以下是使用时钟源设置数字输出的ATSAMD51G19的示例代码：

#include 
#include "wiring_private.h"

#define PIN_LED     PIN_LED2 // LED2 is connected to pin 13 on some boards

void setup() {
  // Set pin mode to OUTPUT
  pinMode(PIN_LED, OUTPUT);

  // Configure GCLK generator 0
  GCLK->GENDIV.reg = GCLK_GENDIV_DIV(1) |          // Divide the clock source by divisor 1: (48MHz / 1) = 48MHz
                     GCLK_GENDIV_ID(0);            // Select generator 0
  while (GCLK->STATUS.bit.SYNCBUSY);               // Wait for synchronization

  // Configure GCLK generator 0 to use external 32.768kHz crystal oscillator
  GCLK->GENCTRL.reg = GCLK_GENCTRL_IDC |           // Set the duty cycle to 50/50 HIGH/LOW
                      GCLK_GENCTRL_GENEN |         // Enable the generator
                      GCLK_GENCTRL_SRC_XOSC32K |   // Set the generator source to external 32.768kHz crystal oscillator
                      GCLK_GENCTRL_ID(0);          // Select generator 0
  while (GCLK->STATUS.bit.SYNCBUSY);               // Wait for synchronization

  // Configure the generic clock 0 output to be connected to pin 13
  PORT->Group[g_APinDescription[PIN_LED].ulPort].PINCFG[g_APinDescription[PIN_LED].ulPin].bit.PMUXEN = 1;
  PORT->Group[g_APinDescription[PIN_LED].ulPort].PMUX[g_APinDescription[PIN_LED].ulPin >> 1].reg |= PORT_PMUX_PMUXO_E;
  
  // Enable the generic clock 0 output
  GCLK->CLKCTRL.reg = GCLK_CLKCTRL_CLKEN |         // Enable the generic clock
                      GCLK_CLKCTRL_GEN_GCLK0 |     // Select generator 0
                      GCLK_CLKCTRL_ID_TC4_TC5;      // Connect the generic clock to Timer/Counter 4 and 5
  while (GCLK->STATUS.bit.SYNCBUSY);               // Wait for synchronization

  // Configure Timer/Counter 4
  TC4->COUNT16.CTRLA.reg = TC_CTRLA_MODE_COUNT16 |  // Set the counter to 16-bit mode
                          TC_CTRLA_PRESCALER_DIV1;  // Set the prescaler to 1: (48MHz / 1) = 48MHz
  while (TC4->COUNT16.STATUS.bit.SYNCBUSY);         // Wait for synchronization

  // Enable Timer/Counter 4
  TC4->COUNT16.CTRLA.bit.ENABLE = 1;
  while (TC4->COUNT16.STATUS.bit.SYNCBUSY);         // Wait for synchronization
}

void loop() {
  // Do nothing
}

这段代码通过配置GCLK generator 0和Timer/Counter 4，将数字输出连接到ATSAMD51G19的引脚13（LED2）。在setup()函数中，首先配置GCLK generator 0以使用外部32.768kHz晶体振荡器作为时钟源，并将其输出连接到引脚13。然后，配置Timer/Counter 4以使用48MHz时钟，并启用它。在loop()函数中，没有任何操作。

请注意，引脚号（如PIN_LED）可能需要根据您实际使用的硬件进行调整。此示例仅用于演示目的，实际应用可能需要根据具体需求进行修改。