I'm not near any hardware to test this in real life so am having to resort to the datasheet and memory.
I want to send a continuous bitstream out of an AVR at high speed. eg the LSB of byte 2 will immediately follow the MSB of byte 1 with no idle time on the data line or in the clock.
My recollection, and it seems to be backed up by the datasheet, is that because the SPI is only single buffered in the transmit direction then using the SPI peripheral won't work.
However if I use the USART in SPI mode then it will work based on this statement...
A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are
succeeding, ending with the most or least significant bit accordingly. When a complete frame is transmitted, a
new frame can directly follow it, or the communication line can be set to an idle (high) state.
Have I remembered correctly?