- About
- DAY 1 : PLL Theory and Tools setup
- DAY 2: PLL Design and post-layout simulations
- Future Scope
- Extra Reference Material
- Acknowledgements
This workshop presents a basic overview of Phase-Locked Loop IC design using Open-Source Google-Skywater 130nm node, using an intuitive approach to designing a simple PLL with little math and without diving into complex frequency domain analysis and control system theory. Starting with basic PLL concepts, this overview spans all the steps in the IC design flow - circuit design, simulation, layout, parasitics extraction, post-layout simulation and finally, it also briefly includes the use of the latest caravel harness to make tape-outs.
PLL is a control system implemented using an analog circuit, which is used in many applications like, FM modulation and demodulation circuits, motor speed controls and tracking filters, frequency shifting decodes for demodulation carrier frequencies, time to digital converters, Jitter reduction, skew suppression, clock recovery, to get a precise clock signal without frequency or phase noise. PLL actually mimics the reference means to have the same or a multiple of the reference frequency and a constant phase difference with it. In this workshop, our focus was to design a PLL for clock signal operations in different SoCs and ICs. PLL uses a Voltage Controlled Oscillator and gives an output that has superior spectral purity nearly equal to that of Quartz Crystal. This figure shows a PLL implemented on a chip with Raven SoC. Second picture shows On Semiconductor's PLL IC on a PCB.
PLL has five main components, and they are:-
- Phase Frequency Detector
- Charge Pump
- Low Pass Filter
- Voltage Controlled Oscillator
- Frequency Divider
It compares the phase difference between the input signal and output signal and generates a pulse signal according to it. The width of the pulse is used to determine the phase of the signal. As for width increases, phase also increases. The PFD output voltage is used to control the VCO such that the phase difference between the two inputs is held constant, making it a negative feedback system. The dead zone is one of a problem with PFD. The phase difference below which PFD output is not able to reach the desired logical level and fails to turn on the charge pump switches is called the dead zone. To tackle this, one should use precision PFDs, which overcome this issue.
The PFD generates a digital signal, but VCO uses an analog signal as input. So, here comes Charge Pump in the picture. The charge pump converts the digital signal of PFD into analog control signal which is given as input to the VCO. This can be done using the current steering circuit. When the UP signal is active the capacitor gets charged, this increases the voltage at charge pump output. When the DOWN signal is active, the capacitor gets discharged through the ground. This output voltage controls the VCO. An increase in voltage, speeds up the oscillator, while a reduction in voltage, slows down the oscillator.
The main issue with the charge pump is charge leakage, it keeps charging the capacitor even when inputs are off. This is tackled by using Low Pass Filter in the output. After that, the low-frequency dc voltage signal is sent into a dc amplifier, which boosts the signal level. After that, the VCO receives the enhanced signal. This will smoothens the output, without LPF, PLL cannot lock and mimic the ref signal. Even though, the primary function of the LPF is to maintain the stability of the system. Mathematically, only one capacitor at the output of the Charge pump makes the system unstable because if we see the frequency domain analysis of the circuit then there will be two poles in the transfer function which makes it oscillating and highly unstable. Thus an RC LPF is added at the output such that output gets stabilize. The value of Cx should be roughly around one-tenth of C. The loop filter bandwidth must be less than one-tenth of the highest output frequency.
VCO is the heart of PLL and it generates the desired high-frequency clock output. Voltage-controlled oscillators are the actual parts that produce alternating digital clock signals. The most common one is the Ring oscillator. It contains an odd number of inverters and flips the output. So, VCO can be implemented using simple inverters. It is necessary to design this VCO such that the range of output frequency we want for the PLL is within the range of frequency the VCO can produce properly. The frequency of this clock signal can be controlled by the input voltage. The frequency depends on delay and delay depends on the current supplied. The LPF output serves as a VCO control signal. Current sources are used at the top and the bottom with the Vctrl voltage to control the ring oscillator. An analog signal is produced by the VCO and its amplitude is proportional to the LPF output amplitude.
A frequency divider is used to convert the whole system into a frequency multiplier. A PLL with a frequency divider on its feedback loop is called a clock multiplier PLL. Such a PLL can make clock signals which are multiples of the reference signals. A toggle flip flop generates the clock which has twice the time period of the input clock given to it, that is we obtain the clock having half the frequency of the input clock provided to the frequency divider. For the 8x clock multiplier, we should divide the output clock by 8 to generate a feedback clock. For obtaining one-eighth of frequency, we should cascade three toggle flip flops. When the frequency difference of each input is 0, which shows a consistent phase difference, the loop is locked. If the two signals are shifted by 180, the output voltage must be at the highest. The output voltage generated will be zero in the absence of an input signal to allow the VCO to function at a fixed frequency. This frequency is referred to as the oscillator's operating frequency. Lock Range: The range of frequencies for which PLL can maintain lock, already in the locked state. Capture Range: The frequencies for which PLL is able to lock in from an unlocked state. Settling Time: The time within which the PLL is able to lock in the form of an unlocked condition.
The beauty of open-source is that it is free and available to everyone. In the whole workshop, I have used only two tools and one PDK. Ngspice is an open-source mixed-signal electronic circuit simulator used in circuit design, pre-layout, and post-layout simulation. Magic VLSI Layout Tool is an open-source circuit layout editor, used in circuit layout. Google SkyWater PDK is an open-source process design kit, used in physical design. To install these open-source tools, I would suggest you go to their websites:-
IT IS RECOMMENDED TO USE LINUX-BASED OS RATHER THAN ANY OTHER KERNEL-BASED OS.
- Technology Node = 130nm
- Supply voltage (VDD) = 1.8V
- Minimum Input Frequency = 5MHz
- Maximum Input Frequency = 12.5MHz
- Multiplier = 8 times
- Jitter (RMS) < 20ns
- Duty cycle = 50%
- Specifications of the IC
- SPICE level circuit development
- Pre-layout simulation
- Layout development
- Parasitic Extraction
- Post-layout simulation
- Tapeout: A brief Introduction
On day 2, we are going to design different components of PLL at the transistor level, then we will simulate it in Ngspice. Check all the desired outputs, and then proceed with Layout in Magic followed by parasitics extraction. After all this, we will again do the post-layout simulation in Ngspice. After all of this, we will combine different components of PLL in a single layout design. At last, we will integrate the layout design of PLL with Caravel SoC. In the end, we will discuss about tape-outs, and know-how any individual can send their own layout design to The Google-Skywater open MPW shuttle program, to be manufactured on a silicon wafer. But it depends on the choice of Google and Skywater to whether manufacture or not.
The transister level design of Phase Frequency Detector, Charge Pump, and Voltage Controlled Oscillator, respectively are as follows:
Pre-Layout Ngspice Simulation of Phase Frequency Detector Circuit, with DOWN and UP Signal:
Red: Clock 2, Blue: Clock 1, Orange: Up Signal, Green: Down Signal
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Pre-Layout Ngspice Simulation of Charge Pump + Low Pass Filter Circuit, with Charging, Discharging, and Leakage:
Red: Charge Pump Output Voltage, Leakage: 40uV increase every 1us
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Pre-Layout Ngspice Simulation of Frequency Divider Circuit:
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Pre-Layout Ngspice Simulation of Voltage Controlled Oscillator:
Pre-Layout Ngspice Simulation of Full PLL Circuit:
Red: Reference Clock, Blue: Output Clock Divided by 8, Yellow: Down Signal, Brown: Up Signal, Pink : ChargePump output
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Close-UP Simulation Result of Full PLL Circuit:
Layout design of Phase Frequency Detector Circuit:
Layout design of MUX:
Layout design of Charge Pump + Low Pass Filter Circuit:
Layout design of Frequency Divider Circuit:
Layout design of Voltage Controlled Oscillator:
Layout design of all components integrated into a single design i.e Full PLL Layout Design
Post-Layout Ngspice Simulation of Phase Frequency Detector Circuit, with DOWN and UP Signal:
Red: Clock 1, Blue: Clock 2, Orange: Up Signal, Green: Down Signal
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Post-Layout Ngspice Simulation of Charge Pump + Low Pass Filter Circuit, with Charging, Discharging, and Leakage:
Orange: Charge Pump Output Voltage, Red: Up Signal, Blue: Down Signal, Leakage: < 0.05V in 100us
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Post-Layout Ngspice Simulation of Frequency Divider Circuit:
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Post-Layout Ngspice Simulation of Voltage Controlled Oscillator:
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Post-Layout Ngspice Simulation of Full PLL Circuit:
Red: Reference Clock, Blue: Output Clock Divided by 8, Yellow: Down Signal, Brown: Up Signal, Pink : ChargePump output
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Close-UP Simulation Result of Full PLL Circuit:
For any design to be tape-out ready there are more requirements than just having model files, simulation results, layout designs, and .gds files.For Example, a proper GPIO (cells that enable the IP to be interfaced with the external world) is needed for connecting the IP pins to the package (the final DIP or Surface Mount case from which the IC comes in from the Fab).To meet these requirements either we need to take care of them individually by ourselves (which may get complicated and time-consuming) or, we can choose a vehicle for enabling our IP to meet the requirements to go through the fabrication process. Here we will be using Efabless Caravel SoC template as the Vehicle. This is the datasheet of the Caravel SoC from Efabless, and these are the parts involved in it:
The Mega Project Area (MPRJ) or 'user_project_wrapper' or in other words 'the container' is where we will place and route our design. Basic Steps Overview:
- Initial setup.
- Place and Route the IP inside the container (keep in mind to not have any DRCs).
- Verify Connectivity.
- Integrate the container onto the Caravel SoC.
- Check if everything is as expected including DRC (Precheck).
- Work on getting specifications and change your designs according to them.
- After changing in your design, you can make this design as your own IP.
- Integrate the IP with Caravel SoC, and send it to Google-Skywater Open MPW Shuttle Program, to get a real IP on a silicon wafer.
- Magic command reference. Here, press the 'Using Magic' button
- Caravel SoC Documentation
- NPTEL Course on Analog IC Design by Nagendra Krishnapura
- NPTEL Course on RF Integrated Circuits by Shouri Chatterjee
- An Improved Performance Ring VCO: Analysis and Design
- A Novel Phase Frequency Detector for a High-Frequency PLL Design
- A pA-leakage CMOS charge pump for low-supply PLLs
- A simple and high performance charge pump based on the self-cascode transistor
- Gain-Boosting Charge Pump for Current Matching in Phase-Locked Loop
- Comparative Analysis of High Speed FBB TSPC and E-TSPC Frequency Divider at 32 nm CMOS process
- Lakshmi S, MS in ECE, Georgia Tech
- Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd.
NOTE:
- I have used my own Ubunutu OS to run all the SPICE Models and Layouts.
- This workshop was conducted successfully for the first time between July 31, 2021, to August 01, 2021.