Month: August 2018

I couldn’t find any usable info about the sampling speed of Espressif’s ESP32 onboard ADC.

The benchmark was performed on the following Hardware:

Platform: Espressif 32 -> NodeMCU-32S
System: ESP32 240MHz 320KB RAM (4MB Flash)

The firmware for the benchmark was build using PlatformIO with the Arduino framework. To get a fast access to the hardware, the Espressif IoT Development Framework was used for all ADC specific function calls.

#include <chrono>
#include <Arduino.h>
#include <driver/adc.h>

void setup() {
  Serial.begin(115200);
  adc1_config_width(ADC_WIDTH_BIT_12);
  adc1_config_channel_atten(ADC1_CHANNEL_0, ADC_ATTEN_DB_11);
}

void loop() {
  const uint16_t samples = 10000;

  auto t1 = std::chrono::system_clock::now();

  for (uint32_t i=0; i<samples; i++) {
    adc1_get_raw(ADC1_CHANNEL_4); // GPIO32
  }

  auto t2 = std::chrono::system_clock::now();

  std::chrono::duration<double> diff = t2 - t1;
  auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(diff).count();

  Serial.printf("Samples: %u\n", samples);
  Serial.printf("Duration: %llums\n", ms);
  Serial.printf("KSPS: %0.4f", static_cast<double>(samples) / ms);
}

The results are showing that the ADC is capable of doing 27.2 ksps (27173 samples per second). This does not change when reducing the resolution to 10 or even 9 bits.

Resolution (bit)	Speed (ksps)
ADC_WIDTH_BIT_9	27.1739
ADC_WIDTH_BIT_10	27.1739
ADC_WIDTH_BIT_11	27.1739
ADC_WIDTH_BIT_12	27.1739

For me this results were unexpected, since a lower resolution should decrease the sample time.

Update

In the Espressif datasheet (topic 4.1.2), the ADC characteristics shows the following maximum sampling rates.

Description	Sampling rate (max)
RTC controller	200 ksps
DIG controller	2 Msps

First question: what is a DIG controller and why has the RTC/DIG controller impact on the sampling speed? Assuming the numbers in the datasheet are valid, the measured speed differ massively from this rates. The implementation of adc1_get_raw is quite complex and does a lot of hardware initialization/locking around adc_convert().

int adc1_get_raw(adc1_channel_t channel)
{
    uint16_t adc_value;
    RTC_MODULE_CHECK(channel < ADC1_CHANNEL_MAX, "ADC Channel Err", ESP_ERR_INVALID_ARG);
    adc1_adc_mode_acquire();
    adc_power_on();

    portENTER_CRITICAL(&rtc_spinlock);    
    //disable other peripherals
    adc1_hall_enable(false);
    adc1_fsm_disable(); //currently the LNA is not open, close it by default
    //set controller
    adc_set_controller( ADC_UNIT_1, ADC_CTRL_RTC );
    //start conversion
    adc_value = adc_convert( ADC_UNIT_1, channel );
    portEXIT_CRITICAL(&rtc_spinlock);
    adc1_lock_release();
    return adc_value;
}

So, the hardware might be capable to provide the claimed speeds but definitely not when using Espressif’s IDF as described in their documentation.