code: func SpiChipSelectPolarity(chip uint8, polarity uint8) {
  	if chip > 2 {
  		return
  	}
  	cspol := uint32(1 << (21 + chip)) // bit 21, 22 or 23 depending on chip
  
  	if polarity == 0 { // chip select is active low
  		spiMem[csReg] &^= cspol
  	} else { // chip select is active hight
  		spiMem[csReg] |= cspol
  	}
  }
docs: // SpiChipSelectPolarity: Sets polarity (0/1) of active chip select
  // default active=0