I planing to build new circuit board with two chips: ATmega16 + ATmega328P and establish connection between them by USART.
But I planing to use ESP8266 WiFi module (it working with USART only).
So my question is...how I should connect ATmega16 + ATmega328P without USART?
I2C - keep only for external modules (such as DS18B20 or other). SPI - for programator use.
...or I should consider to use some new WiFi modules (for example with 1-Wire interface or other interfaces but not USART witch I do not know about)???