Wifi ESP14
There are several solutions if you want to integrate wifi in your Arduino project. An ESP-14 is a complete µcontroller (STM8S003) and a wifi transceiver (ESP8266) in one module. The purpose of this combination is to provide more I/O pins than a regular ESP8266 based board such as ESP-12x, acting as an I/O extender. Unfortunately it needs some rather difficult soldering to make the connections to an Arduino. The ESP modules are 3.3Vdc, so in combination with an Arduino, a level-shifter and an adapted power supply are needed. It looks like an ESP8266xx can be best used stand-alone for IoT applications.

It should be possible to send ATcommands from an Arduino through a serial connection to the ESP-14. M_PD5 (blue=RX) must be connected to Arduino TX(18) and M_PD6 (yellow=TX) must be connected to Arduino RX(19).

According to Benjamin de Jong this module should respond to AT commands. More Gregware info on ESP14. Software example(ESP14-2) for 2560. Another software example and yet another software example(ESP14-1).

Dunno why, but I cannot get it up and running. Seems like I might have destroyed it while trying.

Pin Name Connects Levelshift Arduino
1 M_PC5 STM8 PC5 port -
2 M_PC6 STM8 PC6 port -
3 M_PC7 STM8 PC7 mouth -
5 M_PD3 STM8 PD3 port -
6 M_PD4 STM8 PD4 port -
7 M_NRST STM8 Reset 3 <= 5 DIO out (2)
9 M_PD2 STM8 PD2 port -
10 M_PD5 STM8 PD5 TX port 3 => 5 RX (18=blue)
11 M_PD6 STM8 PD6 RX port 3 <= 5 TX (19=yellow)
12 M_PA1 STM8 PA1 port or crystal input port -
13 M_PA2 STM8 PA2 output port or oscillator -
14 ESP_GPIO0 ESP8266 GPIO0 port 3 <= 5 DIO out
15 M_VCAP STM8 VCAP port 1uFCapacitor 470min 3300max nF to GND
16 M_VDD STM8 power supply pin Buck 3.3Vdc (2.95-5.5)
17 ESP_VDD ESP8266 power supply pin Buck 3.3Vdc
18 M_PA3 STM8 PA3 port -
19 M_PD5 STM8 PB5 port -
20 M_PD4 STM8 PB4 port -
21 M_PC3 STM8 PC3 port -
22 M_PC4 STM8 PC4 port -