Autonomous Fine-Grain Power Gating for Low-Power FPGA

1. Based on: A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. VOL. 19, NO. 8, AUGUST 2011 EE5970 Computer Engineering Seminar Spring 2012 Michigan Technological University Rohit A. Bhatia

2. Contents • About FPGAs • Fine-Grain Power Gating and Related Work • Asynchronous FPGAs • Wave-Pipelining for Bit-Serial FPGAs • Sleep Signal Generation Techniques for Power Gating • Architecture of the proposed FPGA • Overview • Fundamental Principle of Autonomous Fine-Grain Power Gating • Circuit Implementation • Evaluation

3. Field Programmable Gate Array (FPGA) • FPGAs offer greater flexibility than ASICs (Application Specific Integrated Circuits) in terms of usability • Used in various products: • Programmable Logic Controllers (PLCs) • Medical Instruments • Gigabit Ethernet Controllers • Wireless devices • End user can program as per the requirement, enabling incredible customization to the application XILINX FPGA in use Source

4. FPGABasics • FPGAs work using “Look Up Tables” or “LUTs” • There are several logic blocks / cells (that contain LUTs) and programmable interconnects that give FPGAs the feature of being “Field-Programmable” ALTERA FPGA Source

5. About FPGAs • Fine-Grain Power Gating and Related Work • FPGA Power Considerations and LEDR • Wave-Pipelining for Bit-Serial FPGAs • Sleep Signal Generation Techniques for Power Gating • Architecture of the proposed FPGA • Overview • Fundamental Principle of Autonomous Fine-Grain Power Gating • Circuit Implementation • Evaluation

6. Power Gating • Certain logic blocks can be switched-off when not in use to minimize power consumption • Power consumed by sleep controller must be less than that saved by switching off the block • Power Gating: • Fine-Grain: Each LUT has its own sleep transistor and sleep controller • Coarse-Grain: A large number of LUTs share a single sleep controller Overall Architecture [1]

7. Power Considerations • Fine-Grain: Lower standby power but greater dynamic power (sleep transistors) • Coarse-Grain: Lower dynamic power but higher standby power (active blocks) Overall Architecture [1]

8. LEDR (Level Encoded Dual-Rail) • Dual-rail code provides scope for data transmission and its validity • Two wires per bit are used • LEDR or Level Encoded Dual-Rail based architecture achieves lowest dynamic power consumption among all dual-rail architectures considered • Decision to shut down and wake up the power-gated domain is based on its activity • No spacer required: less transitions, more throughput and less power consumption Dual-Rail code [2] [3]

9. Wave-Pipelining for Bit-Serial FPGAs • Allow circuit to process new data set before the previous data set reached the registers. • Used to achieve small area without performance degradation – not to reduce standby power

10. Sleep Signal Generation Techniques for Power Gating • Software Based • Can be used, but suffers from large power and delay overheads • Overheads make this technique not suitable for fine-grain power gating • Hardware Based • A power-gated domain is shut down after it stays idle for a given threshold • Static Sleep Signal Generator (SSSG) – predefined threshold time • Dynamic Sleep Signal Generator (DSSG) – variable threshold time • Both, SSSG and DSSG use instruction level analysis of the activity of the power-gated domains – applicable only to block-level power gating

11. Sleep Signal Generation Techniques for Power Gating (contd.) • Proposed Technique • Fine granularity sleep signal generation technique using two-input and one-output LUT • Power-Gated activity detected by comparing phases of input data with output data • Unlike coarse-grain approach, the fine-grain approach utilizes a sleep controller for each logic block • Each logic block can be turned OFF after operation completion with a small delay comparable to that of few small gates • SSSG employed to reduce power overheads • DSSG consumes more resources – “always-running” global counter!

12. About FPGAs • Fine-Grain Power Gating and Related Work • FPGA Power Considerations and LEDR • Wave-Pipelining for Bit-Serial FPGAs • Sleep Signal Generation Techniques for Power Gating • Architecture of the proposed FPGA • Overview • Fundamental Principle of Autonomous Fine-Grain Power Gating • Circuit Implementation • Evaluation

13. Architecture of the proposed FPGAOverview • Proposed Architecture • Four wires used, two for a data, one for acknowledge (ACK), and one for wake-up • wake-up signal used to wake up the next Logical Block (LB) Overall Architecture [1]

14. Architecture of the proposed FPGAOverview (contd.) • Cell Structure • Four wires used, two for a data, one for acknowledge (ACK), and one for wake-up • Four pass switches and a memory bit make a pass-switch block Cell Structure [4]

15. Architecture of the proposed FPGAOverview (contd.) • Direct Allocation of a Control/Data Flow Graph (CDFG) mapping used • Operations mapped onto the LBs • Input of a logic block is directly connected to the output of another logic block • Reduction in the complexity of the interconnection network between LBs! Operation Data Dependency Direct Allocation [5]

16. Fundamental Principle of Autonomous Fine-Grain Power Gating • On arrival of new data on an LB, the phase of the input data is different from that of the output data • On completion of operation, the phase of the input data and output data are the same • Proposed sleep controller extracts and compares the phases of input and output data. 0 0 1 0 1 1 0 1 0 0 Activity detection using the asynchronous architecture [6]

17. Fundamental Principle of Autonomous Fine-Grain Power Gating (contd.) • On same phase of input and output data, comparator output is 0 and 1 otherwise • Problems: • Sleep Transistor takes time to switch ON on the arrival of new data • Switching power could be more than the “saved” power Phase extraction and wake-up signal generation [7]

18. Fundamental Principle of Autonomous Fine-Grain Power Gating (contd.) • Problems can be solved by introducing a standby state: • wake up the LB before the data arrives; • power OFF the LB only when the data does not come for quite a while (threshold time) Sleep control strategy [8]

19. Fundamental Principle of Autonomous Fine-Grain Power Gating (contd.) Illustration of the proposed power gating method [9]

20. About FPGAs • Fine-Grain Power Gating and Related Work • FPGA Power Considerations and LEDR • Wave-Pipelining for Bit-Serial FPGAs • Sleep Signal Generation Techniques for Power Gating • Architecture of the proposed FPGA • Overview • Fundamental Principle of Autonomous Fine-Grain Power Gating • Circuit Implementation • Evaluation

21. Circuit Implementation • The wake-up signal is used to wake up the LB before new data arrives • Latches retain the wake-up signal until all the input data arrive • Programmable delay is used to delay the sleep signal based on the set threshold time Block diagram of a LB [10]

22. Circuit Implementation (contd.) • Phase comparator – detects data arrival • If Phase-a and Phase-b are different from Phase-out, it means that new data has arrived • The output (cascaded with the programmable delay) controls the sleep transistor for the LUT Phase Comparator [11] Sleep transistor

23. Circuit Implementation (contd.) • Problem comes with invalid inputs – large number of multiplexers is required to keep the previous output Multiplexer Based LUT for LEDR encoding [12]

24. Circuit Implementation (contd.) • Decoders exclude invalid data patterns (ones with different phases) • Number of transistors is reduced since invalid inputs aren’t fed to the multiplexer stage Block diagram of the proposed LUT [13]

25. Circuit Implementation (contd.) Behavior with invalid inputs [14]

26. Circuit Implementation (contd.) Behavior with valid inputs [14]

27. About FPGAs • Fine-Grain Power Gating and Related Work • FPGA Power Considerations and LEDR • Wave-Pipelining for Bit-Serial FPGAs • Sleep Signal Generation Techniques for Power Gating • Architecture of the proposed FPGA • Overview • Fundamental Principle of Autonomous Fine-Grain Power Gating • Circuit Implementation • Evaluation

28. Evaluation • The following results come from HSPICE simulation at 85°C: • Proposed FPGA when compared to conventional LEDR-based FPGA: • Standby power of the cell is reduced by 69% in the sleep state • The area and dynamic power of the logic block are increased by 14% and 6%, respectively • Proposed FPGA when compared to synchronous FPGA: • Standby power of the cell is reduced by 38% in the sleep state and delay is increased by 34% • The area is increased by 170% causing a reduction in available logic by 63% for the same given area

29. Evaluation (contd.) • The concepts demonstrated for a simple data path for a small LUT size can be applied to the complex data path architectures of modern FPGAs • The power and area of the sleep controller are much smaller than those of the data path components such as a logic block and a switch block. In a more complex data path architecture, the overheads of the sleep controller are smaller.

30. Thank you for your time!Questions?

31. Image sources [1] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1397, Fig. 3 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 12, 2012] [2] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1395, Fig. 1 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 12, 2012] [3] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1395, Fig. 2 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 13, 2012] [4] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1397, Fig. 4 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 13, 2012] [5] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1397, Fig. 5 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 14, 2012] [6] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1397, Fig. 6 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 14, 2012] [7] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1398, Fig. 7 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 14, 2012] [8] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1398, Fig. 9 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 14, 2012]

32. [9] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1398, Fig. 10 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012] [10] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1399, Fig. 12 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012] [11] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1399, Fig. 13 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012] [12] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1400, Fig. 15 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012] [13] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1400, Fig. 16 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012] [14] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1400, Fig. 17 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012] [15] Shota Ishihara, Masanori Hariyama, and Michitaka Kameyama. (2011, Aug.). “A Low-Power FPGA Based on Autonomous Fine-Grain Power Gating” IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS. [On-line]. 19(8), pp. 1400, Fig. 18 Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5483137 [Feb. 15, 2012]