SPI delay improvement

[kplus]

No description provided.

[kplus]

It seems the SPI transmission(8 bits data) added about 3us delay, this can be get rid of by moving the actual transmission to another time, and only indicate the daughter board to switch to next LED using NSS pin as generic GPIO. However, the bl_set_led function call itself takes about 6us which is too long. We can try to directly set GPIO or write to BSRR to see if that will be much quicker.

[kplus]

By using NSS pin as GPIO indicator and directly write to GPIO registers, we minimised the delay, and now it's about 200-300ns which should be good enough. And we can see from the diagram that the LEDs on mother and daughter board lights almost at same time, the delay is within 1 sample period.