2.4. Microcontroller ESP32

The ESP32 chip (Figure 7) refers to a microcontroller chip produced by Espressif, which has for some time been regarded as one of the most powerful and renowned microcontrollers on the market; its most powerful features include its processing speed, accessibility, and connectivity, the last of which should be highlighted because of its intelligibility through its Wi-Fi and Bluetooth connection.

Chip ESP32.

The ESP32 was designed with a model that can be either single or have dual-core 32-bits, with two physical processing cores, that can work with clock frequencies of up to 240 (MHz). In addition, it has the huge advantage of possessing a storage capacity that is exponentially more significant than that of the already established Arduino development boards, which has twice as much flash memory as the ATmega 2560 model [29].

One of the crucial features for selecting this chip ESP32-WROOM-32DC is its ADC, which integrates two 12-bit SAR ADCs and supports measurements on 18 channels (analog-enabled pins). The (ULP) co-processor in ESP32 is also designed to measure voltage while operating in sleep mode, which allows low-power consumption. The CPU can be woken up by a threshold setting and/or by other triggers. The ADCs can be configured with appropriate settings to measure the voltage on 18 pins maximally [30].

The ADC resolution can map the input voltage from 0 to 3.3 Vdc depending on the resolution setting (9–12 bits) and the maximum voltage reading configured by the attenuation filter and the recommended ranges for each voltage (Table 1).

Table with the recommended attenuation filters for each range.

Espressif noted that, by default, there is a difference of ±6% in the measured results between chips. This difference takes into account the region of the ADC that has no sensitivity, below 0.14 V, and the region of nonlinearity, above 2.5 V. However, the hardware was developed to operate in the linearity region of the ADC, between the range of 0.14 V and 2.5 V. Even with a low error, as with any equipment that will be launched to the market, it is essential to calibrate the microcontrollers, and one of the guidelines is to select a single reference value for the project [31].