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ADC12CTL0 = REF2_5V | REFON; // Internal 2.5V ref on

Dac12 configuration

The DAC12_0 is configured with 12-bit resolution. The output is updated immediately when a new DAC12 data value is written in straight binary data format to the DAC12_0DAT register.

The full-scale output must be equal to the V REF+ 2.5 V internal reference voltage. Choose a compromise solution between the settling time and current consumption, by selecting a medium frequency and current for both input and output buffers. Configure the following register in order to meet these specifications:

DAC12_0DAT = 0x00; // DAC_0 output 0VDAC12_0CTL = DAC12IR | DAC12AMP_5 | DAC12ENC; // DAC_0 ->P6.6, // DAC_1 ->P6.7, // DAC reference Vref,// 12 bits resolution, // Immediate load,// DAC full scale output, // Medium speed/current,// Straight binary, // Not grouped

Timer_a configuration

Configure Timer_A register to enable an interrupt once every 1 msec. Use the ACLK clock signal as the clock source. This timer is configured in count up mode in order to count until the TAR value reaches the TACCR0 value.

// Before entering in LPM0: TACTL = TACLR | MC_1 | TASSEL_2; // up mode, SMCLK// Timer_A ISR:TAR = 0; // TAR reset TACCR0 = 13600; // Delay to allow Ref to settleTACCTL0 |= CCIE; // Compare-mode interrupt TACTL = TACLR | MC_1 | TASSEL_2; // up mode, SMCLK//*********************************************************// ISR to TACCRO from Timer A //*********************************************************#pragma vector=TIMERA0_VECTOR __interrupt void TimerA0_ISR (void){ DAC12_0DAT++; // Increase DAC's outputif (DAC12_0DAT == 0xfff) DAC12_0DAT = 0; // reset DAC's outputif (flag == 1) // if flag active exite LPM0 {flag = 0; LPM0_EXIT;} }

I/o ports configuration

Port P1 uses the bits P1.0 and P1.2 to activate the ISR whenever the push buttons SW1 and SW2 are activated (low-to-high transition).

// SW1 and SW2 ports configuration P1SEL&= ~0x03; // P1.0 and P1.1 I/O ports P1DIR&= ~0x03; // P1.0 and P1.1 digital inputs P1IFG = 0x00; // clear all interrupts pendingP1IE |= 0x03; // enable port interrupts

DAC12_0 is connected to P6.6. Configure P6 as a special function output:

// P6.6 (DAC12_0 output) // There is no need to configure P6.6 as a// special function output since it was configured in the // DAC12 configuration register (DAC12_0CTL) using// DAC12OPS = 0//********************************************************* // Port1 Interrupt Service Rotine//********************************************************* #pragma vector=PORT1_VECTOR__interrupt void PORT1_ISR (void) {if (P1IFG&0x01) // SW1 generate interrupt DAC12_0DAT += 400; // DAC's output increases if (P1IFG&0x02) // SW2 generate interrupt DAC12_0DAT -= 400; // DAC's output decreases P1IFG = 0x00; // clean all pending interrupts }

Analysis of operation

Observe the analogue signal using an oscilloscope

After compiling the project and starting the debug session, monitor the operation of the application using an oscilloscope probe connected to pin 7 of Header 8 (P6.6).

Measure the current drawn

Assign different values to the bits set in DAC12AMP0. Suspend the execution of the application then directly change the registers. Do not forget that this change requires suspending the operation of the DAC12 by disabling the bit DAC12ENC. Afterwards, this bit must be enabled.

Please note the special cases relating to:

- DAC12 off;

- High impedance output and DAC12 off;

- Output: 0 V.

This example and many others are available on the MSP430 Teaching ROM.

Request this ROM, and our other Teaching Materials here (External Link)

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Source:  OpenStax, Teaching and classroom laboratories based on the “ez430” and "experimenter's board" msp430 microcontroller platforms and code composer essentials. OpenStax CNX. May 19, 2009 Download for free at http://cnx.org/content/col10706/1.3
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