Data Sheet

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3. Functional Description

3.3. Crystal Oscillator

ESP-WROOM-02U and ESP-WROOM-02D use a 26-MHz crystal oscillator. The accuracy

of the crystal oscillator should be ±10 PPM.

When using the download tool, please select the right type of crystal oscillator. In circuit

design, capacitors C1 and C2 which connect to the earth are added to the input and

output terminals of the crystal oscillator respectively. The values of the two capacitors can

be ﬂexible, ranging from 6 pF to 22 pF, however, the speciﬁc capacitive values depend on

further testing of, and adjustment to, the overall performance of the whole circuit. Normally,

the capacitive values of C1 and C2 are within 10 pF for the 26-MHz crystal oscillator.

3.4. Interface Description

Table 3-1. Interface Description

Interface

Functional Description

HSPI

IO12 (MISO), IO13

(MOSI), IO14 (CLK),

IO15 (CS)

Connects to SPI Flash, display screen, and MCU.

PWM

IO12 (R), IO15

(G),IO13 (B)

Currently the PWM interface has four channels, but users can extend it

to eight channels. PWM interface can realize the control of LED lights,

buzzers, relays, electronic machines, etc.

IO14 (IR_T), IO5

(IR_R)

The functionality of the infrared remote control interface can be realized

via software programming. The interface uses NEC coding, modulation,

and demodulation. The frequency of the modulated carrier signal is 38

kHz.

ADC

TOUT

Tests the power supply voltage of VDD3P3 (Pin3 and Pin4) and the input

power voltage of TOUT (Pin6). However, these two functions cannot be

used simultaneously. This interface is typically used in sensors.

I2C

IO14 (SCL), IO2 (SDA)

Connects to external sensors and display screens, etc.

UART

UART0: TXD

(U0TXD),

RXD (U0RXD), IO15

(RTS), IO13 (CTS)

UART1: IO2 (TXD)

Communicates with the UART device.

Downloading: U0TXD + U0RXD or GPIO2 + U0RXD

Communicating: (UART0): U0TXD, U0RXD, MTDO (U0RTS), MTCK

(U0CTS)

Debugging: UART1_TXD (GPIO2) can be used to print debugging

information.

By default, UART0 will output some printed information when you power

on ESP8266EX. If this issue inﬂuences some speciﬁc applications, users

can exchange the inner pins of UART when initializing ESP8266EX, that

is, exchange U0TXD and U0RXD with U0RTS and U0CTS. Users can

connect MTDO and MTCK to the serial port of the external MCU to

realize the communication.

Espressif

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2020.07