RF12 module need to convert some C to Basic

[Art]

Nops in this function are a delay. The function (subroutine as you worked out) sends a single zero value out of the SPI bus.
If you were to set SCK low, and then high again in the very next instruction, the receiving device might miss the signal.
Especially if the external device runs slower than the transmitting device. You can use @nop in PBP.

Since there are several flavours of SPI, it would be better to reproduce this then use PBP SHIFTOUT (unless it’s already working).
They are really only turning two port bits on & off.

writezero:
portb.something = 0’ clear SPI data
portb.something = 0’ clear clock pin, the receiver will look at the value of the SPI data bit now.
delay
portb.something = 1’ set the clock pin, the receiver will look for the next bit on the next rising edge (next time you call this function, or the writeone function).

void Write0( void )
{
  SDI=0;
  SCK=0;
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  NOP();
  SCK=1;
  NOP();
}