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The G2231's ADC can run off of one of several available clock signals of varying speeds. The ADC10 also has a clock divider that can further slow the conversion speed by up to a factor of 8. Once a sample has been captured, it is held ready in the ADC10MEM register for a defined number of clock cycles. Since we are concerned with a low frequency signal, we will want to slow down the ADC10 as much as possible. (Students who have had Elec241 will notice some fundamental flaws in the assumptions made regarding high-frequency noise, but in practice this has very little effect on the final results). Even in its slowest mode, the ADC10 will still sample too quickly, so the use of some kind of moving average will help stabilize its DC readings.

Controlling the adc10 in c

C basics

Your C program will be structured similarly to its assembly language counterparts, but with a much different syntax. Like before, the register names are all pre-defined in the "msp430x20x2.h" header file. To set a register, now just use an equals sign and set it like any other variable. For example, you will want to disable the watchdog timer in the first line of your program. WDTCTL=WDTPW+WDTHOLD; The compiler will execute the void main(void) function first. From that function, you can call any other functions or run any loops that you would like.

C skeleton program

#include "msp430x20x2.h" //Global Variable Declarations//Global Function Declarations void main(void){ WDTCTL = WDTPW + WDTHOLD; // Stop WDT//Other Setup //Your Program Here//Can call other helper functions, loops, etc. }

Configuring the adc10

The ADC10 has two main control registers ADC10CTL0 and ADC10CTL1 , and two analog input enable registers ADC10AE0 and ADC10AE1 ( 10 bit ADC A nalog E nable 0/1 ). These registers control all the timing and conversion aspects of the ADC.

Adc10ctl0

Register diagram of the ADC10CTL0 register.

In the first control register ( ADC10CTL0 ), we only need to change two parameters,

  • ADC10SHTx-- 10 bit ADC S ample H old T ime-- a higher value means each sample will be held for a longer period of time. We want to set this at the max value of ADC10SHT_3 .
  • ADC10ON-- 10 bit ADC ON /OFF--setting this bit to "1" (denoted by the label ADC10ON ) turns on the ADC, a vital step to performing any conversion!
To actually do this in C, just use addition and an equals sign: ADC10CTL0 = ADC10SHT_3 + ADC10ON ;

Adc10ctl1

Register diagram of the ADC10CTL1 register.

In the second control register ( ADC10CTL1 ), we want to again set two parameters, but we will need to use 4 alias labels instead of just two.

  • ADC10DIVx-- 10 bit ADC clock Div ider bit x -- for "more flexibility", you set each bit individually in the three bit ADC10DIVx section of the register. Since we want the maximum divider, we will set all the bits.
    Since some of the bit labeling is inconsistent (ADC10DIV is bit-wise while ADC10SHT is not), it is always good to examine the header file for a controller to see how its aliases are defined before using them in your code.
  • INCHx-- In put Ch annel # -- this 4 bit section determines which of the possible input channels the ADC will actually convert in single convert mode. In series mode, this determines the highest channel to be converted in the series (all channels below this number will also be converted).
ADC10CTL1 = ADC10DIV0 + ADC10DIV1 + ADC10DIV2 + INCH_X;

Lastly, the ADC10 has the ADC10AE0/1 registers that enable analog input on the different pins. These act as gates to prevent leakage current from flowing from a pin set as an output through the ADC to ground-- a substantial waste of power. To enable the ADC for your desired GPIO pin, just set the corresponding bit in ADC10AE0 to "1". ADC10AE0 |= BIT#;

For more info about the ADC10's configuration options, see the MSP430 manual starting on page 609.

Using the adc

To read a sample from the ADC, just read from the ADC10MEM register after the sample has completed. my_var= ADC10MEM; Remember that we have setup the ADC for single conversion and hold, so if you want another value, you will have to tell it to sample and convert again. You do so by modifying two values in ADC10CTL0:

  • ENC-- En able C onversion-- locks in the ADC settings and stabilizes it for conversion.
  • ADC10SC-- 10 bit ADC S tart C onversion-- setting this bit to one actually triggers the ADC's conversion cycle.
ADC10CTL0 |= ENC + ADC10SC;
Be sure to use OR equal (|=) so that the configuration bits you set before don't get overridden.
Also, don't forget to configure P1 as usual. You will need to set the pins you wish to use as ADC inputs to input mode at the P1DIR register as well as the ADC10AE0 register. You can configure the P1 registers using aliases and variable assignments just like with the ADC registers.

Assignment details

Using Code Composer Studio 4, write a C language program turning your MSP430 LaunchPad into a simple 10 level voltmeter. Your program should divide the 0-3.3V input range of the ADC into 10 zones, and then output from a 0 to a 9 on the LED display depending on the input voltage. Don't worry about a value landing on the boundary between two zones, just deal with it consistently. Test your volt meter by attaching it to some of the variable power supplies around the room. DO NOT EXCEED AN INPUT VOLTAGE OF 3.3V . You will damage your circuits and destroy your MSP430.

Your Program should consist of:

  • A "void main(void)" function that drives your program
  • A successful setup routine that properly configures the ADC10
  • An output routine that successfully re-scales the 1024 ADC possibilities to 10 zones

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Source:  OpenStax, Intro to computational engineering: elec 220 labs. OpenStax CNX. Mar 11, 2013 Download for free at http://cnx.org/content/col11405/1.2
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