  Calibration is an important step towards building reliable IoT systems. For
example, accurate sensor reading requires ADC calibration, and power monitoring
chips must be calibrated before being used for measuring the energy consumption
of IoT devices. In this paper, we present ProCal, a low-cost, accurate, and
scalable power calibration tool. ProCal is a programmable platform which
provides dynamic voltage and current output for calibration. The basic idea is
to use a digital potentiometer connected to a parallel resistor network
controlled through digital switches. The resistance and output frequency of
ProCal is controlled by a software communicating with the board through the SPI
interface. Our design provides a simple synchronization mechanism which
prevents the need for accurate time synchronization. We present mathematical
modeling and validation of the tool by incorporating the concept of Fibonacci
sequence. Our extensive experimental studies show that this tool can
significantly improve measurement accuracy. For example, for ATMega2560, the
ADC error reduces from 0.2% to 0.01%. ProCal not only costs less than 2\% of
the current commercial solutions, it is also highly accurate by being able to
provide extensive range of current and voltage values.
