// +build nrf52

package ws2812

// This file implements the WS2812 protocol for 64MHz Cortex-M4
// microcontrollers.

import (
	"device/arm"
)

// Send a single byte using the WS2812 protocol.
func (d Device) WriteByte(c byte) error {
	// For the Cortex-M4 at 64MHz
	portSet, maskSet := d.Pin.PortMaskSet()
	portClear, maskClear := d.Pin.PortMaskClear()

	// See:
	// https://wp.josh.com/2014/05/13/ws2812-neopixels-are-not-so-finicky-once-you-get-to-know-them/
	// T0H: 17-19 cycles or  265.63ns -  296.88ns
	// T0L: 54-56 cycles or  843.75ns -  875.00ns
	//   +: 71-75 cycles or 1109.38ns - 1171.88ns
	// T1H: 39-41 cycles or  609.38ns -  640.63ns
	// T1L: 30-32 cycles or  468.75ns -  500.0ns
	//   +: 69-73 cycles or 1078.13ns - 1140.63ns
	// A branch is treated here as 1-3 cycles, because apparently it might get
	// speculated. This is more of a guess than hard fact, because the only docs
	// by ARM that state this are now considered superseded (by what?).
	value := uint32(c) << 24
	arm.AsmFull(`
	1: @ send_bit
		str   {maskSet}, {portSet}     @ [2]   T0H and T0L start here
		lsls  {value}, #1              @ [1]
		nop                            @ [13]
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		bcs.n 2f                       @ [1-3] skip_store
		str   {maskClear}, {portClear} @ [2]   T0H -> T0L transition
	2: @ skip_store
		nop                            @ [22]
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		str   {maskClear}, {portClear} @ [2]   T1H -> T1L transition
		nop                            @ [26]
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		nop
		subs  {i}, #1                  @ [1]
		bne.n 1b                       @ [1-3] send_bit
	`, map[string]interface{}{
		"value":     value,
		"i":         8,
		"maskSet":   maskSet,
		"portSet":   portSet,
		"maskClear": maskClear,
		"portClear": portClear,
	})
	return nil
}