Wide-range, reference free, on-chip voltage sensor for variable Vdd operations

In future systems with relatively unreliable and unpredictable energy sources such as harvesters, the system Vdd may become non-deterministic. Reliable and accurate on-chip voltage sensors are therefore indispensible for the power and computation management of such systems. Stable and known references are also difficult to obtain in this environment. This paper describes a reference-free voltage sensor implemented using a speed independent (SI) SRAM cell and an inverter chain. It can work under a wide range of Vdd, and provides accurate measurements of Vdd over this operating range with a precision range from 50mV to 10mV. Unlike existing methods, the voltage information is directly generated as a digital code without any analog circuits. This is realized by exploiting the inherently different latency behaviors of different types of circuits under different Vdd.