ADC Sample Speed

[NuclearPhoenixx]

Hello there again,

in the datasheet of the RP2040 it is stated that the ADC has a sample rate of 500 ksps. The ADC is much slower, though, if we use the standard analogRead() function that is implemented via the Pico SDK. Is the Pico SDK just much slower than the best possible sample rate, i.e. is there just so much programming overhead that it cannot reach the 500 ksps? Or can you get close to this via any of the Pico SDK functions? Maybe using the ADC FIFO would work faster? I'm not really sure, sorry if I'm just ignorant here.

arduino-pico/cores/rp2040/wiring_analog.cpp

Lines 125 to 142 in 895ffce

	extern "C" int analogRead(pin_size_t pin) {
	CoreMutex m(&_adcMutex);
	pin_size_t maxPin = max(A0, A3);
	pin_size_t minPin = min(A0, A3);
	if ((pin < minPin) || (pin > maxPin) || !m) {
	DEBUGCORE("ERROR: Illegal analogRead pin (%d)\n", pin);
	return 0;
	}
	if (!adcInitted) {
	adc_init();
	adcInitted = true;
	}
	adc_gpio_init(pin);
	adc_select_input(pin - minPin);
	return (_readBits < 12) ? adc_read() >> (12 - _readBits) : adc_read() << (_readBits - 12);
	}

[earlephilhower]

Using a simple test

void setup() {
  Serial.begin();
  delay(5000);
  uint32_t start = millis();
  int sum = analogRead(A0);
  for (int i = 0; i < 1000000; i++) {
    sum += analogRead(A0);
  }
  uint32_t end = millis();
  Serial.printf("ADC reads per second: %0.0f\n", 1000000.0 / ((end - start) * 0.001));
}

void loop() {
}

With the current core I get

ADC reads per second: 184298

Not 500k, but not completely atrocious.

With the simple optimizations in #1072 I get about 20% speed improvement:

ADC reads per second: 226347

The remaining difference is down to the SDK/manual reading of the ADC inputs which we have no control over.

[NuclearPhoenixx]

Thanks a lot for your testing and the explanation as always. You're doing a really great job, also thank you for #1072!

[earlephilhower]

@Phoenix1747 can you give #1072 a quick run? You've obviously got an app using the ADC so it would be a good sanity check.

[NuclearPhoenixx]

@earlephilhower Yup, looks fine for me. It's also a bit faster indeed. Nice!

[earlephilhower]

Thx for the update, merged for the next release!