Wired ADC Signal when using I2S DMA

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Hi guys,

I was trying to use ADC for super fast sampling with I2S and DMA. The I2S config is as below:

Code: Select all

void i2sInit()
{
   i2s_config_t i2s_config = {
    .mode = (i2s_mode_t)(I2S_MODE_MASTER | I2S_MODE_RX | I2S_MODE_ADC_BUILT_IN),
    .sample_rate =  400000,              
    .bits_per_sample = I2S_BITS_PER_SAMPLE_16BIT, 
    .channel_format = I2S_CHANNEL_FMT_ONLY_LEFT,
    .communication_format = I2S_COMM_FORMAT_I2S_MSB,
    .intr_alloc_flags = ESP_INTR_FLAG_LEVEL1,
    .dma_buf_count = 4,
    .dma_buf_len = 16, // small dma buffer to ensure the i2s sampling rate is equal to real ADC sampling rate
    .use_apll = false,
    .tx_desc_auto_clear = false,
    .fixed_mclk = 0
   };
   adc1_config_channel_atten(ADC1_CHANNEL_0,ADC_ATTEN_DB_11);
   adc1_config_width (ADC_WIDTH_12Bit);
   i2s_driver_install(I2S_NUM_0, &i2s_config, 0, NULL);
   i2s_set_adc_mode(ADC_UNIT_1, ADC_INPUT);
}

...
// then using i2s_read() to read from dma buffer
...

I use a PWM to test ADC. However, when the frequency of signal exceeds 1kHz, some wired peaks appears at each rising. Take a 10kHz signal for example:

T-1ms_400k.png (62.96 KiB) Viewed 1661 times

The result from oscilloscope shows there is no peak actually, so something must be wrong with ADC or I2S. Is there anyone who meets the same problem?