Phil Lehwalder ECE526 Summer 2011 Dr. Chiang

Transcription

1 Phil Lehwalder ECE526 Summer 2011 Dr. Chiang

2 PLL (Phase Lock Loop) Dynamic system that produces a clock in response to the frequency and phase of an input clock by varying frequency of an internal oscillator. DLL (Delay Lock Loop) - Dynamic system that produces a clock in response to the frequency and phase of an input clock by varying frequency the delay of an internal delay line. Can be modeled as linearized system for simulation if: Near locked condition minimizes non-linear behavior Loop BW << input f in sampled system therefore Nyquist applies

3 V0ltage Controlled Oscillator (VCO) f out V control When in or near lock: f out changes quickly with VCTRL, but f error takes time to correct Gain of this block is Hz/V f V out CTRL K VCO Ring oscillator common: V CTRL ( t) V V ( t) fout( s) 2 K V ( s) s CTRL LOCK VCO CTRL f out = inverse of the 2x round trip delay f out controlled via inverter current, voltage, or capacitance Problem: Tuning and gain varies widely with PT

4 All methods vary R, C or I Used for PLL VCO control and for DLL delay line control

5 Divider lowers feedback frequency by factor of N VCO has to run N time higher than f in to lock This is one way to make a clock multiplier N=1 f out = f in & output tracks input in frequency & phase N=1 makes a zero delay buffer Divider must be able to function at VCO f max of PLL may fail to lock at startup

6 Compares phase of f in & feedback signal Generates PWM error signal to loop filter based on phase delta of f in & f out over 180 degree phase window Output %D min = 0% f in & f out in phase Output %D max = 100% f in & f out 180 out of phase Becomes VCO control signal post loop filter Sensitive to clock jitter and %D variation V f PD ERR ( s) K ( s) PD VDD

7 Sequential PD drives the loop filter based on phase delta of f in & f out Makes UP & DOWN signals to drive charge pump that drives loop filter Signals are only high until both are high, then asynchronously reset. Good over +/-360 phase window Less sensitive to jitter & duty cycle variation since flops are edge triggered Preferred phase detector for clock generation Faster to lock & more reliably than XOR PD

8 I PD ( s) I CP K PD f ( s) 2 ERR ( s ) ( s ) Phase Frequency Detector drives charge pump V I CTRL PD K s I K P 1 sc R R,C, & C2 form shunt filter converts charge pump I OUT to VCO V CTRL R&C set the bandwidth C2 smooth charge pump ripple avoids charge pump induced jitter C2 << C C2 can be ignored since RC set the loop BW roll off point Loop filter sets VCO adjustment per unit phase error Kp term is proportional instantaneous change Ki term is fine error correction over time Integral Filter forms a PI control system that sets BW & system stability

9 PLL system is a 2 nd order system Loop is biquadratic low pass BW selected depending on dominant jitter/noise source On chip noise set BW high to compensate Off chip clock noise lower BW to average out or reject noise BW should be f in /10 so continuous time rules apply Avoids alias induced jitter since this is really a sampled system? Damping factor < to avoid ringing on input step change

10 Similar overall structure to PLL, but VCO replaced with adjustable delay and no FB divider Cannot perform frequency multiplication, but allows for multiple skewed or phase clock outputs Delay gain = seconds/v Can be build using the same elements used to make the VCO adjustable DLL system is a 1 st order system Filter is single pole (integrator) not prone to instability Output = delay version of input prone to input noise issues

11 Communication BW increasing over time Market driving smaller devices Cannot just add pins Speeds > 1GHz becoming common place Significant challenges of high speed I/O Generate fast pulses & detect logic state reliably No ideal transmission line effect in play (lossy) Distinguishing one bit from next when transmitted in succession

12 Point to point becoming more common vs. multi-drop configurations Easier to achieve good signal integrity (SIE) PCIe, SATA, USB, Fire Wire, GB LAN Paralleled point to point lanes increase BW Low Frequency (channel length << l) system = lumped Channel ~ideal t-line effects die out in small time relative to signal period Conductor act like equipotential net signal same at both ends of the channel Tx Driver see Rx load impedance High Frequency(channel length > l) system = distributed t-line effects in play tf & tf << signal propagation delay Signal propagate like a wave signal not same at both ends of the channel Tx driver see t-line Z 0 (characteristic t-line Impedance) as load Impedance matching critical to signal integrity (SIE)

13 Signal Propagation: n m e e e r relative permittivity known as dielectric constant of material (e.g.fr4) e 0 permittivity of free space m 0 relative permeability of free space Channel Characteristic Impedance (Z 0 ): Structure dependent Microstrip routed on outer layers of package or board 0.475k w t Stripline - routed on inner layers of package or board 1 o r 0 e r 1 m e Z 60 4h ln o 8 Z 60 4h ln o k w t o 0 c e r

14 Z TERMINATION Z O (Impedance mismatched case) Signal energy not completely absorbed at load Some energy reflected back towards the source Reflected signal set by reflection coefficient G Z L < Z O (-) reflection Ring back & non-monotonicity Z L > Z O (+) reflection Overshoot, undershot & non-monotonicity At high data rates (bit time << t PROPAGATION ) reflection from one bit can interfere other bits Inter-symbol Interference (ISI) Z TERMINATION = Z O (Impedance matched Case) No reflections Tx can send bit n+1 before bit has reached the Rx no ISI V G V REFLECTED INCIDENT Z Z L L Z Z O O

15 Dispersion: Frequency dependent attenuation of signal Results in distorted signal at Rx due to attenuation of high frequency content Source of issue is dielectric loss & skin effect Depending on signal pattern signal distortion can result in ISI Can be corrected with de-emphasis (equalization)

16 Crosstalk: Capacitive or inductive coupling between near by signals where energy from an aggressor net is coupled into a victim net. Single ended systems: Coupling must be removed to fix routing change. Can reduce with guard ground traces to shield signals of aggressors Differential signaling crosstalk is common mode so cancels out at Rx Error correction coding, error detecting codes, and sequencing detection can be used to detect data in the presence of crosstalk Less common in HS I/O used in digital communications systems.

17 Return Path Noise: Power path that results in difference between the Tx & Rx sides due to non-ideal VCC and GND same effect as noise at the Rx input Ground Bounce: Ground noise induced by driver conduction that has the same effect as noise at the Rx input. VCC Droop: VCC noise induced by driver conduction that has the same effect as noise at the Rx input. Return path noise manifest itself in many ways Signal jitter, V OH transient droop, VOL transient ring back Multi-signal interfaces can suffer from simultaneous switching noise (SSN) Return path noise from signal(s) interferes with other signals

18 Driver Z OUT varies widely over the transfer characteristic Need for external termination to minimize reflections and ISI a). Pull only driver - current mode signal converted to voltage by R=Z O (ex. GTL). b). Push-Pull CMOS - Standard series termination where Rout << Z O R=Z O c). Pull Only Terminated at both ends, but swing is cut in half for a given current mode signal Buffers can be RCOMP trimmed to make buffer Rout = Z O

19 Differential output signals 180 out of phase Increases noise immunity Common mode noise canceled out at Rx Need for external termination to minimize reflections and ISI Typical swing < 1V: USB2.0 = SATA (Gen2) = 3Gbpd PCIe = 1.2Gbps LVDS = up to 3.125Gbps a). Pull only differential current mode driver Steers Isw between D+ and D- outputs Signal converted to voltage by R=Z O at Rx end b). Push-Pull CMOS driver Steers Isw between D+ and D- outputs Signal converted to voltage by R=Z O at Rx across D+ and D- lines\ D+ transistors on for output =1 D- transistors on for output =0

20 AC Coupling due to unmatched DC operating point between Tx & Rx May require 8b/10b encoding to minimize LF information & signal loss across the caps Programmable Drive strength Switch in or out parallel legs to change drive strength of buffer Allows for output swing adjustment Can aid in compensating for signal loss Can help with signal integrity issues Programmable Slew Rate (edge control): Parallel output legs switched in over time Reduced edge rates lower EMC emissions Transmitter De-emphasis (Equalization): Attenuates LF information to make frequency response in loss t-line flat Corrects distortion due to dispersion Secondary buffer not active for consecutive data (attenuates low frequency information)

21 Multi-transmitter time-interleaving Multiple transmitters in parallel driven by shifted clocks Example: 4 x 4Gbps transmitter in parallel with clock driven in quadrature Each runs at 4Gbps combined Tx rate is 16Gbps

22 Multi-level Transmitter Single transmitter encodes multiple bits Uses paralleled buffers with weighted buffer strengths Example: 4 level transmitter encodes 2 bits on a single wire

23 Receivers: Core of receiver is a flip-flop that samples the data at the appropriate time Sub-1v HS differential interfaces may use an SA-FF low signal swing FF Time interleaving I/O uses staggered receivers based on tapped DLL clocks Multi-level signaling interfaces use ADC as Rx Bit Error Rate (BER): Main measure of performance for I/O Statistical measure of the probability of a transferring an erroneous bit Typical rates for HS I/O are in the to Number is very low because most logic has little redundancy

24 Low frequency interfaces typically use handshaking for bit transmission Tx sends bit and Rx notifies or ACKs that bit is received Only one on transmission path at any point in time HS I/O use time a marker for bits Tx clock source is a PLL or DLL minimize clock edge variation (absolute jitter) Bit transmitted at a constant time intervals fixed bit time No handshaking signals between the Tx and Rx Tx and Rx must agree on the timing of each bit bit time precisely controlled Rx must sync with the transmitter to sample data at middle of signal eye Example: NRZ encoded differential HS I/O Logical 1 = high (+ polarity) Logical 0 = low level (- polarity)

25 Clock transmitted with the data on separate on separate I/O channel Clock is typically common to several I/O channels Discrepancies in channel propagation delays lead to timing errors at Rx Forces tight layout matching common to DDR, SATA, PCIe, USB, Firewire etc Single Data Rate System (SDR) data samples on one edge of clock Double Data Rate System (DDR) data sampled on both clock edges Clock in quadrature (90 out of phase) with data for SDR & DDR

26 Clocks typically need to be buffered on Rx side Conventional buffers Generates internal skew shifting data sample point away from middle of eye Variation in delay across buffers results in substrate noise that can increase jitter Fix: Use a PLL /DLL to lock to source CLK acts like zero-delay buffer Interface clock may be transmitted in phase with data PLL with multi-tap VCO used to recover proper clock to data phase

27 At very high transfer rates keeping CLK & DATA aligned becomes difficult Bit time small minor clock to data skew is a significant portion bit time Skew due to systematic variations in tolerances in Z O, propagation delays etc Phase is not reliable, but frequency is reliable need Rx that can realign clock to data phase

28 In some cases it may be hard to accommodate the added clock channel Instead Tx & Rx agree on a bit transfer rate and Rx uses local clock to recover data Local Rx clock has tolerance that causes it to vary over time This system has uncertain phase and uncertain clock frequency at Rx side Common method is for Rx to use a PLL to generate a sampling clock Clock acts as oversampling clock that is centered on the data eye Example: Rs232 & RS422 serial interfaces (30+ years old)

29 True Random Number Generator uses thermal noise driving a VCO to generate random bits that are sampled via a shift register to form a random bit sequence Chip ID: Utilizes process variation as a chip finger print good for tracking illegal re-branding and counterfeiting Cross coupled inverters who's final state after reset de-assertion is set by device mistmatcing due to process variation, but is consistent for a specific part.

30 Phil Lehwalder ECE526 Summer 2011 Dr. Chiang

31 Can be modeled in terms of their phase: CLK = 1 for f(t)mod 2 < CLK = 0 for f(t)mod 2 Signal phase is linearly accumulated over time Neglects non-ideal effects (jitter, Power supply noise..etc)