A u t o m a t i n g S e q u e n t i a l C l o c k G a t i n g

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1 WHITEPAPER 1 A u t o m a t i n g S e q u e n t i a l l o c k G a t i n g Mitch Dale alypto Design Systems Introduction lock gating is a common Register Transfer Level (RTL) power optimization. Today, RTL synthesis tools identify and automate simple, combinational clock gating. However, greater power savings can be achieved through sequential clock gating optimizations. Until recently, sequential clock gating required manual identification and implementation by expert hardware designers. Now, with the availability of RTL power optimization tools, designers have access to advanced automated, low-power design techniques, eliminating the need for the often difficult and error-prone manual methods. This article describes sequential analysis and its application to clock gating. An example of sequential clock gating is given as well as a case study of reducing power in a digital signal correlation block using an automated RTL power optimization tool. The power of sequential clock gating Total power has both static and dynamic components. These are a function of voltage, load capacitance, switching frequency and static current across all nodes in a circuit. Power optimization targets reducing one or more of these variables. Understanding the cost / benefit of different power optimization techniques is difficult because of the complex interdependency between timing, area and power. A successful low-power design strategy ensures a cumulative reduction in total power without compromising timing and area requirements. Automated power optimization tools have the advantage of being able to evaluate numerous transformations against multiple constraints simultaneously. Reducing power in digital design is achieved through a variety of techniques including: lowering voltages, running at reduced frequencies, applying multiple v th cells and gating of clocks to name but a few. Of these, clock gating is the most common RTL optimization for reducing dynamic power.

2 WHITEPAPER 2 Most RTL designers understand how to write code so that RTL synthesis tools can recognize and insert combinational clock gating cells. Even after combinational optimization there remains additional power saving opportunities from sequential clock gating. Sequential clock gating takes advantage of existing inefficiencies in the RTL such as unused computations, data dependent functions and don t-care cycles. There are many forms of sequential clock gating transformations. These include observability based conditions such as data being written to registers that will not be used in subsequent clock cycles. Sequential analysis recognizes these unnecessary writes and eliminates them with clock gates. Sequential clock gating saves dynamic power by reducing power dissipation in the clock tree and associated registers. Additionally, switching activity in downstream combinational logic and registers is eliminated further reducing power dissipation. The keys to sequential clock gating are understanding the sequential nature of the design and identifying the correct enable conditions. Sequential Analysis of RTL designs Sequential analysis is the process of observing functional behavior over time. Applied to RTL, sequential analysis computes the temporal relationships between design states across multiple clock cycles. These relationships can be exploited to reduce power. Sequential clock gating uses sequential analysis to identify enable conditions that span multiple cycles. These conditions can become very complex, making them difficult to identify and implement. As a consequence, manual coding of sequential clock gating requires experienced engineers with considerable design knowledge. To demonstrate sequential clock gating, the diagram in figure 1 shows a non-optimized and clock gated datapath. In the example, data flows through two computational stages before being latched into the output register dout. The output of dout is held based on the signal vld_2. The clock gate on dout is a simple combinational substitution of the feedback loop. Sequential clock gating on d_1 and d_2 requires sequential analysis to propagate the data hold condition backwards, disabling the unused computations in previous cycles.

3 WHITEPAPER 3 d_1 d_2 din dout vld Non-optimized vld_1 vld_2 din d_1 d_2 vld dout With clock gating vld_1 vld_2 ombinational Analysis Sequential Analysis Figure 1: lock gated datapath By looking at the waveform corresponding to the clock gated datapath in figure 2, the yellow check marks show the cycles during which clock to the register dout is gated. Similarly the red check marks show the additional switching eliminated by sequential clock gating on d_1 and d-2. ase study Figure 2: Timing Diagram of lock-gated Design A correlator function is commonly used in pattern recognition algorithms. The correlator measures the similarity of two signals. In this case, it was used to find features in an unknown signal by comparing it to a known one at different times. The original design consumed 964 uw and already had 44% of the registers clock gated.

4 WHITEPAPER 4 To reduce power in the correlator, the design team added sequential clock gating to the RTL code using PowerPro G from alypto Design Systems. This automated RTL power optimization tool uses sequential analysis technology to identify clock-gating optimizations. Its cost-driven optimizations take into account area, timing and static power while evaluating sequential transformations. The design team used the software to run the correlator block and many sequential clock gating opportunities were identified. Figure 3 shows one of the sequential clock gating transformations found in the correlator. In this case the output of register Q is used only when output of register B is zero. Reg B The value of register B comes from register A in the previous cycle. Reg A Reg Q Figure 3 Sequential Analysis of correlator RTL code Understanding the temporal relationship between register A and register Q, it becomes clear that register Q can be clock gated whenever register B is zero. Identifying this sequential relationship and recognizing the opportunity for clock gating requires sequential analysis; power aware RTL synthesis tools won t find these clock gating opportunities. The tool generated new, functionally equivalent RTL code with register A driving the enable logic of register Q. Overall, more than 50 sequential transformations were implemented to produce a lowpower version of the correlator RTL. This code was run through RTL synthesis to measure power, timing and area. The results showed 24% power reduction with number of clock gated registers increasing by 60%. Results from RTL synthesis showed the total area was unchanged and timing slack went from 1402 ps in the original design to 1378 ps in the new design. onclusions Sequential analysis of RTL identifies powerful sequential clock gating optimizations that reduce dynamic power without changing functionality or impacting timing. PowerPro

5 WHITEPAPER 5 G automates the sequential clock gating process, reducing power without impacting design area or timing.