CLK1

CLK2

ΔT

Example ΔT = 4.3 τ

SAR TDC Operation

Digital Output：4

20/40

Digital Output：4

t t t t t t t

CLK1

Multiplexerr

D QCLK2SAR Logic

Dout

3

3

select

101

1

100

100

STEP4

(Stable)

ΔT ΔT ΔT

CLK1

CLK2

ΔT

Example ΔT = 4.3 τ

SAR TDC Operation

21/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

22/40

Trigger Circuit

Output starts to oscillate

at rising timing edge of input

Output waveform with no transient change

23/40

T/H Circuit

24/40

Trigger Circuit Waves

25/40

Combination of Trigger Circuit & SAR TDC

Proposed Circuit

START

STOP

One-shot signal

∆𝑇

W1

W2

∆𝑇

clk1

clk2

Repetitive signals

∆𝑇 ∆𝑇

26/40

Simulation Results

2 trigger circuits can generate

repetitive signals

One-shot timing

can be measured

with SAR TDC

27/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

28/40

Ring Oscillator

Single-shot signal Timing signal

Self replicating

D Q

select

3

3Dout1

SAR Logic

𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏

Multiplexer

𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏

Mu

ltiplexer

Mu

ltiplexer

START

STOP

𝜏𝑖

𝜏𝑖

Ring OscillatorⅠ

Ring OscillatorⅡ

29/40

Ring Oscillator

∆𝑇 ∆𝑇 ∆𝑇 ∆𝑇

START

STOP

Ring

OscillatorⅠ

Ring

OscillatorⅡ 𝑇𝑅𝑂2 𝑇𝑅𝑂

2

𝑇𝑅𝑂2

𝑇𝑅𝑂2 𝑇𝑅𝑂

2

𝑇𝑅𝑂2 𝑇𝑅𝑂

2 𝑇𝑅𝑂2

𝜏 𝜏 𝜏

𝜏𝑖

Signal

In 3-bit case

𝑇𝑅𝑂 2 = 7𝜏 + 𝜏𝑖

If τ = 1ns and τi = 1ns,

∆𝑇 ≤ 𝑇𝑅𝑂 2 = 7 + 1 = 1ns

⇒ Resolution of 1ns

𝑇𝑅𝑂 = 2(𝑛𝜏 + 𝜏𝑖)

∆𝑇 ≤ 𝑇𝑅𝑂 2

Periodic time 𝑇𝑅𝑂

30/40

Frequency Repetitive Clock Measurement

10𝑛𝑠 × 10 = 100𝑛𝑠

Ring Oscillator

2GHz

High Frequency

Low Frequency

0.01Hz

10𝑛𝑠 × 10 = 100𝑛𝑠

Ring Oscillator

High Frequency

Low Frequency

Ready

CLK1

(CLK2)

Ring Oscillator I

(Ring Oscillator II)

START

(STOP)

Ready

CLK1

(CLK2)

Ring Oscillator I

(Ring Oscillator II)

START

(STOP)

31/40

Simulated Circuit

D Q

CLR

D Q

CLR

D Q

CLR

D Q

CLR

D Q

CLR

D Q

CLR

D Q

CLR

D Q

CLR

D Q

D Q

D Q

D Q

D Q

Q

Q

Q

Q

START STOP

D0D1D2

Ring OscillatorⅠ Ring OscillatorⅡ

SAR Logic

Control Unit

MUX

D-FF(COMP)𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏 𝜏

𝜏𝑖𝜏𝑖

32/40

STA

RT

ST

OP

One-Shot Input Signal

∆𝑇 =5.2ns

33/40

Simulated Ring Oscillator WaveformsR

ing

Oscill

ato

rⅠR

ing

Oscill

ato

rⅡ

∆𝑇 ∆𝑇 ∆𝑇 ∆𝑇 ∆𝑇 ∆𝑇 ∆𝑇 ∆𝑇∆𝑇

34/40

D-FF Output Signal at Each Step

Δ𝑇 > 4𝜏= 100

Δ𝑇 < 6𝜏= 110

Δ𝑇 > 5𝜏= 101 Δ𝑇 =5.2ns

1

D0

D1

D2

1 1 1

0

0

1

0 1 1

0 0

35/40

Input Output Linearity

Proposed SAR TDC input & output have linear relationship

𝑇[ns]

Output

000

001

111

110

010

011

100

101

36/40

Outline

Research Objective

Conventional TDC Architecture

- Flash TDC

SAR TDC Architecture & Operation

Proposed SAR TDC – Analog Centric

Proposed SAR TDC – Digital Centric

- Architecture & Operation

- High Frequency

- Low Frequency

- Simulation

Conclusion

37/40

Discussion (1)

SAR TDC operates with external clock

Operation speed depends on measured clock

witch may be variable

Employing ring oscillators

Frequency is fixed

Reasonable as mixed-signal circuit design

38/40

Discussion (2)

Assumption

Two ring oscillators are completely matched

(oscillation frequencies are the same)

In reality they are not

Future work

Mismatch compensation or self-calibration method

(e.g. redundancy SAR algorithm)

39/40

Conclusion

Trigger circuit can generate repetitive signals

Ring oscillator helps the realization of full digital FPGA

One shot timing can be measured with SAR TDC

The remaining work Taking care of mismatches between two ring oscillators

40/40

Time is GOLD !!

TDC is a key.

Time continues indefinitely.