1 Low Power System-On-Chip-Design Chapter 12: Physical Libraries Friedemann Wesner
2 Outline Standard Cell Libraries Modeling of Standard Cell Libraries Isolation Cells Level Shifters Memories Power Gating Strategies and Structures Fine-Grain Power Gating Coarse-Grain Power Gating Power Gating Cells
3 Standard Cell Libraries Standard cell libraries may be optimized for Performance Power consumption Required chip area Choice and mix of used libraries therefore has significant impact on power, timing and area
4...Standard Cell Libraries Key characteristic of cell libraries is cell height, measured in tracks (1 track = pitch of a Metal one wire) Tall track height libraries: 11-12 tracks Optimized for performance, high leakage power Low track height libraries: 7 8 tracks Optimized for area efficiency Standard track height libraries 9 10 tracks Designed to give trade-off between performance and area efficiency Used in majority of designs
5...Standard Cell Libraries Transistors with different threshold voltage can be used for building a library High threshold voltage: Lowest leakage power, lower performance Low threshold voltage: High performance, higher leakage power Regular threshold voltage: Between high and low threshold voltage libraries
6 Modeling of Standard Cell Libraries Library level provides abstract views of underlying transistor level models Commercially sensitive internals are hidden Detailed physical views of cells are switched in for manufacturing Required abstract views: Timing models Physical models Functional models Power models Test models
7 Isolation Cells Implemented either in powered down domain (output isolation) or active power domain (input isolation) 3 types of isolation circuits: Clamp signal to 0 Clamp signal to 1 Clamp signal to last value
8...Isolation Cells Clamping signal to 0 uses NAND gate and inverter with active low isolation control signal: Similar clamping to 1 by using NOR gate and inverter and active high control signal:
9...Isolation Cells Clamp to last value: use retention latch to retain state of output signal. Multiplexer switches between logic cell output and value of retention latch
10 Output vs. Input Isolation Advantages of output isolation: Only one isolation cell required for output signal going to different power domains Isolation cells of one domain share a single control signal Drawback: Custom isolation cells required, since connection to an always-on power supply must be provided
11 Recommendations Output isolation should be preferred over input isolation If custom isolation cells are not available, standard cells may be used when placed in an always-on area created for this purpose Insert isolation cells only when necessary since they introduce delay penalty
12 Level Shifters Level down shifter realized through simple inverter or buffer: Level up shifter requires special amplifier circuit:
13...Level Shifters Level up shifter can also be extended to provide shift and isolation functionality together:
14 Memories Usually generated by memory compilers, but for special cases optimized memories may be built as well Different memory architectures: Single or multi ported RAM RAM array or register file Performance-optimized Area-optimized Power-optimized Like for logic cells, high (for decreased leakage) or low (increased performance) threshold voltage transistors can be used for implementation
15...Memories In multi-voltage designs, up shifting of RAM inputs and down shifting of RAM outputs is often required because: Some logic blocks are run at lower voltage to save power, but...... below 90nm RAMs typically have no voltage headroom and must be run at full voltage To support power gating, RAM inputs must be clamped to retain the RAM contents correctly Example of a multi-voltage RAM interface (level shifters and isolation are part of the RAM):
16 Power Gating Strategies and Structures 2 common approaches for power gating: Multi-Threshold CMOS (MTCMOS) Use high threshold voltage switches to turn off power (considered in the following) Multi-Voltage CMOS (MVCMOS) Use low threshold voltage switches to turn off power Complex to support Rarely used in commercial designs
17 Fine-Grain Power Gating A sleep transistor is inserted into every standard cell (called Multi-Threshold CMOS (MTCMOS)) Power switch has to be very large to minimize influence on performance of the cell When mixing power gated and always-on cells, pull-down / up transistor is required to clamp cell output
18...Fine-Grain Power Gating Advantages: Not sensitive to ground noise injection Small wake up latency Built-in clamp transistors eliminate all crowbar currents MTCMOS library cells can be modelled in same way as standard cell libraries, thus conventional ASIC tools can be used for synthesizing and analysing Drawbacks: Significant area penalty: due to sleep transistors, cell can be up to 3 times as big Requires specially designed MTCMOS library Needs significant routing and buffering resources to distribute sleep control signal to all cells
19 Coarse-Grain Power Gating Sleep transistors are connected in parallel between permanent power and virtual power network:
20...Coarse-Grain Power Gating Advantages: Significantly smaller area overhead than fine-grain power gating Less sensitive to process, voltage and temperature variation in the sleep transistors than in fine-grain power gating Can utilize existing standard cells, only few special cells need to be added (sleep transistors, isolation cells, retention registers) Drawbacks: Requires complex power network, power network synthesis becomes challenging Requires in-rush current control on wake-up to prevent power supply noise Longer wake-up latency due to charging up the virtual power network Imposes special constraints in logic and physical synthesis
21 Power Gating Cells A library should provide header and footer switches in different sizes and strengths to allow different switch network designs Physically, a switch cell consists of several parallel switch transistors:
22 References All images are taken from: M. Keating, et al., Low Power Methodology Manual for System-on-Chip Design, Springer, 2007