ESP32 ULP ADC Support

[caseyWebb]

Hello! I've been exploring the espulp module with a project to read some analog sensors using the ULP and I believe I've hit a wall. As far as I'm able to tell, the ADCs have to be initialized from the main processor before they can be used by the ULP.

RISC-V:

https://github.com/espressif/esp-idf/blob/630c2724fc8c69eeaaa1bb025de52b99c5cb11aa/examples/system/ulp_riscv/adc/main/ulp_riscv_adc_example_main.c#L60-L66

ulp_riscv_adc_cfg_t cfg = {
    .channel = EXAMPLE_ADC_CHANNEL,
    .width   = EXAMPLE_ADC_WIDTH,
    .atten   = EXAMPLE_ADC_ATTEN,
};

ESP_ERROR_CHECK(ulp_riscv_adc_init(&cfg));

FSM:

https://github.com/espressif/esp-idf/blob/630c2724fc8c69eeaaa1bb025de52b99c5cb11aa/examples/system/ulp_fsm/ulp_adc/main/ulp_adc_example_main.c#L66-L72

/* Configure ADC channel */
/* Note: when changing channel here, also change 'adc_channel' constant
    in adc.S */
adc1_config_channel_atten(ADC1_CHANNEL_6, ADC_ATTEN_DB_11);
#if CONFIG_IDF_TARGET_ESP32
adc1_config_width(ADC_WIDTH_BIT_12);
#elif CONFIG_IDF_TARGET_ESP32S2
adc1_config_width(ADC_WIDTH_BIT_13);
#endif
adc1_ulp_enable();

---

I'd be happy to work on a PR to add this functionality. The API I have in mind is:

ulp = espulp.ULP()
ulp.run(code, adc_pin=board.IO11)

I think it makes sense to limit the ULP to using a single pin on ADC1 because:

• Wi-Fi interferes with ADC2. Trying to manage this seems like complexity that is outside of the scope of CircuitPython, but I could be wrong.
• AFAICT, the ADC needs to be configured on a per-channel basis with each pin using a different channel, and the ULP is unable to re-calibrate the ADC. This means if multiple pins were used by the ULP, only the last one configured will be accurate (this is my understanding of the code, at least)

[anecdata]

ADC calibration is baked into all but the oldest chips and the same for all channels. It should be possible to configure different channels / pins, but single pin probably covers most use cases. Also it looks like only ADC1 is allowed in ULP, but in any case, that's a very reasonable limitation.