samx5x: support external clock sources

Adds two board config define's, which can be added in a board's
mpconfigboard.h: BOARD_XOSC_FREQ_HZ, and BOARD_XOSC_IS_CRYSTAL, which
are passed to clock_init(). External clock sources are currently only
implemented for SAM_D5X_E5X series chips, so defining BOARD_XOSC_FREQ_HZ
for a SAMD21 board will emit an error.

Signed-off-by: Qyriad <qyriad@qyriad.me>

Author: Qyriad

ports/atmel-samd/mpconfigport.h

@@ -173,6 +173,30 @@
 #define BOARD_HAS_CRYSTAL (0)
 #endif
 
+#ifndef BOARD_XOSC_FREQ_HZ
+// 0 Indicates XOSC is not used.
+  #define BOARD_XOSC_FREQ_HZ (0)
+#else
+// For now, only allow external clock sources that divide cleanly into
+// the system clock frequency of 120 MHz.
+  #if (120000000 % BOARD_XOSC_FREQ_HZ) != 0
+    #error "BOARD_XOSC_FREQ_HZ must be an integer factor of 120 MHz"
+  #endif
+#endif
+
+#if BOARD_XOSC_FREQ_HZ != 0
+// External clock sources are currently not implemented for SAMD21 chips.
+  #ifdef SAMD21
+    #error "BOARD_XOSC_FREQ_HZ is non-zero but external clock sources are not yet supported for SAMD21 chips"
+  #endif
+  #ifndef BOARD_XOSC_IS_CRYSTAL
+    #error "BOARD_XOSC_IS_CRYSTAL must be defined to 0 or 1 if BOARD_XOSC_FREQ_HZ is not 0"
+  #endif
+#else
+// It doesn't matter what the value is in this case.
+  #define BOARD_XOSC_IS_CRYSTAL (0)
+#endif
+
 // if CALIBRATE_CRYSTALLESS is requested, make room for storing
 // calibration data generated from external USB.
 #ifndef CIRCUITPY_INTERNAL_CONFIG_SIZE

ports/atmel-samd/peripherals

@@ -353,10 +353,10 @@ safe_mode_t port_init(void) {
     if (strcmp((char *)CIRCUITPY_INTERNAL_CONFIG_START_ADDR, "CIRCUITPYTHON1") == 0) {
         fine = ((uint16_t *)CIRCUITPY_INTERNAL_CONFIG_START_ADDR)[8];
     }
-    clock_init(BOARD_HAS_CRYSTAL, fine);
+    clock_init(BOARD_HAS_CRYSTAL, BOARD_XOSC_FREQ_HZ, BOARD_XOSC_IS_CRYSTAL, fine);
     #else
     // Use a default fine value
-    clock_init(BOARD_HAS_CRYSTAL, DEFAULT_DFLL48M_FINE_CALIBRATION);
+    clock_init(BOARD_HAS_CRYSTAL, BOARD_XOSC_FREQ_HZ, BOARD_XOSC_IS_CRYSTAL, DEFAULT_DFLL48M_FINE_CALIBRATION);
     #endif
 
     rtc_init();