hi,

by the way - must it be 21 clocks only? You see, both SPI and UART mode 0 send a bit stream based on 8-bit (byte) base. So with it, you may obtain only 8,16,24 etc clocks during data sent. If you need with 21 clock exactly then I think you should use software-made shift register (for example, based on a timer or PCA).

Regards,
Oleg