Custom digitalReadFast()

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someteen

Well-known member
Hello,

As writen in the core_pins.h header file, this is the digitalReadFast() function content:

Code: 
static inline uint8_t digitalReadFast(uint8_t pin)
{
	if (__builtin_constant_p(pin)) {
		if (pin == 0) {
			return (CORE_PIN0_PINREG & CORE_PIN0_BITMASK) ? 1 : 0;
		} else if (pin == 1) {
			return (CORE_PIN1_PINREG & CORE_PIN1_BITMASK) ? 1 : 0;
		} else if (pin == 2) {
			return (CORE_PIN2_PINREG & CORE_PIN2_BITMASK) ? 1 : 0;
		} else if (pin == 3) {
			return (CORE_PIN3_PINREG & CORE_PIN3_BITMASK) ? 1 : 0;
		} else if (pin == 4) {
			return (CORE_PIN4_PINREG & CORE_PIN4_BITMASK) ? 1 : 0;
		} else if (pin == 5) {
			return (CORE_PIN5_PINREG & CORE_PIN5_BITMASK) ? 1 : 0;
		} else if (pin == 6) {
			return (CORE_PIN6_PINREG & CORE_PIN6_BITMASK) ? 1 : 0;
		} else if (pin == 7) {
			return (CORE_PIN7_PINREG & CORE_PIN7_BITMASK) ? 1 : 0;
		} else if (pin == 8) {
			return (CORE_PIN8_PINREG & CORE_PIN8_BITMASK) ? 1 : 0;
		} else if (pin == 9) {
			return (CORE_PIN9_PINREG & CORE_PIN9_BITMASK) ? 1 : 0;
		} else if (pin == 10) {
			return (CORE_PIN10_PINREG & CORE_PIN10_BITMASK) ? 1 : 0;
		} else if (pin == 11) {
			return (CORE_PIN11_PINREG & CORE_PIN11_BITMASK) ? 1 : 0;
		} else if (pin == 12) {
			return (CORE_PIN12_PINREG & CORE_PIN12_BITMASK) ? 1 : 0;
		} else if (pin == 13) {
			return (CORE_PIN13_PINREG & CORE_PIN13_BITMASK) ? 1 : 0;
		} else if (pin == 14) {
			return (CORE_PIN14_PINREG & CORE_PIN14_BITMASK) ? 1 : 0;
		} else if (pin == 15) {
			return (CORE_PIN15_PINREG & CORE_PIN15_BITMASK) ? 1 : 0;
		} else if (pin == 16) {
			return (CORE_PIN16_PINREG & CORE_PIN16_BITMASK) ? 1 : 0;
		} else if (pin == 17) {
			return (CORE_PIN17_PINREG & CORE_PIN17_BITMASK) ? 1 : 0;
		} else if (pin == 18) {
			return (CORE_PIN18_PINREG & CORE_PIN18_BITMASK) ? 1 : 0;
		} else if (pin == 19) {
			return (CORE_PIN19_PINREG & CORE_PIN19_BITMASK) ? 1 : 0;
		} else if (pin == 20) {
			return (CORE_PIN20_PINREG & CORE_PIN20_BITMASK) ? 1 : 0;
		} else if (pin == 21) {
			return (CORE_PIN21_PINREG & CORE_PIN21_BITMASK) ? 1 : 0;
		} else if (pin == 22) {
			return (CORE_PIN22_PINREG & CORE_PIN22_BITMASK) ? 1 : 0;
		} else if (pin == 23) {
			return (CORE_PIN23_PINREG & CORE_PIN23_BITMASK) ? 1 : 0;
		} else if (pin == 24) {
			return (CORE_PIN24_PINREG & CORE_PIN24_BITMASK) ? 1 : 0;
		} else if (pin == 25) {
			return (CORE_PIN25_PINREG & CORE_PIN25_BITMASK) ? 1 : 0;
		} else if (pin == 26) {
			return (CORE_PIN26_PINREG & CORE_PIN26_BITMASK) ? 1 : 0;
		}
		#if defined(CORE_PIN27_PINREG)
		  else if (pin == 27) {
			return (CORE_PIN27_PINREG & CORE_PIN27_BITMASK) ? 1 : 0;
		} else if (pin == 28) {
			return (CORE_PIN28_PINREG & CORE_PIN28_BITMASK) ? 1 : 0;
		} else if (pin == 29) {
			return (CORE_PIN29_PINREG & CORE_PIN29_BITMASK) ? 1 : 0;
		} else if (pin == 30) {
			return (CORE_PIN30_PINREG & CORE_PIN30_BITMASK) ? 1 : 0;
		} else if (pin == 31) {
			return (CORE_PIN31_PINREG & CORE_PIN31_BITMASK) ? 1 : 0;
		} else if (pin == 32) {
			return (CORE_PIN32_PINREG & CORE_PIN32_BITMASK) ? 1 : 0;
		} else if (pin == 33) {
			return (CORE_PIN33_PINREG & CORE_PIN33_BITMASK) ? 1 : 0;
		}
		#endif
		  else {
			return 0;
		}
	} else {
		#if defined(KINETISK)
		return *portInputRegister(pin);
		#else
		return (*portInputRegister(pin) & digitalPinToBitMask(pin)) ? 1 : 0;
		#endif
	}
}

Apart from some preprocessor directives which slighty reduce the length of code, there are _alot_ of time consuming "else if" instructions.

In my current project, I only need to read two digital pins (2 and 3). I thought of writing a custom "warpRead" or "blazingFastRead" function to eliminate those unuseful steps.

What do I actually need from the function above? By example, if I want to read pin 2, the above function returns:

Code: 
return (CORE_PIN2_PINREG & CORE_PIN2_BITMASK) ? 1 : 0;

That's it? I only have to do this registry operation?
 
Thanks, Frank! But for the record, it only takes that registry operation?

So this two lines:

Code: 
pin2 = (CORE_PIN2_PINREG & CORE_PIN2_BITMASK) ? 1 : 0;

pin2 = digitalReadFast(2);

should be identical (compiler wise)?
 
Yes, the generated code should be identical.

From a source code perspective, this makes your code less readable and more difficult to modify for other pins. Hardly seems like progress.
 
Thanks for confirmation, Paul. If the generated code is identical, case closed.

(as for the source code, I thought about using some inline #define directive for that registry operation)
 
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