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ADC/DMA Fails at sampling rates above 40kHz

Question asked by whata on Dec 28, 2018
Latest reply on Jan 9, 2019 by whata



I'm attempting to sample signals at a rate of 80kHz, but for some reason I'm able to achieve only 40kHz. My set-up is a follows:


- MKL17Z128VFM4

- Running on LIRC

- Core Clock : 8MHz

- Bus Clock : 4 MHz

- ADC Dma code is taken from \demo_apps\adc16_low_power_async_dma and adapted to use PIT instead of LPTMR


I've calculated that with my current ADC Setup conversion time is abou 9.25us, while sampling time is between 1.5-2.5us which gives maximum rate of around 88kHz. Setting the pit interval to 100 ticks (4M/100 = 40kHz) works like a charm every time, while setting the tick count to 50 (4M/50=80kHz) yields interesting results:


1. noise_dma_read is executed

2. DMA0_IRQHandler is hit where the contents of DMA_DSR_BCRn are equal to  (1<<24) which means that transfer is complete and no errors are detected.

3. Immediately after DMA_ClearChannelStatusFlags(DMA0, 0, kDMA_TransactionsDoneFlag); is executed  the contents of register DMA_DSR_BCRn are equal to  (1<<24) | (1<<30) 

4. The code then gets stuck in DMA0_IRQHandler 


I'm not quite sure what is causing this behavior and how to handle it. Prior to clearing channel status flag DMA_DSR_BCRn clearly states that transfer is complete and there are no errors and no bytes left to transmitt.


static uint32_t                 noiseDmaBuf[1024];
static adc16_config_t           nDmaCfg;
static adc16_channel_config_t   nDmaChCfg;
static dma_handle_t             nDmaHandle;
static dma_transfer_config_t    nDmaXferCfg;
static volatile bool            noiseDmaDone;

static void noise_dma_adc_cfg       (void);
static void noise_dma_cfg           (void);
static void noise_dma_pit_cfg       (void);
static void noise_dma_sim_cfg       (void);
static void noise_dma_get_freq      (void);

void noise_dma_init (void)
    /* Configure ADC */
    noise_dma_adc_cfg ();

    /* Configure DMA */
    noise_dma_cfg ();

    /* Configure Timer Source */
    noise_dma_pit_cfg ();

    /* Link Timer Source to ADC Hw Trigger */
    noise_dma_sim_cfg ();

    /* Initially transfer is complete */
    noiseDmaDone = true;

uint32_t *noise_dma_read (void)

    noiseDmaDone = false;
    PIT_StartTimer(PIT, kPIT_Chnl_0);
    while (!noiseDmaDone);
    return (uint32_t*)&noiseDmaBuf[0];

static void noise_dma_adc_cfg   (void)
    nDmaChCfg.channelNumber = 1;
    nDmaChCfg.enableDifferentialConversion = false;
    nDmaChCfg.enableInterruptOnConversionCompleted = false;


    nDmaCfg.resolution = kADC16_Resolution16Bit;
    nDmaCfg.enableContinuousConversion = false;
    nDmaCfg.clockSource = kADC16_ClockSourceAlt0;           /** Run directly on bus-clock */
    nDmaCfg.clockDivider = kADC16_ClockDivider1;            /** No divider, raw bus-clock in */
    nDmaCfg.enableHighSpeed = true;
    nDmaCfg.longSampleMode = kADC16_LongSampleDisabled;
    nDmaCfg.enableLowPower = false;
    nDmaCfg.referenceVoltageSource = kADC16_ReferenceVoltageSourceValt;

    ADC16_Init(ADC0, &nDmaCfg);
    noise_dma_get_freq ();
    ADC16_DoAutoCalibration (ADC0);
    ADC16_SetChannelConfig(ADC0, 0, &nDmaChCfg);
    ADC16_SetHardwareAverage(ADC0, kADC16_HardwareAverageDisabled);
    ADC16_EnableHardwareTrigger(ADC0, true);
    ADC16_EnableDMA(ADC0, true);

static void noise_dma_cfg       (void)
    /* Configure DMAMUX */
    DMAMUX_SetSource(DMAMUX0, 0U, kDmaRequestMux0ADC0); /* Map ADC source to channel 0 */
    DMAMUX_EnableChannel(DMAMUX0, 0U);

    DMA_CreateHandle(&nDmaHandle, DMA0, 0U);
    DMA_PrepareTransfer(&nDmaXferCfg, (void *)(uint32_t)(&ADC0->R[0]), sizeof(uint32_t),
                        (void *)noiseDmaBuf, sizeof(uint32_t), sizeof(noiseDmaBuf),
    /* Setup transfer */
    DMA_SetTransferConfig(DMA0, 0U, &nDmaXferCfg);
    /* Enable interrupt when transfer is done. */
    DMA_EnableInterrupts(DMA0, 0U);
    /* Enable async DMA request. */
    DMA_EnableAsyncRequest(DMA0, 0U, true);
    /* Forces a single read/write transfer per request. */
    DMA_EnableCycleSteal(DMA0, 0U, true);
    /* Enable transfer. */
    /* Enable IRQ. */

* @brief           Configures ADC Sampling frequency.
* @note            Sampling frequency is set to 40kHz
*                  while bus clock is at 4Mhz, yielding
*                  tick time of 4MHz/40kHz = 100 ticks
* @param   none    None
* @retval  none    None

static void noise_dma_pit_cfg   (void)
    pit_config_t pitCfg;

    PIT_Init(PIT, &pitCfg);
    PIT_SetTimerPeriod(PIT, kPIT_Chnl_0, 50);

static void noise_dma_sim_cfg   (void)

void DMA0_IRQHandler (void)
    /* Stop trigger */
    PIT_StopTimer(PIT, kPIT_Chnl_0);
    /* Inidicate that transfer is complete */
    noiseDmaDone = true;
    /* Clear transaction done interrupt flag */
    DMA_ClearChannelStatusFlags(DMA0, 0, kDMA_TransactionsDoneFlag);
    /* Setup transfer */
    DMA_PrepareTransfer(&nDmaXferCfg, (void *)(uint32_t)(&ADC0->R[0]), sizeof(uint32_t),
                        (void *)noiseDmaBuf, sizeof(uint32_t), sizeof(noiseDmaBuf),
    DMA_SetTransferConfig(DMA0, 0U, &nDmaXferCfg);