Lecture #34 Page 1 ECE 4110–5110 Digital System Design Lecture #34 Agenda 1.Timing 2.Clocking Techniques Announcements 1.n/a

Lecture #34 Page 2 Timing Pipelined Logic - we can break up the combinational logic delay by inserting registers between each level. - this reduces combinational logic delay, but we don't get the information right away Latency - the time you need to wait for the data to come out of the pipeline Activity - the percentage of time that the signals are switching. More activity means that the pipeline will continually output data and Latency is not a problem. - if the signals are not very active (i.e, a transition here and there), then the pipeline overhead might not be worth it.

Lecture #34 Page 3 Timing Pipelined Logic - for a pipeline to improve data throughput, the timing for a burst of data must be better than: (T orig )·(n) < (T pipe )·(L c + n -1) where: T orig = period of fastest clock frequency with unbroken combinational logic T pipe = improved period when pipelining (typically considers one logic level) L c = the number of latency clock cycles necessary for the data to appear at the output of the pipeline n= number of consecutive pieces of meaningful data - In an ideal case, the data inputs would always be meaningful (which is sometimes the case) This would mean that you would only incur Lc once at the start-up of the system - However, some protocols start and stop the system to save power. This means you continually have to incur Lc each time you start the system.

Lecture #34 Page 4 Clocking Clocking - in a synchronous system, the clock is the trigger for all data movement & manipulation - the clock is assumed to arrive at the CLK inputs of each Flip-Flop at the same time - in reality, this is not the case. Physical factors create mismatches in when the clock arrives at each register input - this timing error is called "Clock Skew" - this can be caused by: 1) Trace mismatching 2) Process variation- traces are wider on one side = different RC 3) Power Supply Variation- clocks distributed using buffers are sensitive to power

Lecture #34 Page 5 Clocking Clock Trees - an H-Tree is a technique to distribute clocks to all regions of a chip with equal delay - the rule is that each time an H is added to any end-node, an H is added at every other end-node. - this keeps the RC's the same for all paths

Lecture #34 Page 6 Clocking Clock Buffering (Clock Repeating) - as traces get small, their Resistance and Capacitance changes - we use a "Scaling Factor" (S) to describe the change in characteristics as we scale IC feature sizes - S is > 1 and typically between 1 and 2 (if S=2, then we reduce all sizes by 50%) BeforeAfter QuantityScalingScaling Widthww’ = w/S Spacingss’ = s/S Thicknesstt’ = t/S Interlayer oxide heighthh’ = h/S ws t h

Lecture #34 Page 7 Clocking Clock Buffering (Clock Repeating) - we can use S to see how the RC delay of traces scales - interconnect delay can be considerable and dominating in modern IC's Resistance scales following : S 2 w t h h Capacitance scales following : 1 Delay scales following : S 2 Horrible!!! OK Horrible!!!

Lecture #34 Page 8 Clocking Clock Buffering (Clock Repeating) - R & C delay is also proportional to the Length of a trace (L) R = L·(  /t·w) C = L·(  r w/h)  int  RC  L 2 - this means there is a quadratic dependency between delay and trace length - this is a major problem in clock trees w t h h L

Lecture #34 Page 9 Clocking Clock Buffering (Clock Repeating) - a technique to break up the delay of long traces is to insert "repeaters" - each repeater and trace segment has a fixed delay - this allows the total delay of the trace to scale linearly t total = n·(t repeater + t race ) where n= the number of repeater/trace segments t trace = delay of the trace segment t repeater = delay of the buffer - optimal sizing is where t trace = t repeater

Lecture #34 Page 10 Clocking Clock Buffering (Clock Repeating) - advantages of clock repeating: 1) linear scaling of delay with length 2) signal strength at end-node is good - disadvantages of clock repeating power consumption of active buffers