yes, you´re right. I`m using the non-os compiler from Espressif. When I started with the ESP8266, I used Arduino too. But I could see, in this environment is the time between the GPIO-interrupt trigger and ISR-start higher. And sometimes the ISR execution time is sometimes longer, so that the syncronization between MasterClock and Slave was lost. In the SDK of Espressif is this behavior quite better. I want to avoid a Slave-Clock-Streching and decide to use this development tool.
But unfortunately the dokumentation which I could find on the Espressif site is not sufficient - at least for me. I was spoiled by Microchip
Seems I have to study it more deeply;-)
Thank you for yor support.