diff --git a/io.hpp b/io.hpp
index ccdec66..95dee00 100644
--- a/io.hpp
+++ b/io.hpp
@@ -127,17 +127,23 @@ enum class Bus {
 namespace detail {
 
 /*
-The following works in avr-gcc 5.4.0, but is not legal C++, because ptr's are not legal constexpr's
+The following works in avr-gcc 5.4.0, but is not legal C++, because ptr's are not legal constexpr's:
   constexpr auto *foo = ptr;
 
-Workaround is to store the the address of the ptr in a uintptr_t and reinterpret_cast it at call site
-For this the sfr_defs.h provides a macro which returns the address of a register
+Workaround is to store the the address of the ptr in a uintptr_t and reinterpret_cast it at call site.
+The _SFR_ADDR macro in sfr_defs.h would give the address, but it does that by taking the address of the dereferenced
+pointer and casts it to uint16_t, which is still not a legal constexpr.
+The workaround therefore is to disable the pointer cast and dereference macro _MMIO_BYTE temporarily.
 */
 
+#pragma push_macro("_MMIO_BYTE")
+#undef _MMIO_BYTE
+#define _MMIO_BYTE
+
 #ifdef PORT_A_AVAILABLE
-static constexpr uintptr_t PORT_A_DIR_REG_ADDR = _SFR_ADDR(DDRA);
-static constexpr uintptr_t PORT_A_OUTPUT_REG_ADDR = _SFR_ADDR(PORTA);
-static constexpr uintptr_t PORT_A_INPUT_REG_ADDR = _SFR_ADDR(PINA);
+static constexpr uintptr_t PORT_A_DIR_REG_ADDR = DDRA;
+static constexpr uintptr_t PORT_A_OUTPUT_REG_ADDR = PORTA;
+static constexpr uintptr_t PORT_A_INPUT_REG_ADDR = PINA;
 #else
 static constexpr uintptr_t PORT_A_DIR_REG_ADDR = 0;
 static constexpr uintptr_t PORT_A_OUTPUT_REG_ADDR = 0;
@@ -146,9 +152,9 @@ static constexpr uintptr_t PORT_A_INPUT_REG_ADDR = 0;
 
 #ifdef PORT_B_AVAILABLE
 
-static constexpr uintptr_t PORT_B_DIR_REG_ADDR = _SFR_ADDR(DDRB);
-static constexpr uintptr_t PORT_B_OUTPUT_REG_ADDR = _SFR_ADDR(PORTB);
-static constexpr uintptr_t PORT_B_INPUT_REG_ADDR = _SFR_ADDR(PINB);
+static constexpr uintptr_t PORT_B_DIR_REG_ADDR = DDRB;
+static constexpr uintptr_t PORT_B_OUTPUT_REG_ADDR = PORTB;
+static constexpr uintptr_t PORT_B_INPUT_REG_ADDR = PINB;
 #else
 static constexpr uintptr_t PORT_B_DIR_REG_ADDR = 0;
 static constexpr uintptr_t PORT_B_OUTPUT_REG_ADDR = 0;
@@ -156,9 +162,9 @@ static constexpr uintptr_t PORT_B_INPUT_REG_ADDR = 0;
 #endif
 
 #ifdef PORT_C_AVAILABLE
-static constexpr uintptr_t PORT_C_DIR_REG_ADDR = _SFR_ADDR(DDRC);
-static constexpr uintptr_t PORT_C_OUTPUT_REG_ADDR = _SFR_ADDR(PORTC);
-static constexpr uintptr_t PORT_C_INPUT_REG_ADDR = _SFR_ADDR(PINC);
+static constexpr uintptr_t PORT_C_DIR_REG_ADDR = DDRC;
+static constexpr uintptr_t PORT_C_OUTPUT_REG_ADDR = PORTC;
+static constexpr uintptr_t PORT_C_INPUT_REG_ADDR = PINC;
 #else
 static constexpr uintptr_t PORT_C_DIR_REG_ADDR = 0;
 static constexpr uintptr_t PORT_C_OUTPUT_REG_ADDR = 0;
@@ -166,15 +172,17 @@ static constexpr uintptr_t PORT_C_INPUT_REG_ADDR = 0;
 #endif
 
 #ifdef PORT_D_AVAILABLE
-static constexpr uintptr_t PORT_D_DIR_REG_ADDR = _SFR_ADDR(DDRD);
-static constexpr uintptr_t PORT_D_OUTPUT_REG_ADDR = _SFR_ADDR(PORTD);
-static constexpr uintptr_t PORT_D_INPUT_REG_ADDR = _SFR_ADDR(PIND);
+static constexpr uintptr_t PORT_D_DIR_REG_ADDR = DDRD;
+static constexpr uintptr_t PORT_D_OUTPUT_REG_ADDR = PORTD;
+static constexpr uintptr_t PORT_D_INPUT_REG_ADDR = PIND;
 #else
 static constexpr uintptr_t PORT_D_DIR_REG_ADDR = 0;
 static constexpr uintptr_t PORT_D_OUTPUT_REG_ADDR = 0;
 static constexpr uintptr_t PORT_D_INPUT_REG_ADDR = 0;
 #endif
 
+#pragma pop_macro("_MMIO_BYTE")
+
 static constexpr auto getBus(const P pin)
 {
 	// Upper 4 bits of pin encode which port this pin is on