A Fast-Locked All-Digital Phase-Locked Loop
for Dynamic Frequency Scaling
Dian Huang
Ying Qiao
Motivation

CMOS IC technology keeps further scaling


SoC benefits from All-Digital PLL (ADPLL) designs
Dynamic frequency scaling in CPU

Fast-locked phase-locked loop (PLL) for clock generation

Tradeoffs between locking time and clock jitter

We will focus on


ADPLL design with bang-bang phase detector (BBPHD)
Digitally controlled oscillator (DCO) frequency-search using algorithms
with Successive-Approximation Registers (SAR)
UC Berkeley - 2
ADPLL Architecture

Conventional vs. Proposed ADPLL Architecture
Conventional BBPHD ADPLL
SAR Delay-Search
Ref_clk
MUX2
MUX1
DCDL
β
Ref_clkd
BBPHD
+
α
BBPHD
1
1
/SET
D
Q /CLR
SAR Delay-Search
DCO
0
+
Q
UserDefined
Clock_out
0
0
1
MUX3
reset
PI Controler
activate
0
1
Frequency
Serch Mode
SAR Frequency Search
Divide by 16
Proposed BBPHD ADPLL with SAR
UC Berkeley - 3
Design Considerations

Tradeoff exists between frequency
phase locking time and output
clock jitter performance
𝑡𝑙𝑜𝑐𝑘 =
𝜋
1
2𝜋
× 𝛽𝑘𝑣𝑐𝑜 − 𝑓𝑜𝑓𝑓 𝑓𝑟𝑒𝑓
𝑓𝑟𝑒𝑓
𝑓𝑟𝑒𝑓 – reference clock frequency
𝑓𝑜𝑓𝑓 – initial frequency error
𝛽𝑘𝑣𝑐𝑜 – system loop gain
𝛼 1 + 2𝐷 β - Proportional path gain
𝑞= 𝛽−
2
α – Integral path gain
𝑁𝑘𝑣𝑐𝑜
Δ𝑡𝑝𝑝 =
1 + 𝐷 4 𝛼 3 + 4 1 + 𝐷 3 𝛼 2 𝑞 + 8 1 + 𝐷 2 𝛼𝑞 2 + 8 1 + 𝐷 𝑞 3
2
4𝑞
UC Berkeley - 4
Fast-locking Techniques


Simultaneous frequency and phase locking

Yang, JSSC ’10 – adaptive loop gain

Hung, Trans Circuit & Syst. ’11 – modified bang-bang algorithm
Detangled frequency and phase locking

Chung, JSSC ’11 – BSA frequency search + TDC phase locking
UC Berkeley - 5
Proposed ADPLL Architecture
SAR Delay-Search
MUX2
MUX1
Ref_clk
DCDL
β
Ref_clkd
BBPHD
+
α
BBPHD
1
1
/SET
D
Q /CLR
SAR Delay-Search
DCO
0
+
Q
UserDefined
Clock_out
0
0
1
MUX3
reset
PI Controler
activate
0
1
Frequency
Serch Mode
SAR Frequency Search
Divide by 16
UC Berkeley - 6
SAR-based Frequency Search
Set DCO[MSB]=1
ref_clk=0
1->MSB
activate DCO & divider
Tref_clk>TDivider?
ref_clk=1
0->MSB
deactivate DCO & divider
Reference clock
Set DAC[MSB-1]=1
ref_clk=0
1->[MSB-1]
activate DCO & divider
Tref_clk>VDivider?
0->[MSB-1]
Divider output
Oscillator output
1->[LSB]
Tref_clk>VDivider?
ref_clk=1
deactivate DCO & divider
Frequency serch done
UC Berkeley - 7
0->[LSB]
BBPHD UP signal
SAR-based Delay Search



Falling edge of divider output does not align with that of
reference clock due to delay.
Add extra delay to reference clock
Once frequency search is done, CPU designer can choose
whether input clock of PLL is reference clock or its delay
version based on jitter and locking requirement.
DELAYN[0]
DELAYN[1]
DELAYN[2]
REF_CLK
DELAY[0]
DELAY[1]
UC Berkeley - 8
DELAY[3]
Vdd
REF_CLKD
DELAY[2]
Locking Procedure



2 cycles delay-search, 10 cycles frequency-search for a 10 bit DCO.
Remained frequency error and phase error are tiny.
Locks at 790ns
UC Berkeley - 9
Five Stage DCO
 DCO consists of 960 tri-state buffer: 64 row with
each row has 15 buffers.
 Five extra tri-state buffer are used to drive each to
node to either Vdd or ground during reset for fast
start-up
 DCO Frequency Range: 0.42GHz ~ 12GHz
UC Berkeley - 10
PI Controller



With proposed frequency-search algorithm, small 𝛃 and 𝛂 can
be chosen.
𝛃 needs to be several time larger than 𝛂 for stability, but want 𝛃
to be 1 or 2 to minimize the quantization noise.
Integral path code increment by 1 only when it can increment
by 4
UC Berkeley - 11
Performance
Key Parameter
Technology
45nm
Locking Time
790ns
Jitter RMS
1.32ps
Jitter peak-to-peak
4.56ps
Power
16mW@4.5GHz


Achieves 790ns locking time while maintaining 1.32ps rms jitter.
Peak-to-peak jitter is too optimistic.
UC Berkeley - 12
Comparison
[10] Hsu
[8] Kim
[9] Chung
[2] Tierno
This Work
CMOS Process
0.18µm
0.13µm
65nm
45nm
45nm
Core Area
0.14 mm2
0.2 mm2
0.07mm2
0.07 mm2
N/A
Power
26.7mW@600MHz
16.5mW@1.35GHz
1.81mW@520MHz
NA
16mW@4.5GHz
Output Range
62~616MHz
0.3~1.4GHz
90~527MHz
0.8~12GHz
0.42~12GHz
Locking Time
NA
3.5µs
NA
*46 µs
790ns
Jitter RMS
7.28ps @600MHz
3.7ps @1.35GHz
8.64ps @527MHz
1ps @5GHz
1.32ps @4.5GHz
Jitter peak-to-peak
56ps @600MHz
32ps @ 1.35GHz
NA
NA
4.56ps @4.5GHz
UC Berkeley - 13
Conclusion



Proposed ADPLL realizes fast-locking without sacrificing jitter
performance.
790ns locking time demonstrates that it is suitable to dynamic
frequency scaling.
Future work includes ADPLL with smooth frequency change
so that CPU does not needs to stall its instructions.
UC Berkeley - 14