Changing sampling rate of ADC in cc2541

Hi Krishna,

Yes, you can use a timer to lower the effective sample rate (the rate at which you get/read samples). However, you don't have to write an ISR and manually trigger the ADC conversion. Instead, setup the ADC to trigger on Timer 1 Channel 0 compare event. Then setup Timer 1 in either up/down or modulo mode to get periodic triggers. You can control the sample rate then via Timer 1. See ADCCON1.STSEL = 0x10

Could you please provide a code snippet of how to setup both the timer and the adc to do what you suggest?

We need to sample at 125hz (~ every 8 milliseconds).

I have been struggling with this point for about 2 weeks and I am not able to get an adc interrupt and I even get stuck when  I read the fifo.

Thanks

Hi Aharon,

I created this code based on the document www.ti.com/lit/swru191

#include "hal_dma.h" // Which you can find in most installers

#define NUMBER_OF_ADC_CHANNELS 1 // Maximum 8
#define NUMBER_OF_SAMPLES_PER_NOTIFICATION 2 // We can let the DMA perform a few samples before we decide to notify the CPU
#define TRANSFERCOUNT (NUMBER_OF_ADC_CHANNELS * NUMBER_OF_SAMPLES_PER_NOTIFICATION)
static volatile uint8 pingOrPong = 0;
static uint16 adcSampleStorage_Ping[TRANSFERCOUNT]; // Number of samples to store in ping buffer
static uint16 adcSampleStorage_Pong[TRANSFERCOUNT]; // Number of samples to store in pong buffer
#define DMA_CHANNEL 0
halDMADesc_t dmaCh0;

void startADC()
{
halDMADesc_t *ch = &dmaCh0;
HAL_DMA_SET_SOURCE(ch, &X_ADCL); // Address of ADC data low
HAL_DMA_SET_DEST(ch, (uint8 *)adcSampleStorage_Ping); // Copy samples to global buffer
HAL_DMA_SET_VLEN(ch, HAL_DMA_VLEN_USE_LEN); // Use length field configured next
HAL_DMA_SET_LEN(ch, TRANSFERCOUNT); // Number of words to transfer before issuing DMA done interrupt
HAL_DMA_SET_WORD_SIZE(ch, HAL_DMA_WORDSIZE_WORD); // Transfer 16 bit per word
HAL_DMA_SET_TRIG_MODE(ch, HAL_DMA_TMODE_SINGLE); // ADC will generate one trigger per sample
HAL_DMA_SET_TRIG_SRC(ch, HAL_DMA_TRIG_ADC_CHALL); // Trigger on any sample conversion
HAL_DMA_SET_SRC_INC(ch, HAL_DMA_SRCINC_0); // Always read from the same address
HAL_DMA_SET_DST_INC(ch, HAL_DMA_DSTINC_1); // Increment with 1 word in destination per transfer
// We implement interrupt service routine for the DMA done interrupt
HAL_DMA_SET_IRQ(ch, HAL_DMA_IRQMASK_ENABLE);
HAL_DMA_SET_PRIORITY(ch, HAL_DMA_PRI_HIGH);
HAL_DMA_CLEAR_IRQ(DMA_CHANNEL);
// Enable DMA interrupt
DMAIE = 1;
// Arm DMA
HAL_DMA_ARM_CH(DMA_CHANNEL);

// Now the DMA is setup, setup ADC
#include "hal_adc.h"

// Configure ADC (in this example we only setup a sequence of two channels (CH0 and CH1)
// Setup ADC pin for channel 0 and 1
APCFG = (APCFG & 0xFC) | 0x03; // These settings override P0SEL settings
// Use internal 1.15V as reference as reference, resolution 12 bits, scan up to and including channel 1
ADCCON2 = (HAL_ADC_RESOLUTION_12 << 4) | HAL_ADC_CHANNEL_1;
// Start ADC conversion on Timer 1 Channel 0 compare event
ADCCON1 = 0x23; // ADCCON1[5:4] = ADCCON1.STSEL = 0x10 --> Timer 1 Channel 0 compare event triggers sample. Two LSBs must be set to 0x03

// Now ADC is setup, kick of everything by setting up Timer 1.
// First setup tick frequency to 250000kHz. This allows us to get 125Hz from the 16 bits counter (250kHz/2000 = 125Hz)
T1CTL = 0x0C; // This will set tick frequency to Tick/128. Without clock tick generator modifications this means 250kHz. It also suspends operation.
// Clear count, just in case timer was running
T1CNTL = 0;
// Setup Channel 0 in compare mode, no capture, interrupt enabled
T1CCTL0 = 0x40 | 0x04; // In order: interrupt enable, compare mode
// Setup period, based on timer in modulo mode. Write low byte first, this is latched when high byte is written.
T1CC0L = (2000 & 0x00FF);
T1CC0H = ((2000 & 0xFF00) >> 8) & 0x00FF;

// Now kick off everything by starting timer!
T1CTL |= 0x02; // Count repeatedly from 0 to T1CC0 which has been setup to 2000 --> period 125Hz
}

#include "hal_mcu.h"
/* It takes about 20-100us for the interrupt to be handled after interrupt is fired. This depends on what else is
* running at the time, for example if some routine is currently disabling interrupts.
* The interrupt service has (1/125) seconds to execute, to avoid overflow.
* This is solved by limiting what needs to be done here. We try to limit it by simply changing destination buffer.
* An event should here be sent to another task that can be executed outside of interrupt context.
* The event has (1/125 * NUMBER_OF_SAMPLES_PER_NOTIFICATION) seconds to use the samples, to avoid missing any samples.
*/
HAL_ISR_FUNCTION(dmaIsr, DMA_VECTOR)
{
HAL_ENTER_ISR();

halDMADesc_t *ch = &dmaCh0;

// Clear interrupt
HAL_DMA_CLEAR_IRQ(DMA_CHANNEL);

// Change between ping and pong buffers
if (pingOrPong == 0)
{
// Currently using Ping, change to Pong
HAL_DMA_SET_DEST(ch, (uint8 *)adcSampleStorage_Pong);
pingOrPong = 1;
}
else
{
// Currently using Pong, change to Ping
HAL_DMA_SET_DEST(ch, (uint8 *)adcSampleStorage_Ping);
pingOrPong = 0;
}

// Re-Arm DMA, Timer keeps triggering ADC so this needs to happen before the next sample
HAL_DMA_ARM_CH(DMA_CHANNEL);

HAL_EXIT_ISR();
}

Thanks Torbjorn. I will try the method and come back :)

Hi,

Those are defined in the IAR include file for your device; "ioCC25xx.h" (ioCC2541 in your case, because you're using CC2541?)