Wrong sample values due to adc_clk_div

[Armannas]

Basic Infos

Hardware

Hardware: ESP8266 - Wemos D1 R2 & mini
Core Version: 2.1.0(deb1901) (obtained through system_get_sdk_version() )

Description

I am trying to investigate the possiblity to use the internal ADC with sampling frequency >10kS/s (preferably 50) based on code found in #2304, for this I am using the system_adc_read_fast() function, as the Arduino AnalogRead is too slow. Using the adc_clk_div at 16, I am getting ~116kSamples, far more than I need, and I wanted to at least get lower sampling frequencies by setting the clock divider to 32. However, this causes all my measurements to have 1024 as value. This only happens when I change the clock divider higher than 16 (at 24 it also does something similar, ~900). Otherwise, at 16, the measurements give something I am 'used to' (~4/5 due to some mysterious offset).

I could also just use this sampling frequency and systematically remove every other sample to artificially reduce it, but it's dirty. Would there be a clean way to get a lower sampling frequency, either by solving the problem in the above paragraph or another method?

Mind you, I want to use this together with WiFi, however, I am only going to grab 100 measurements in bursts with some time in between. At these sampling times well below 50 ms, I would expect that the ADC reads do not interfer with the periodic necessary processes to maintain WiFi.

Settings in IDE

Arduino Version: 1.8.6 Hourly build 2018/01/25
Module: Wemos D1 R2 & mini
Flash Size: 4M(1M SPIFFS)
CPU Frequency: 80Mhz
Flash Mode: ?
Upload speed: 115200
Upload Using: SERIAL
Reset Method: ?

Sketch

#ifdef ESP8266
extern "C" {
#include "user_interface.h"
}
#endif
int i=0;
long total = 0;
long tim = 0;
//ADC_MODE(ADC_TOUT);
void setup() {
// put your setup code here, to run once:
Serial.begin(115200);
delay(1000);
}

float frequencies[100] = {0};

void loop() {
// put your main code here, to run repeatedly:
#ifdef ESP8266

uint16_t adc_addr[100];
uint16_t adc_num = 100;
uint8_t adc_clk_div = 16;

uint16_t adc_addr_warm[10];

//Serial.println(system_get_sdk_version());
// Warming up for a few iterations, because the first
// few measurements are wrong
system_adc_read_fast(adc_addr_warm, 10, adc_clk_div);
long start= micros();
system_adc_read_fast(adc_addr, adc_num, adc_clk_div);
// Normal ADC read
//for(int iter = 0; iter < adc_num; iter++) {
//system_adc_read();
//}
int tot = micros() - start;
tim += tot;

// Keep track of sampling frequencies for each iteration
// to calculate mean and deviation later
frequencies[i] = 100000000.0/tot;
total += 100000000.0/tot;
i++;

/* Print ADC measurements
for(int m = 0; m<adc_num; m++) {
Serial.print(adc_addr[m]);
Serial.print(", ");
}*/
Serial.println();
delay(10);
if(i == 100){
Serial.print("Sampling frequencies: ");
// Print sampling frequencies
for(int f = 0; f<adc_num; f++) {
Serial.print(frequencies[f]);
Serial.print(", ");
}
Serial.print("Average Sampling rate: ");
Serial.println(total/100);
Serial.print("It lasted: ");
Serial.println(tim/100);
i = 0;
tim = 0;
total = 0;
}
#endif

}

Debug Messages

// Clock divider set at 16
measurements = [4, 4, . . . , 4]

/ Clock divider set at 32
measurements = [1024, 1024, . . . , 1024]

[devyte]

@Armannas This is the Arduino core repo. You are using the SDK API, so this is not the right place to ask this. I suggest asking at the Espressif bbs or the NONOS repo.
Closing.