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Secrets of the DCM: Part 2 Steven Knapp General Products Division (v1.2, 11-OCT-2004) © 2004 by Xilinx, Inc. All rights reserved. NOTICE: This is an early draft of this presentation. Please visit the Xilinx Sales Partner Web (SPW) for the latest version.

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Secrets of the DCM (Part I) 2 Continuing On … This is a continuation of Secrets of the DCM: Part 1 that covered the following topics … – Overview of the DCM and its applications – Basic Delay Locked-Loop (DLL) operation – Clock Wizard – Clock Jitter

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Secrets of the DCM (Part I) 3 Workshop Objectives By the end of this class, you will … Understand how the DCM can phase shift clocks Understand how to generate other clock frequencies Learn how to build high-speed data interfaces Overcome various DCM limitations Legitimately say DCMs D ont C onfuse M e

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Lesson Four Phase Shifting

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Secrets of the DCM (Part I) 5 DCM DCM Block Diagram Digital Frequency Synthesizer Phase Shifter (PS) Input Stage Output Stage Delay Taps Status Logic Delay-Locked Loop (DLL) PSINCDEC PSEN PSCLK CLKIN CLKFB RST PSDONE STATUS[7:0] LOCKED CLK0 CLK90 CLK180 CLK270 CLK2X CLK2X180 CLKDV CLKFX CLKFX180

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Secrets of the DCM (Part I) 6 Phase Shifting The DCM provides various phase shifting options Dedicated phase shift outputs – These signals always maintain their relationship – Quadrant: CLK0, CLK90, CLK180, CLK270 – Half-Period: CLK2X/CLK2X180, CLKFX/CLKFX180 Fixed or Variable phase shifting – Adjust the phase relationship of all DCM clock outputs – Requires the DCMs DLL function

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Secrets of the DCM (Part I) 7 Quadrant Phase Shifts CLK90 and CLK270 only available in low-frequency mode 0 ° 1T CLK0 ° 90 ¼T CLK90 T 180 ° ½ CLK ° ¾T CLK ° 0 Phase Shift (degrees) Delay (fraction of clock period) Clock Period (T)

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Secrets of the DCM (Part I) 8 Half-Period Phase Shift CLK2X/CLK2X180 and CLKFX/CLKFX180 are similar 180 ° 0 ° ½T1T CLK0 CLK ° 0 Phase Shift (degrees) Delay (fraction of clock period) Clock Period (T) Highly useful for high-performance Dual-Data Rate (DDR) applications Guarantees precise half-period timing

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Secrets of the DCM (Part I) 9 DDR Example Clock Inversion at I/O Block Introduces Duty-cycle Distortion < 150 MHz 150 MHz Local inversion Precise Timing with Half-Period Phase Shifting Complementary

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Secrets of the DCM (Part I) 10 Review: The DLL DLL shifts the feedback until the Clock and the Feedback are in phase (0° phase shift) DLL controls the phase relationship, can be other than 0° Phase Detector Delay Line Clock Feedback Clock Feedback Positive Phase ShiftNegative Phase Shift

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Secrets of the DCM (Part I) 11 Fixed Phase Shifting Fixed phase shift value set during configuration, unchangeable Affects all DCM clock outputs Clock Outputs 0 Fixed Phase Shift + Limit Fixed Phase Shift – Limit The PHASE_SHIFT attribute determines the initial phase shift position. DCM initially asserts LOCKED with this phase shift value. DCM returns to this value upon RESET. CLKIN Phase Shift Type: Value: 23 Enter the Fixed phase shift value ns Degrees Resulting fixed phase shift in nanoseconds and degrees of phase shift NONE FIXED Choose FIXED Clock Wizard

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Secrets of the DCM (Part I) 12 Checking If Youre Awake A DCM has a FIXED phase shift of 10° What is the phase relationship between CLK0 and CLK90? What is the phase relationship between CLKIN and CLK0? What is the phase relationship between CLKIN and CLK90? Always 90° 100° = 90°+ 10° 10° = 0°+ 10°

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Secrets of the DCM (Part I) 13 Delay Line and Frequency Delay line has 255 taps, each 30 ps to 60 ps Maximum guaranteed delay line is 10 ns, ~40 ps per tap Clock inputs above 100 MHz can be shifted a full period (360°) – Shift resolution defined by individual tap delay Phase Detector Delay Line Clock Feedback Clock LIMIT

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Secrets of the DCM (Part I) 14 Shift Limits < 100 MHz Clock inputs 10 ns) Can only shift the clock a fraction of the clock period (<360°) Phase Detector Delay Line Clock Feedback Clock BEYOND LIMIT

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Secrets of the DCM (Part I) 15 Phase-Shift Limits T CLKIN > FINE_SHIFT_RANGE (Frequency < 100 MHz) T CLKIN = Period of CLKIN input clock T CLKIN FINE_SHIFT_RANGE (Frequency > 100 MHz) FINE_SHIFT_RANGE = Guaranteed length of delay line = 10 ns (per data sheet)

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Secrets of the DCM (Part I) 16 Variable Phase Shifting Clock Outputs The PHASE_SHIFT attribute determines the initial phase shift position. DCM initially asserts LOCKED with this phase shift value. DCM returns to this value upon RESET. CLKIN 0 Fixed Phase Shift + Limit Increment Phase Shift Value Decrement Phase Shift Value 0 Dynamic Phase Shift - Limit Dynamic Phase Shift + Limit After the DCM asserts LOCKED, the FPGA application can increment or decrement the present phase shift value using the Dynamic Phase Shift Control logic. PSEN PSINCDEC PSCLK PSDONE STATUS[0] Enable Increment/Decrement Phase Shift Clock Phase Shift Done Variable Phase Shift Overflow DCM Variable Phase Shift Control Fixed Phase Shift + Limit

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Secrets of the DCM (Part I) 17 Variable Phase Shift Timing LOCKED remains asserted during a variable phase shift operation

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Secrets of the DCM (Part I) 18 Phase Shift Mathematics Convert Negative Phase Shift to Positive Phase Shift Convert Phase Shift in Degrees to Phase Shift in Nanoseconds Alternate Phase Shift Solutions

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Lesson Five Clock Synthesis

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Secrets of the DCM (Part I) 20 Frequency Synthesis FPGA DCM F n -bits wide F n F High-speed serial data down-converted to slower parallel data Clock Division DCM F F m m -bits wide F Slower parallel data up-converted to high- speed serial data Clock Multiplication F Overclocked, time-shared logic DCM F F x Clock Synthesis Low cost design technique: Utilize full performance of the FPGA

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Secrets of the DCM (Part I) 21 Output clocks are phase aligned when using clock feedback via the CLKFB input. Frequency Synthesis CLKINCLK0 CLKFB DCM CLK2X CLK2X180 F=2 F CLKIN Clock Doubler 50% duty cycle Output available only when DLL_FREQUENCY_MODE=LOW De-skewed Clock F=F CLKIN 50% duty cycle when DUTY_CYCLE_CORRECTION=TRUE CLKDV Clock Divider F= F CLKIN CLKDV_DIVIDE Usually 50% duty cycle, depending on conditions CLKFX CLKFX180 Frequency Synthesizer F=F CLKIN CLKFX_MULTIPLY CLKFX_DIVIDE 50% duty cycle Does not require CLKFB input

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Secrets of the DCM (Part I) 22 Clock Synthesis Options Function DCM Output(s)Frequency Functional Unit Feedback Required? Clock Doubler CLK2X CLK2X180 2*F CLKIN DLLYes Clock Divider CLKDVF CLKIN /DIVDLLYes Frequency Synthesizer CLKFX CLKFX180 F CLKIN *M/DDFSOptional

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Secrets of the DCM (Part I) 23 Clock Divider CLKDV output requires the DLL and consequently the CLKFB feedback For Spartan-3, the CLKDV_DIVIDE attribute can be … – 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 9, 10, 11, 12, 13, 14, 15, 16 Lower output jitter when CLKDV_DIVIDE is an integer 50% duty-cycle output usually – Except when operating in High Frequency mode and CLKDV_DIVIDE is a non-integer ( i.e., 1.5)

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Secrets of the DCM (Part I) 24 Digital Frequency Synthesizer (DFS) CLKFX/180 output frequency controlled by fraction of two attributes – CLKFX_MULTIPLY = {2,3,4 32} – CLKFX_DIVIDE = {1,2,3,4 32} – Reduce to least common multiple CLKIN and CLKFX/180 outputs are more limited when also using the DLL – Using the DLL with the DFS limits the frequency range When used, DDL and DFS are phase aligned every – CLKFX_DIVIDE cycles of CLKIN – CLKFX_MULTIPLY cycles of CLKFX (from General Setup window)

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Secrets of the DCM (Part I) 25 Period Jitter on CLKFX Maximum period jitter for CLKFX output is characterized – Depends on multiply and divide values – Depends on input and output frequencies – Room temperature (25°C) – Nominal voltages – 50% CLBs and 40 simultaneous outputs switching at 100 MHz Jitter reported by Clock Wizard Spartan-3 jitter is effectively same as Virtex-II Pro

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Secrets of the DCM (Part I) 26 Spartan-3 CLKFX Jitter Equations CLKFX/CLKFX180 Output Frequency Peak-to-peak Period Jitter (Unit Interval) Peak-to-peak Period Jitter (ns) B = 0.05 when F CLKIN > 8 MHz, else = 0.04 K =CLKFX_MULTIPLY when CLKFX_MULTIPLY > CLKFX_DIVIDE, else = CLKFX_DIVIDE

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Secrets of the DCM (Part I) 27 Jitter on CLKFX (from General Setup window)

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Secrets of the DCM (Part I) 28 Cascading DCMs DCMs can be cascaded, but there is no dedicated routing available for this – The specified output jitter from the first DCM must not exceed the specified input jitter of the second DCM – CLKFX output practically eliminated for the first DCM stage due to jitter Keep second DCM reset until first DCM locks – WARNING: CLKDV does not toggle until LOCKED 1 – Requires three flip-flops to synchronize reset Function also available in the DCM Wizard

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Secrets of the DCM (Part I) 29 Clock Wizard Cascade DCMs

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Lesson Six System vs. Source Synchronous Design

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Secrets of the DCM (Part I) 31 System Synchronous Assumes a single, system-wide clock DLL path includes delay to compensate for clock path – Ensures no hold time requirement at the receiver System synchronous is the default assumption

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Secrets of the DCM (Part I) 32 Source Synchronous Clock generated by same source as data Typically, clock and data have same phase No compensation for clock path delay Receiver hold time is not an issue – Clock MUST be phase shifted in the receiver

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Secrets of the DCM (Part I) 33 System vs. Source Synchronous Timing DATA_IN SOURCE_SYNCHRONOUSSYSTEM_SYNCHRONOUS SOURCE_SYNCHRONOUS + Fixed or Dynamic Phase Shift Data capture window or data eye A little extra delay guarantees no hold time requirement Application phase shifts clock to middle of data eye

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Secrets of the DCM (Part I) 34 Deskew Adjustment in Clock Wizard (from General Setup window)

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Lesson Seven Optional Divide by 2

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Secrets of the DCM (Part I) 36 Dividing Input Clock by 2 Optional divide-by-2 function on CLKIN input – Dedicated high-speed toggle flip-flop inside the DCM Divide down a high-frequency clock to the range acceptable to the DCM Clean up a non-50% duty-cycle input to guarantee a clean 50% clock input – 50% to 60% duty-cycle improves DLL performance – Low/high duty cycles will stop the DCM working See data sheet for specs

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Secrets of the DCM (Part I) 37 Divide-by-2 in Clock Wizard (from General Setup window)

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Lesson Eight 622 Mbps Interface: Pulling it all together

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Secrets of the DCM (Part I) 39 Spartan-3 DCM Errata CLK2X feedback – Fixed on XC3S50 and XC3S1000 – Coming on remainder of Spartan-3 family in 2005 Negative phase shift (No longer an issue) Maximum DLL frequency in High Frequency mode

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Secrets of the DCM (Part I) 40 Problem Statement We want to build a LVDS transmitter operating at 622 Mbps with a 311 MHz input clock Issues: – Probably NONE DCM not required as long as the input clock has roughly 50% duty cycle Transmitter macros with embedded FIFO are available for the Spartan-3 in 4:1, 6:1, 7:1 and 8:1 SERDES factors Transmitter macros are also available without the FIFO for designs that use the DCM

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Secrets of the DCM (Part I) Mbit/second DDR Transmitter Example Clock output shown – data outputs are similar BUFG D0 D1 CE C0 C1 Q FDDRCPE OBUFDS 311 MHz FPGA IBUFGDS_DIFF_OUT 311 MHz 100 No resistors required for differential transmitters.

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Secrets of the DCM (Part I) 42 Problem Statement We want to build a LVDS receiver operating at 622 Mbps with a 311 MHz input clock Issues: – We have and need a 311 MHz clock – We need phase shifting, which requires the DLL – It is a DDR application Worry about jitter and duty-cycle distortion M I S S I O N I M P O S S I B L E ?

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Secrets of the DCM (Part I) 43 Typical 622 Mbps DDR DCM Configuration CLK0 CLK180 CLKFB CLKIN Phase shift control 311 MHz Exceeds DLL maximum frequency 311 MHz Exceeds DLL maximum phase shift frequency of 165 MHz DDR interface uses half-rate clock

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Secrets of the DCM (Part I) 44 Fundamental Problem 311 MHz forwarded clock coming right at us! Spartan-3 DCM supports shifting if CLKIN 165 MHz What to do? – Use the dedicated divide by 2 option at the input of the DCM Clock applied to the DLL in the DCM will be MHz Guarantees 50% duty-cycle DCM Friendly! – Phase shifting is performed with DCM in low-frequency mode – Use the 2X and 2X180 DCM outputs to reproduce to the required 311 MHz once phase shift has been applied

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Secrets of the DCM (Part I) 45 Example 622 Mbps DDR DCM Configuration CLK2X CLK2X180 CLKFB CLKIN Phase shift control CLKIN_DIVIDE_BY_2=TRUE 311 MHz Clock double reproduces original input frequency 311 MHz Reduces 311 MHz incoming clock to <165 MHz, placing DCM in low-frequency mode Because input clock is 311 MHz and further reduced with optional divide-by-2, DCM support phase shifting DDR interface uses half-rate clock CLK2X output supports up to 330 MHz! NOTE: Double jitter as well CLK2X output supports up to 330 MHz! NOTE: Double jitter as well Some devices require additional BUFG due to CLK2X feedback errata

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Secrets of the DCM (Part I) 46 CLK0 DCM CLK2X CLKIN CLKFB BUFG CLK2X180 CLKIN_DIVIDE_BY_2= TRUE CLK_FEEDBACK= 1X DLL_FREQUENCY_MODE= LOW MHz 311 MHz BUFG The CLKIN_DIVIDE_BY_2 option reduces the effective DCM frequency to MHz, which is within the DCMs frequency limits. IBUFDS Phase shifter operates at up to 165 MHz. D CE C FDCPE Q D CE C FDCPE_1 Q CLK2X, CLK2X180 outputs limited to 330 MHz, maximum. CLK0 feedback required for specific part numbers. Optionally, use general-purpose interconnect but phase alignment not guaranteed. 622 Mbps 100 LVDS/RSDS receiver termination resistor. FPGA IBUFGDS 311 MHz 100 DETAILED VIEW

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Secrets of the DCM (Part I) 47 Problem Re-Statement We want to build a LVDS receiver operating at 622 Mbps with a 311 MHz input clock Issues solved : – Phase shifting is indirectly available at 311 MHz – Receiver macros are available for Spartan-3 at 1:4, 1:6, 1:7 and 1:8 SERDES factors – Setup and hold eye parameters do still require characterization

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Secrets of the DCM (Part I) 48 XAPP462: The DCM Reference A comprehensive 68- page tree killer Updated for ISE 6.3i and latest Spartan-3 DCM knowledge

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Secrets of the DCM (Part I) 49 Questions?

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Secrets of the DCM (Part I) 50 Please Fill Out and Return the Feedback Forms! Steve Knapp Secrets of the DCM: Part 2 Forms are in the back of your FAE conference book Please return at back of the room Thank You!

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Secrets of the DCM (Part I) 51 Jump Point Return to last slide viewed Lesson 4: Phase Shifting Lesson 5: Frequency Synthesis Lesson 6: System vs. Source Synchronous Lesson 7: Optional CLKIN divide-by-2 Lesson 8: Spartan-3 622Mbps Design Example Session Evaluation Forms

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