Analog-to-Digital conversion via the Texas Instruments MSP430 Microcontroller

Ian McCall November 6, 2013

ECE 480 Design Team 7

Abstract

The MSP430 is a line of microcontrollers (MCUs) designed and manufactured by Texas Instruments. These MCUs, which are inexpensive and consume little power, feature several peripherals that can be utilized to accomplish a wide variety of tasks. One example is an analog-to-digital converter (ADC) that can measure and capture analog signals so they can be then processed by the MCU. The ADCs that are included in the various MSP430 iterations have multiple channels to handle multiple analog input signals as well as several modes of operation.

Introduction

The ADC featured on the MSP430 line of microcontrollers is an eight channel, ten bit analog-to-digital converter. This means that the MSP430 can perform analog-to-digital conversions on up to eight input signals with ten bit resolution. This enables the MCU to measure analog input signals, respond to these inputs, and subsequently perform different tasks using these inputs, which serves as a rather powerful tool in many applications. For example, in a dual-parachute rocket system, the MSP430 can trigger the ejection parachutes given voltage readings from the on-board electronics (accelerometer, altimeter, etc).

Figure 1 - MSP430 Launchpad

Objective

The objective of this application note is to educate users regarding the ADC peripherals of the MSP430 line of microcontrollers. Topics that will be covered include the different operating modes and MCU registers associated with the ADC peripheral, and software configuration of the ADC with examples of code relating to the various aspects of the ADC.

Hardware and Software

Along with the MSP430 microcontroller, Texas Instruments also provides a development board that is an essential part of creating MSP430 applications, called the LaunchPad. It features on-board emulation for debugging the MSP430. To develop code, TI produced an integrated development environment called Code Composer Studio (CCS). The LaunchPad can connect to a personal computer via a mini-USB cable included with the LaunchPad, which powers the board and allows the user to program and debug (using CCS) the MSP430. Once it has been programmed, the MCU can be removed from the LaunchPad and installed in whatever system it was programmed for.

ADC Modes

The ADC present on the MSP430 has four modes of operation: single channel single-conversion, sequence-of-channels, repeat single-channel, and repeat sequence-of-channels. Below is a table giving a brief description of each mode.

Figure 2 - ADC Conversion Modes

ADC Conversion Process

The general order of actions that the ADC performs for each conversion is exhibited in the single channel single-conversion mode:

1. The conversion mode is selected

2. The ADC is turned on

3. The input channel is selected

4. The ADC waits to be enabled or instructed to start conversion

5. When a conversion is triggered, the voltage at the selected input is sampled

6. The voltage is then converted to a digital value

7. This value is written to the ADC memory register

8. An ADC interrupt is triggered

9. The ADC waits again to be instructed to start a conversion

The process described above is illustrated on the next page. In the other conversion modes, this process differs only in that multiple channels are being sampled, or individual channels are sampled repeatedly.

Figure 3 - Conversion Process for Single Channel Single-Conversion mode

ADC Registers

The MSP430 has several control registers that are used to configure various operating parameters of the peripheral that they correspond to. The registers that are used by the ADC are listed in the table on the next page.

Figure 4 - ADC Registers

The registers that are used to configure the operation of the ADC are the control registers ADC10CTL0 and ADC10CTL1. The figures below show the contents of these registers:

Figure 5 - ADC10CTL0 Register

Figure 6 - ADC10CTL1 Register

On the next page is a table giving a brief description of the portions of these control registers that are essential for basic use of the ADC.

Figure 7 - Description of key control register sections

Configuring the ADC

The ADC can be configured with only a few lines of code. An example of simple configuration, written in the C++ programming language, is implemented in the block of code below.

Figure 8 - ADC Configuration Code Example

The symbols such as ADC10ON and INCH_5 that can be seen in the above example stand for binary values that configure the ADC control registers. The different configuration options and corresponding symbols are defined in the MSP430 User’s Guide, which is linked to in the references section. In this particular example, the ADC is turned on and channel five is selected as the input channel. The SMCLK is selected as the ADC clock and the ADC interrupt is enabled. The ADC is instructed to hold its samples for 64 clock ticks before making a conversion. By default, the ADC is in single channel single-conversion mode. When instructed, the ADC begins conversion and the CPU will wait until the ADC10BUSY bit is cleared before continuing program execution. The ADC triggers an interrupt every time it performs a conversion, therefore it is necessary to write an interrupt service routine (ISR) to handle the interrupt for the ADC to function properly. An example of an ISR is shown below:

Figure 9 - ADC ISR example

This code can be executed at time intervals defined by a timer peripheral on the MSP430 to achieve the desired sample rate.

Conclusion

The analog-to-digital converter included on the MSP430 microcontroller can be a powerful tool for developers of applications using this device. It allows users to measure analog signals such as sensor outputs for instrumentation or control applications. This application note introduced users to the different operating modes and registers associated with the ADC and showed how it can be configured with software for the desired application.

References

1. MSP430 User’s Guide – http://www.ti.com/product/msp4302553

2. MSP430 LaunchPad Wiki – http://processors.wiki.ti.com/index.php/MSP430_LaunchPad_(MSP-EXP430G2)