Low Cost Digital I/O Based Analog-to-Digital Converter.

摘要

Analog to digital converters (ADCs) are an important part of most embedded system. Traditionally, performing analog measurements has required the use of specialized hardware, such as current sources, comparators, and dedicated ADC peripherals. This work examines the possibility of simplifying the hardware requirements for an ADC to allow for simple microcontrollers to directly measure voltages using only general purpose I/O (GPIO) and no specialized hardware.;At the core of all single-slope ADCs is a linear voltage-to-time converter (VTC). Previous work has always implemented the VTC using a constant current source and a capacitor. While this is a very straightforward way to achieve a linear voltage to time relationship, constant current sources are generally not built into low cost microcontrollers. What the microcontrollers do have is controllable constant voltage sources in the form of GPIO.;This work examines VTC implementations that can be realized using only GPIO and discrete passive components. The various implementations build up to a general voltage source based VTC circuit with good linearity and simple hardware requirements. The properties of the resulting ADC circuits are analyzed, such as speed, resolution, and accuracy. The software effort required to utilize these circuits as part of an ADC within the context of a larger real time embedded system is also analyzed. Finally, an example implementation is presented which uses these methods to implement a lithium cell battery charger.;An extension of this ADC methodology is the creation of an analog comparator using only GPIO and discrete passive components. The comparator triggers a GPIO input to change states when the input voltage crosses the desired threshold, similar to how a hardware comparator operates. An analysis and example implementation of this methodology is also presented.