Introduction
This laboratory gives examples of the uses of the ADC types available in the hardware development kits. A different laboratory is developed for each kit, taking into account that both the ADC10 and the SD16_A laboratories implement a temperature data logger. The ADC12 laboratory also uses operational amplifiers to perform the analogue signal conditioning.
Overview
This laboratory ( Lab1_ADC.c) implements a temperature data logger using the hardware kit’s integrated temperature sensor. The device is configured to perform an acquisition each minute for one hour. Each temperature’s (ºC) value is transferred to flash info memory segment B and C. When the microcontroller is not performing any task, it enters into low power mode.
Resources
The ADC10 module uses VREF+ = 1.5 V as the reference voltage.
It is necessary to configure the ADC10 to use the integrated temperature sensor (A10) as an input. Timer_A generates an interrupt once every second that starts conversion in the ADC10. At the end of a conversion, an interrupt is requested by the converter and the temperature value is written to flash memory.
The voltage value is converted into temperature following the equation provided in ADC10 section of the MSP430 User’s Guide <slau144e.pdf>. After transferring the value to the flash memory, the system returns to low power mode LPM3.
The resources used by the application are:
- ADC10;
- Timer_A;
- Ports I/O;
- Interrupts;
- Low power mode.
Software application organization
The application starts by stopping the Watchdog Timer.
The system checks for calibration constants on info memory segment A. The CPU execution will be trapped if it does not find this information.
Digitally controller oscillator (DCO) is set to 1 MHz, providing clock source for MCLK and SMCLK, while the Basic Clock System+ is configured to set ACLK to 1.5 kHz.
Controller’s flash timing is obtained from MCLK divided by three to comply with the device specifications.
Port P1.0 is configured as an output and will blink the once LED every second.
The ADC10 is configured to use the input channel corresponding to the on-chip temperature sensor (channel A10). The configuration includes the activation of the internal reference voltage VREF+ = 1.5 V and the selection of ADC10OSC as clock signal. The converter is configured to perform a single-channel single-conversion. At the end of conversion, an interrupt is requested.
The Timer_A is configured to generate an interrupt once every second. ACLK/8 is selected as the clock signal using the VLOCLK as clock source and will count until the TACCR0 value is reached (in up mode). The system then enters into low power mode and waits for an interrupt.
Flash memory pointers and interrupt counters are initialized. The Timer_A ISR increments the variable counter and when this variable reaches the value 60 (1 minute), the software start of conversion is requested. At the end of this ISR, the system returns to low power mode.
When the ADC10 ends the conversion, an interrupt is requested. While variable min is lower than 60, the temperature is written to flash memory. The memory pointer is increased by two (word). When min = 60, the system stops operation.
System configuration
DCO configuration
Adjust the DCO frequency to 1 MHz by software using the calibrated DCOCTL and BCSCTL1 register settings stored in information memory segment A.
if (CALBC1_1MHZ == 0xFF || CALDCO_1MHZ == 0xFF)
{
while(1); // If calibration constants erased
// do not load, trap CPU!!
}
DCOCTL = CALDCO_1MHZ; // Set DCO to 1 MHz
Basic Clock module+ configuration
Set MCLK and SMCLK to 1 MHz. Use the internal very low power VLOCLK source clock to ACLK/8 clock signal as low frequency oscillator (12 kHz):
BCSCTL1 = DIVA_3; // ACLK = 1.5 kHz
BCSCTL3 = LFXT1S_2; // Set VLOCLK (12 kHz)
ADC10 configuration
The ADC10’s input channel is the integrated temperature sensor (A10) and it uses the signal VREF+ (1.5 V) as reference voltage. The ADC10 clock source is ADC10OSC, the clock signal being ADC10CLK/4. Configure the ADC10 sample-and-hold time: 64xADC10CLKs, to perform a single-channel single-conversion and enable its interrupts. What are the values to write to the configuration registers?
ADC10CTL1 = INCH_10 + ADC10DIV_3; // Temp Sensor (A10),
// ADC10CLK/4,
// clock source: ADC10OSC
ADC10CTL0=SREF_1 + ADC10SHT_3 + REFON + ADC10ON +ADC10IE;
// Internal reference voltage Vref+ = 1.5 V
// ADC10 sample-and-hold time: 64 x ADC10CLKs
// Reference-generator voltage = 1.5 V
// ADC10 on + ADC10 interrupt enable
//*********************************************************
// ADC10 Interrupt Service Routine
//*********************************************************
#pragma vector=ADC10_VECTOR
__interrupt void ADC10ISR(void)
{
unsigned int temperature;
if (min <= 60)
{
temperature = ((ADC10MEM - 673) * 423) / 1024;
write_int_flash(memo_ptr,temperature);
memo_ptr += 2;
}
else
{
_NOP();
}
}
Timer_A configuration
Configure Timer_A register to enable an interrupt once every second. Use the ACLK clock signal as the clock source. This timer is configured in up counter mode in order to count until the TAR value reaches the TACCR0 value.
TACCTL0 = CCIE; // TACCR0 interrupt enabled
TACCR0 = 1500; // this count corresponds to 1 sec
TACTL = TASSEL_1 | MC_1 | ID_0;// ACLK, up mode to TACCR0
//**********************************************************
// Timer_A Interrupt Service Routine
//**********************************************************
#pragma vector=TIMERA0_VECTOR
__interrupt void TimerA0_ISR (void)
{
counter++;
P1OUT ^= 0x01; // LED toogle
if (counter == 60)
{
min++;
counter = 0;
ADC10CTL0 |= ENC + ADC10SC; // Sampling/Conversion start
}
}
Analysis of operation
After compiling the project, start the debug session and before running the application, put a breakpoint at the line of code with the _NOP() instruction. Go to breakpoint properties and set action to Write data to file. Name the file as Temp.dat and define the data format as integer. The data starts at address 0x01040, with a length of 3C. Run the application and let the temperature data logger acquire the values for 1 hour. Use a heater or a fan to force temperature variations during the measurement period. When execution reaches the breakpoint, the file will be available in your file system. Construct a graph in Excel or a similar tool, in order to plot the temperature variation obtained by the data logger.
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