Lec-12_Clocking-a

Lec-12_Clocking-a - EE M216A .:. Fall 2010 Lecture 12...

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EE M216A .:. Fall 2010 Lecture 12 Clocking Issues: Distribution, Energy Prof. Dejan Markovi ć ee216a@gmail.com Clock Distribution ± Goals: Deliver clock to all memory elements with acceptable skew Deliver clock edges with acceptable sharpness ± Clock network design is another one of the big challenges in the design of a large chip ± Clocks are generally distributed via wiring trees ± Want to use low resistance interconnect to minimize delay D. Markovic / Slide 2 Want to use low resistance interconnect to minimize delay ± Use multiple drivers to distribute driver requirements Use optimal sizing principles to design buffers Clock lines can create significant crosstalk EEM216A .:. Fall 2010 Lecture 12: Clocking Issues | 2
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Issues in Clock Distribution Network ± Skew Process, voltage, temp. Data dependence Skew from Clk distribution Noise coupling Load balancing ± Power, CV 2 f – (no ½ or α ) Clock gating ± Flexibility/Tunability Compactness D. Markovic / Slide 3 fit into existing layout/design ± Reliability Electromigration See videos from P. Restle (IBM) on classwiki EEM216A .:. Fall 2010 Lecture 12: Clocking Issues | 3 Clock Distribution Methods ± RC Tree Less capacitance More accuracy Fl ibl i i ± Grids Reliable Less data dependency Flexible wiring Tunable (late in design) D. Markovic / Slide 4 Shown here for final stage drivers driving F F loads EEM216A .:. Fall 2010 Lecture 12: Clocking Issues | 4
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RC Trees, Clock Distribution ± Observe: only relative skew is important H Tree X Binary D. Markovic / Slide 5 EEM216A .:. Fall 2010 CLOCK H-Tree Network Asymmetric trees that match RCs can be used Lecture 12: Clocking Issues | 5 More Realistic H Tree D. Markovic / Slide 6 EEM216A .:. Fall 2010 [Restle98] Lecture 12: Clocking Issues | 6
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The Grid System ± No RC matching ± Large power GCLK clock DEC Alpha Examples Driver Driver GCLK GCLK grid 21064 21164 D. Markovic / Slide 7 EEM216A .:. Fall 2010 Driver GCLK 21264 Lecture 12: Clocking Issues | 7 Examples of Distribution H Tree Asymmetric RC Tree Grids DEC [Alphas] Serpentines Intel x86 [Young ISSCC97] IBM D. Markovic / Slide 8 EEM216A .:. Fall 2010 Lecture 12: Clocking Issues | 8
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Examples of Processor Chips DEC Alpha 21064 clock spines DEC Alpha 21064 RC delays Note: reverse Z axis DEC Alpha 21164 RC delays DEC Alpha 21164 RC local D. Markovic / Slide 9 EEM216A .:. Fall 2010 for global distribution Spine + Grid delays Lecture 12: Clocking Issues | 9 Example: EV5 (Alpha 21164) Clocking (1995) t rise = 0.35ns t skew = 150ps t cycle = 3.3ns ± Single phase clocking Clock Drivers Single phase clocking ± 2 distributed driver channels Reduced RC delay/skew Improved thermal distribution 3.75 nF clock load 58 cm final driver width D. Markovic / Slide 10 EEM216A .:. Fall 2010 ± Local inverters for latching ± Conditional clocks in caches to reduce power ± More complex race checking ± Device variation Lecture 12: Clocking Issues | 10
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