如何使用 gcc 内联汇编器代码访问成员变量

Question

所以我最近偶然发现了这篇博客文章 NeoPixel 揭示：如何(不需要(生成精确定时的信号并支持 github 项目，现在我试图将此代码的核心封装到 c++ 类中，以便我可以从多个 arduino uno 数字引脚访问各种 neopixel 字符串。

为此，我创建了一个受保护的成员变量 (m_PixelChannel(，用于存储访问浅色字符串所需的引脚。但是，我无法让程序集代码识别成员变量。下面是我尝试工作的代码(它或多或少是 github 项目中原始代码的直接复制粘贴，并在它之前添加了类名(：

// send a bit to the string. We must to drop to asm to enusre that the complier does
// not reorder things and make it so the delay happens in the wrong place.
inline void fastNeoPixels::sendBit(bool bitVal) {
if (bitVal) { // 0 bit
asm volatile(
"sbi %[port], %[bit] nt" // Set the output bit
".rept %[onCycles] nt"   // Execute NOPs to delay exactly the specified number of cycles
"nop nt"
".endr nt"
"cbi %[port], %[bit] nt" // Clear the output bit
".rept %[offCycles] nt"  // Execute NOPs to delay exactly the specified number of cycles
"nop nt"
".endr nt" ::
[port] "I"(_SFR_IO_ADDR(PIXEL_PORT)),
[bit] "r"(m_PixelChannel),
// [bit] "I" (PIXEL_STRING0),
[onCycles] "I"(NS_TO_CYCLES(T1H) - 2), // 1-bit width less overhead  for the actual bit setting, note that this delay could be longer and everything would still work
[offCycles] "I"(NS_TO_CYCLES(T1L) - 2) // Minimum interbit delay. Note that we probably don't need this at all since the loop overhead will be enough, but here for correctness
);
} else { // 1 bit
// **************************************************************************
// This line is really the only tight goldilocks timing in the whole program!
// **************************************************************************
asm volatile(
"sbi %[port], %[bit] nt" // Set the output bit
".rept %[onCycles] nt"   // Now timing actually matters. The 0-bit must be long enough to be detected but not too long or it will be a 1-bit
"nop nt"                 // Execute NOPs to delay exactly the specified number of cycles
".endr nt"
"cbi %[port], %[bit] nt" // Clear the output bit
".rept %[offCycles] nt"  // Execute NOPs to delay exactly the specified number of cycles
"nop nt"
".endr nt" ::
[port] "I"(_SFR_IO_ADDR(PIXEL_PORT)),
[bit] "r" (m_PixelChannel),
// [bit] "I" (PIXEL_STRING0),
[onCycles] "I"(NS_TO_CYCLES(T0H) - 2),
[offCycles] "I"(NS_TO_CYCLES(T0L) - 2)
);
}  // if (bitVal)...
// Note that the inter-bit gap can be as long as you want as long as it doesn't exceed the 5us reset timeout (which is A long time)
// Here I have been generous and not tried to squeeze the gap tight but instead erred on the side of lots of extra time.
// This has thenice side effect of avoid glitches on very long strings becuase
}

我确信m_PixelChannel变量导致了问题;我想这与约束有关，因为我可以通过取消注释PIXEL_STRING0行代码来使其再次工作。或者，我可以将值作为参数传递给方法并使用"n"约束代码使其工作(正如我成功所做的那样(，但我认为我不应该将参数传递给可以访问该值的方法已经...

我尝试了以下约束代码，但没有运气："n"，"o"，"I"，"m"，"+m"，"r"和"g"。

显然我错过了一些东西。请有人指出我正确的方向来完成这项工作吗？

Answer 1

问题是 SBI 指令的操作数必须是常量(即时值(。 因此，唯一有效的约束是I并且该值必须是常量。 无法设置可变位。

如果要设置可变位，则必须使用类似于 switch 语句的内容来选择 8 个不同的指令之一。

Answer 2

我决定我真的不喜欢我在评论克里斯·多德的回应时使用的模板<>方法。因此，经过多次迭代，我能够弄清楚如何使其工作......

void sendBit( bool bitVal ) {
volatile uint8_t _hi   = *m_PixelPORT |  m_PinMask;
volatile uint8_t _lo   = *m_PixelPORT & ~m_PinMask;
if ( bitVal ) {
asm volatile (
"st %a[port], %[hi]"  "nt"        // Set the output bit
".rept %[onCycles]"  "nt"   // Execute NOPs to delay exactly the specified number of cycles
"nop"                "nt"   
".endr"              "nt"  
"st %a[port], %[lo]"  "nt"   // Clear the output bit
".rept %[offCycles]" "nt"   // Execute NOPs to delay exactly the specified number of cycles
"nop"                "nt"
".endr"              "nt"
: [port] "+e" (m_PixelPORT)
: [hi] "r" (_hi),
[lo] "r" (_lo),
[onCycles] "I" (NS_TO_CYCLES(T1H) - 2), // 1-bit width less overhead  for the actual bit setting, note that this delay could be longer and everything would still work
[offCycles] "I" (NS_TO_CYCLES(T1L) - 2) // Minimum interbit delay. Note that we probably don't need this at all since the loop overhead will be enough, but here for correctness
);
} else {
asm volatile (
"st %a[port], %[hi]"  "nt"        // Set the output bit
".rept %[onCycles]"  "nt"   // Execute NOPs to delay exactly the specified number of cycles
"nop"                "nt"   
".endr"              "nt"  
"st %a[port], %[lo]"  "nt"   // Clear the output bit
".rept %[offCycles]" "nt"   // Execute NOPs to delay exactly the specified number of cycles
"nop"                "nt"
".endr"              "nt"
: [port] "+e" (m_PixelPORT)
: [hi] "r" (_hi),
[lo] "r" (_lo),
[onCycles] "I" (NS_TO_CYCLES(T0H) - 2),
[offCycles]   "I" (NS_TO_CYCLES(T0L) - 2)
);
}
}  

其中m_PinMask = _BV(digital_pin(;

请注意，对sbi/cbi的调用已替换为对st的调用和约束类型的更改。

应用这些更改使代码完全按照我想做的事情，同时保持在位爆炸过程的计时要求范围内。

再次感谢克里斯为我指出正确的方向！