More efficient ADC sample reading in espressif port

[kevinjwalters]

I've been looking at some ESP32-S2 ADC phenomena recently and ended up comparing CircuitPython and Arduino code.

The CircuitPython code looks like it's initialising quite a few things per read (use of the value property on AnalogIn object) that only need to be done once. Insides common_hal_analogio_analogin_get_value() there are calls to adc_oneshot_new_unit() and adc_oneshot_config_channel() which the Arduino code only does once or when configuration options change.

circuitpython/ports/espressif/common-hal/analogio/AnalogIn.c

Lines 82 to 89 in 1727222

	CHECK_ESP_RESULT(adc_oneshot_new_unit(&adc_config, &adc_handle));
	adc_oneshot_chan_cfg_t channel_config = {
	.atten = ATTENUATION,
	.bitwidth = DATA_WIDTH
	};
	adc_channel_t channel = (adc_channel_t)self->pin->adc_channel;
	adc_oneshot_config_channel(adc_handle, channel, &channel_config);

I note the return code from adc_oneshot_config_channel() is checked in the Arduino code.

Perhaps the calibration initialisation could also be done just once when AnalogIn is instantiated.

[dhalbert]

The ADC is a shared resource, and the assumption was that the ADC may be used for other functions in between AnalogIn uses. For instance on atmel-samd, the ADC is also used when doing microcontroller.cpu.temperature. On ESP32, ADC2 is used by WiFi (see, for instance, https://rectronx.com/blog/esp32-adc2-wifi-problem-fix).

We could notice some of these uses in the peripheral/ code and only re-init as needed.