no ping testing. so i guess that is 12ms for 400? The UART buffer probably fills, then the sketch yields and u get a wdt? As there is a UART buffer, could you do this async? Fill the buffer, loop the sketch fill it again? if the buffer is 256 bytes, 4 per pixel, 64 pixels total = 1.920 ms. so if you can loop round again and fill the buffer in under 1.92ms there will not be a gap? I'm not familiar with any of this so forgive me if I'm talking nonsense?