The issue with the ESP8266 is that it is multitasking, and only 80% of the CPU time is available to developers. This manifests as an outage of 100-200us every 500ms or so, which rather complicates all sorts of software serial interfaces, such as I2C, SPI and UART. As a bus master the ESP8266 hides these issues as it is responsible for the bus clock, but as a bus slave it needs to halt data transfer for the duration of these random events.
A typical solution for this inter-CPU issue is to provide an external RAM cache to act as a mailbox, where each CPU will periodically check to see if any actions are outstanding and leave any requested data.
Has anyone explored this option for the Arduino to ESP8266 interface?
