Overview

MSP stands for "mixed signal processor" and most MSP430s are used in applications that handle both analog and digital data. We already saw how pulse-width modulation is used to provide a substitute for a "real" analog output. A few devices contain a 12-bit digital-to-analog converter (DAC12), which is used when PWM is unacceptable.

Most of this chapter is concerned with analog inputs and how these are converted to digital values that can be stored, processed, and transmitted to other systems. The MSP430 offers three methods of conversion with quite different characteristics. The first is a do-it-yourself approach that requires only a simple peripheral on the chip while the others employ a full analog-to-digital converter (ADC):

• Comparator: Simple and cheap module that cannot perform a conversion by itself but is usually used with Timer_A to measure the time-constant of an external RC circuit. There are two versions, Comparator_A and Comparator_A+.

• Successive-approximation ADC: The general-purpose type of ADC for many years. It is fast and relatively straightforward to understand. There are two versions, ADC10 and ADC12, which give 10 and 12 bits of output.

• Sigma delta ADC: A more complicated ADC that works in a quite different way to give higher resolution (more bits) but at a slower speed. There are two versions, SD16 and SD16_A, both of which give a 16-bit output.

Each module has several analog inputs. In most cases these are multiplexed to a single converter, which means that conversions are performed sequentially. A few...