diff --git a/Marlin/src/HAL/HAL_SAMD51/fastio.h b/Marlin/src/HAL/HAL_SAMD51/fastio.h
index e00fa77a7c..f6a2675de0 100644
--- a/Marlin/src/HAL/HAL_SAMD51/fastio.h
+++ b/Marlin/src/HAL/HAL_SAMD51/fastio.h
@@ -37,7 +37,7 @@
 /**
  * Magic I/O routines
  *
- * Now you can simply SET_OUTPUT(STEP); WRITE(STEP, HIGH); WRITE(STEP, LOW);
+ * Now you can simply SET_OUTPUT(IO); WRITE(IO, HIGH); WRITE(IO, LOW);
  */
 
 // Read a pin
diff --git a/Marlin/src/HAL/HAL_SAMD51/inc/SanityCheck.h b/Marlin/src/HAL/HAL_SAMD51/inc/SanityCheck.h
index ddea64f2d0..04c137c344 100644
--- a/Marlin/src/HAL/HAL_SAMD51/inc/SanityCheck.h
+++ b/Marlin/src/HAL/HAL_SAMD51/inc/SanityCheck.h
@@ -23,6 +23,10 @@
  * Test SAMD51 specific configuration values for errors at compile-time.
  */
 
+#if ENABLED(EEPROM_SETTINGS) && NONE(SPI_EEPROM, I2C_EEPROM)
+  #warning "Did you activate the SmartEEPROM? See https://github.com/GMagician/SAMD51-SmartEEprom-Manager/releases"
+#endif
+
 #if defined(ADAFRUIT_GRAND_CENTRAL_M4) && SD_CONNECTION_IS(CUSTOM_CABLE)
   #error "No custom SD drive cable defined for this board."
 #endif
@@ -42,11 +46,3 @@
 #if ENABLED(FAST_PWM_FAN)
   #error "FAST_PWM_FAN is not yet implemented for this platform."
 #endif
-
-#if ENABLED(EEPROM_SETTINGS) && NONE(SPI_EEPROM, I2C_EEPROM)
-  #warning "Did you activate the SmartEEPROM? See https://github.com/GMagician/SAMD51-SmartEEprom-Manager/releases"
-#endif
-
-#if HAS_TMC_SW_SERIAL
-  #error "TMC220x Software Serial is not supported on this platform."
-#endif
diff --git a/Marlin/src/HAL/HAL_SAMD51/timers.cpp b/Marlin/src/HAL/HAL_SAMD51/timers.cpp
index 99839cd48a..3eb021c25b 100644
--- a/Marlin/src/HAL/HAL_SAMD51/timers.cpp
+++ b/Marlin/src/HAL/HAL_SAMD51/timers.cpp
@@ -42,7 +42,7 @@ const tTimerConfig TimerConfig[NUM_HARDWARE_TIMERS+1] = {
   { {.pTc=TC1},  TC1_IRQn, TC_PRIORITY(1) },  // 1 - stepper (needed by 32 bit timers)
   { {.pTc=TC2},  TC2_IRQn, TC_PRIORITY(2) },  // 2 - tone (framework)
   { {.pTc=TC3},  TC3_IRQn, TC_PRIORITY(3) },  // 3 - servo
-  { {.pTc=TC4},  TC4_IRQn, TC_PRIORITY(4) },
+  { {.pTc=TC4},  TC4_IRQn, TC_PRIORITY(4) },  // 4 - software serial
   { {.pTc=TC5},  TC5_IRQn, TC_PRIORITY(5) },
   { {.pTc=TC6},  TC6_IRQn, TC_PRIORITY(6) },
   { {.pTc=TC7},  TC7_IRQn, TC_PRIORITY(7) },
@@ -145,12 +145,12 @@ void HAL_timer_start(const uint8_t timer_num, const uint32_t frequency) {
 }
 
 void HAL_timer_enable_interrupt(const uint8_t timer_num) {
-  IRQn_Type irq = TimerConfig[timer_num].IRQ_Id;
+  const IRQn_Type irq = TimerConfig[timer_num].IRQ_Id;
   NVIC_EnableIRQ(irq);
 }
 
 void HAL_timer_disable_interrupt(const uint8_t timer_num) {
-  IRQn_Type irq = TimerConfig[timer_num].IRQ_Id;
+  const IRQn_Type irq = TimerConfig[timer_num].IRQ_Id;
   Disable_Irq(irq);
 }
 
@@ -160,7 +160,7 @@ static bool NVIC_GetEnabledIRQ(IRQn_Type IRQn) {
 }
 
 bool HAL_timer_interrupt_enabled(const uint8_t timer_num) {
-  IRQn_Type irq = TimerConfig[timer_num].IRQ_Id;
+  const IRQn_Type irq = TimerConfig[timer_num].IRQ_Id;
   return NVIC_GetEnabledIRQ(irq);
 }
 
diff --git a/Marlin/src/HAL/HAL_SAMD51/timers.h b/Marlin/src/HAL/HAL_SAMD51/timers.h
index 073239c431..4b21e47162 100644
--- a/Marlin/src/HAL/HAL_SAMD51/timers.h
+++ b/Marlin/src/HAL/HAL_SAMD51/timers.h
@@ -57,16 +57,16 @@ typedef uint32_t hal_timer_t;
                                : (t == TEMP_TIMER_NUM) ? 6                        \
                                : 7
 
-#define _TC_HANDLER(t)        void TC##t##_Handler()
-#define TC_HANDLER(t)         _TC_HANDLER(t)
-#define HAL_STEP_TIMER_ISR()  TC_HANDLER(STEP_TIMER_NUM)
+#define _TC_HANDLER(t)            void TC##t##_Handler()
+#define TC_HANDLER(t)             _TC_HANDLER(t)
+#define HAL_STEP_TIMER_ISR()      TC_HANDLER(STEP_TIMER_NUM)
 #if STEP_TIMER_NUM != PULSE_TIMER_NUM
-  #define HAL_PULSE_TIMER_ISR()  TC_HANDLER(PULSE_TIMER_NUM)
+  #define HAL_PULSE_TIMER_ISR()   TC_HANDLER(PULSE_TIMER_NUM)
 #endif
 #if TEMP_TIMER_NUM == RTC_TIMER_NUM
-  #define HAL_TEMP_TIMER_ISR()  void RTC_Handler()
+  #define HAL_TEMP_TIMER_ISR()    void RTC_Handler()
 #else
-  #define HAL_TEMP_TIMER_ISR()  TC_HANDLER(TEMP_TIMER_NUM)
+  #define HAL_TEMP_TIMER_ISR()    TC_HANDLER(TEMP_TIMER_NUM)
 #endif
 
 // --------------------------------------------------------------------------
diff --git a/platformio.ini b/platformio.ini
index 89afb91235..c57c1818bd 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -771,20 +771,20 @@ src_build_flags = -Wall -IMarlin/src/HAL/HAL_LINUX/include
 build_unflags   = -Wall
 lib_ldf_mode    = off
 lib_deps        =
-extra_scripts   =
 src_filter      = ${common.default_src_filter} +<src/HAL/HAL_LINUX>
 
 #
 # Adafruit Grand Central M4 (Atmel SAMD51P20A ARM Cortex-M4)
 #
 [env:SAMD51_grandcentral_m4]
-platform      = atmelsam
-board         = adafruit_grandcentral_m4
-build_flags   = ${common.build_flags} -std=gnu++17
-extra_scripts = ${common.extra_scripts}
-build_unflags = -std=gnu++11
-src_filter    = ${common.default_src_filter} +<src/HAL/HAL_SAMD51>
-debug_tool    = jlink
+platform       = atmelsam
+board          = adafruit_grandcentral_m4
+build_flags    = ${common.build_flags} -std=gnu++17 -Wno-register
+build_unflags  = -std=gnu++11
+src_filter     = ${common.default_src_filter} +<src/HAL/HAL_SAMD51>
+lib_deps       = ${common.lib_deps}
+  SoftwareSerialM=https://github.com/FYSETC/SoftwareSerialM/archive/master.zip
+debug_tool     = jlink
 
 #
 # RUMBA32