Abstract
We present an ultra-low power (ULP) 1 KB SRAM macro for Internet of Things (IoT) battery-less systems-on-chip (SoCs) operating under varying energy harvesting conditions. The unique combination of features within this array allows battery-less SoCs to retain important information for a significantly longer period of time when energy harvesting conditions are poor. The array uses 8T high-threshold (high-VT) static random access memory (SRAM) cells with word line boosting to eliminate write failures coupled with a read-before-write scheme to address read-disturb in half-selected cells. Due to the reduced on current in high-VT devices, read word line boosting is implemented to improve the drive strength of the read buffer, and to eliminate read failures. Leakage currents through the unselected cells during a read operation is addressed by boosting the footer virtual VSS (VVSS) of the read port to the supply voltage (VDD). To reduce the power consumption of instruction memories in battery-less SoCs, two features were utilized in this array: a read burst mode is used when reading consecutive addresses to reduce the read energy, and instructions with higher percentages of “1” data are defined since reading a “1” is less costly than reading a “0” in 8T cells. The proposed array can operate at a wide range of supply voltages (350–700 mV) and has two ULP modes: standby with retention (1.5 pW/bit) and shutdown without retention (0.13 pW/bit). Aggressive power gating of all peripherals during the standby state reduces the array power consumption down to 12.29 nW/KB at 320 mV with data retention. Compared to previously published 8T arrays, the proposed design provides the lowest standby power. The complete shutdown of the array allows further reduction down to 1.09 nW/KB and is suitable for reducing the power consumption of data memories in battery-less SoCs. The measured results from a commercial 130 nm chip show that the proposed array consumes a minimum of 6.24 pJ/access with a 17.16 nW standby power at 400 mV. The read burst mode allows up to 22% reduction in energy/access at 400 mV.
Highlights
The huge drive towards the internet of things (IoT) has led to the development of ultra-low power (ULP) systems-on-chip (SoCs) that are capable of operating on harvested energy [1,2]
The circuits within such SoCs must operate reliably under varying harvesting conditions, and their energy and power consumption must be kept at a minimum
One way to guarantee low power and energy consumption is to scale down the supply voltage to the sub-threshold region [3]
Summary
The huge drive towards the internet of things (IoT) has led to the development of ultra-low power (ULP) systems-on-chip (SoCs) that are capable of operating on harvested energy [1,2]. The circuits within such SoCs must operate reliably under varying harvesting conditions, and their energy and power consumption must be kept at a minimum. The proposed combination of techniques implemented in this paper can be ported into more advanced technologies to reduce the power consumed in the SRAM arrays. The trade-offs between the two approaches need to be assessed for more advanced technologies to determine the more energy-efficient solution (this is outside the scope of the presented work)
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