S100 Slave - ESP8266


The S100 Slave - ESP8266 module is a certified Rhomb.io module that delivers highly integrated Wi-Fi solution to meet users’ continuous demands for efficient power usage, compact design and reliable performance in the Internet of Things industry.

This module has been build around the ESP-WROOM-02 module from Espressif. Its complete and self-contained Wi-Fi networking capabilities can be controlled easily by AT commands through the UART interface.

The S100 Slave - ESP8266 module integrates a printed circuit antenna with its switches, RF balun, power amplifier, low noise receive amplifier and filters, all of them adjusted to get the best RF performance. The ESP-WROOM-02 is a FCC and CE certified module and you can use it to develop it straight into consumer products.

The next figure show a 3D view for the S100 Slave - ESP8266.

S100 Slave - ESP8266 Top.png

           S100 Slave - ESP8266 Bottom.png


  • Internet of Things
  • Home automation
  • Smart lighting
  • Security alarms
  • Monitor and scales
  • Mesh Networks
  • Industrial Wireless Control
  • Wereable electronics
  • WiFi Position System Beacons

Module Specifications

The S100 Slave - ESP8266 module mounts the well-known ESP-WROOM-02 module, which integrates the Espressif ESP8266EX SoC. With the complete and self-contained Wi-Fi networking capabilities, this Rhomb.io module can perform as a host for standalone applications. The ESP8266EX chip is a low-power 32-bit MCU with TCP/IP network stacks and HSPI/UART/PWM/I2C/I2S interfaces embedded. ESP-WROOM-S2 has a 2 MB SPI flash connected to HSPI, working as SDIO/SPI slave, with the SPI speed being up to 8 Mbps.

As said previously, the module can be controlled easily by AT commands through the UART interface, or connecting it to a computer through USB. I2C and SPI interfaces are also accesible, and a bunch of GPIOs, interruptions and PWM signals make their way to the Rhomb.io connectors.

The module implements TCP/IP, full 802.11 b/g/n WLAN MAC protocol and Wi-Fi Direct specification. It supports not only basic service set (BSS) operations under the distributed control function (DCF) but also P2P group operation compliant with the latest Wi-Fi P2P protocol. Low level protocol functions are handled automatically by the integrated ESP8266EX.

The following figure identifies the main components onboard:

S100 Slave - ESP8266 Description Top.png

The next figure shows the block diagram for the S100 Master - ESP8266.

S100 Slave - ESP8266 Block Diagram.png
ESP8266 Features
Microcontroller Espressif ESP8266EX, 32-bit Tensilica L106, 80-160 MHz
Internal Memory 2 MB Flash, 50 KB SRAM
ID Memory 64-bit Unique-ID Memory with 112 B User EEPROM
Connectivity 802.11b/g/n 2.4 GHz, up to 72.2 Mbps; FCC/CE(RED)/TELEC(MIC)/KCC/SRRC/IC/NCC; WPA/WPA2
Rhomb.io config. UART, SPI, I2C, 2xINT, 5xGPIO
MCU I/O 12xI/O (4xPWM, 11xINT)
Op. Voltage 2.8 V / 3.3 V
Op. Temperature -40 ºC to +85 ºC
Hardware Specifications
Communication protocols Wi-Fi 802.11 b/g/n
Frequency range 2.4 GHz ~ 2.5 GHz (2400 MHz ~ 2483.5 MHz)
Peripheral interface UART/I2C/SPI/GPIO
Operating voltage 2.8V or 3.3V
Operating current Average: 80 mA
Software Specifications
Wi-Fi mode Station/SoftAP/SoftAP + Station
Security WPA/WPA2
Encryption WEP/TKIP/AES
Firmware upgrade UART Download / OTA (via network) / Download and write firmware via host
Software development Supports Cloud Server Development / SDK for custom firmware development
Network protocols IPv4, TCP/UDP/HTTP/FTP
User configuration AT Instruction Set, Cloud Server, Android/iOS app

User Interfaces

The following table indicates the available serial interfaces of the Rhomb.io Standard used in this module. This table relates the interfaces of the Rhomb.io Standard with the net names of the schematic and with the components to which they are connected.

Rhomb.io Interface Schematic Signal Component Component Pin Comments
I2C-A I2C-A_SCL ESP8266 (U2) IO14 SJ12 must be closed
I2C-A_SDA IO2 SJ13 must be open
SPI-A SPI-A_MISO ESP8266 (U2) IO12 SJ11 must be open
SPI-A_CLK IO14 SJ12 must be open
SPI-A_CS0 IO15 -
UART-A_CTS ESP8266 (U2) IO13 SJ8 must be closed
UART-A_RTS IO15 SJ7 must be closed
UART-B UART-B_TXD ESP8266 (U2) IO2 SJ13 must be closed
  • I2C-A: It can be connected to external sensors, display screens, etc. To enable this function, SJ12 should be closed. By default, "I2C-A_SDA" line is tied to "DVCC" through a 10k resistor, you can open the SJ10 solder-jumper to undo this connection.
  • SPI-A: Functions of these pins can ve implemented via hardware. In the ESP8266 module, this interface is known as HSPI. To enable this function SJ7, SJ8, SJ12, should be opened. Also, SJ11 connects SPI-A_MISO and GPIO-A_IO1, this solder-jumper should be opened if GPIO1 is used.
  • UART-A: data transfers to/from UART interface can be implemented via hardware. The data transmission speed via UART interface reaches 115200 x 40 (4.5 Mbps). By default, UART outputs some printed information when the device is powered on and booting up. To enable full UART-A interface, SJ7 and SJ8 should be closed.
  • UART-B: can be used to print debugging information. To enable this function, SJ13 should be closed.


S100 Master - ESP8266 Interface Solder Jumpers.png

GPIOs and Control Signals

The following table summarizes the GPIOs and Control Signals used on the S100 Slave - ESP8266 module. This table relates the signals of the Rhomb.io standard with the net names of the schematic and with the components to which they are connected.

Rhomb.io Signal Schematic Signal Component Component Pin Comments
#NMI #NMI ESP8266 (U2) IO0 -
1WIRE 1WIRE ESP8266 (U2) IO16 SJ6 must be closed
ID Memory (U1) IO -
INT0 INT0 ESP8266 (U2) IO0 SJ9 must be closed
IO IO0 ESP8266 (U2) IO4 -
IO1 IO12 SJ11 must be closed
IO2 IO5 SJ5 must be open
IO3 IO16 SJ1 and SJ6 must be open
PWM PWM0 ESP8266 (U2) IO15 -
User LED (LED1) - SJ2 must be closed
RESET_OUT RESET_OUT ESP8266 (U2) IO5 SJ5 must be closed.
  • IO0 / #NMI: When #NMI signal is asserted during power up, the ESP-WROOM-02 enters in UART Download Mode. It can be asserted externally or by the "non-button" upload circuit. In normal operation, this signal could be used as GPIO.
  • IO2 / RESET_OUT: This signal can be used as RESET_OUT if sJ5 Solder-jumper is closed.
  • IO3 / 1WIRE: This signal could be on-board connected to #RESET_IN Signal through the Deep Sleep solder-jumper. When done this, IO16 pin can be used for Deep-Sleep wake-up. This signal can be used to read the ID Memory if SJ6 solder-jumper is closed.

Nevertheless, the versatility of the ESP-WROOM-02 module lies in the multifunction of all of its pins. The above table is an adaptation of the module pinout to the Rhomb.io standard pinout. Be sure that most of the pins of the S100 Slave - ESP8266 module have way more functions than the ones shown in the schematics.

For more details, check the module schematics and the ESP-WROOM-02 manufacturer documentation.

S100 Master - ESP8266 GPIO&Control Solder Jumpers Top.png

           S100 Master - ESP8266 GPIO&Control Solder Jumpers Bottom.png


The S100 Slave - ESP8266 module can use the 2.8 V rail or the 3.3 V rail. You can choose the voltage shorting the corresponding solder-jumper. Make sure these rails is enabled on the motherboard you are going to plug this Module.

Working at 3.3 V, the average consumption of the module is 80 mA, with peaks of 170 mA when transmitting and only 20 μA in deep-sleep mode.

WROOM Power Consumption.png



Click the image below to download the schematic files.

Bill Of Materials

Click the image below to download the BOM files.

Fabrication Data

Click the image below to download the fabrication files.

Mechanical Specifications

S100 Slave - ESP8266 Dimensions.png


  • Precaution against Electrostatic Discharge. When handling Rhomb.io products, ensure that the environment is protected against static electricity. Follow the next recommendations:
  1. The users should wear anti-static clothing and use earth band when manipulating the device.
  2. All objects that come in direct contact with devices should be made of materials that do not produce static electricity that would cause damage.
  3. Equipment and work table must be earthed.
  4. Ionizer is recommended to remove electron charge.
  • Contamination. Be sure to use semiconductor products in the environment that may not be exposed to dust or dirt adhesion.
  • Temperature/Humidity. Semiconductor devices are sensitive to environment temperature and humidity. High temperature or humidity may deteriorate semiconductor devices characteristics. Therefore avoid storage or usage in such conditions.
  • Mechanical Shock. Care should be exercised not to apply excessive mechanical shock or force on the connectors and semiconductors devices.
  • Chemical. Do not expose semiconductor device to chemical because reaction to chemical may cause deterioration of device characteristics.
  • Light Protection. In case of non-EMC (Epoxy Molding Compound) package, do not expose semiconductor IC to strong light. It may cause devices malfunction. Some special products which utilize the light or have security function are excepted from this specification.
  • Radioactive, Cosmic and X-ray. Semiconductor devices can be influenced by radioactive, cosmic ray or X-ray. Radioactive, cosmic and X-ray may cause soft error during device operation. Therefore semiconductor devices must be shielded under environment that may be exposed to radioactive, cosmic ray or X-ray.
  • EMS (Electromagnetic Susceptibility). Note that semiconductor devices characteristics may be affected by strong electromagnetic waves or magnetic field during operation.


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