我正在使用Atmel SAML21微处理器在定制板上实现USB CDC设备功能。由于ASF库已经为用户提供了内置的USB CDC方法，我需要做的是相应地修改源代码以使其与我的主板兼容。但是，我最担心的是该板没有外部32kHz振荡器（SYSTEM_CLOCK_SOURCE_XOSC32K），它用作闭环DFLL的参考时钟源，以产生48 MHz USB外设时钟。我改为使用32kHz内部时钟（SYSTEM_CLOCK_SOURCE_OSC32K）并修改conf_clock.h的内容，如下所示：
#include <clock.h>
# ifndef CONF_CLOCKS_H_INCLUDED
# define CONF_CLOCKS_H_INCLUDED
/* System clock bus configuration */
# define CONF_CLOCK_CPU_CLOCK_FAILURE_DETECT false
# define CONF_CLOCK_FLASH_WAIT_STATES 2
# define CONF_CLOCK_CPU_DIVIDER SYSTEM_MAIN_CLOCK_DIV_1
# define CONF_CLOCK_LOW_POWER_DIVIDER SYSTEM_MAIN_CLOCK_DIV_1
# define CONF_CLOCK_BACKUP_DIVIDER SYSTEM_MAIN_CLOCK_DIV_1
/* SYSTEM_CLOCK_SOURCE_OSC16M configuration - Internal 16MHz oscillator */
# define CONF_CLOCK_OSC16M_FREQ_SEL SYSTEM_OSC16M_16M
# define CONF_CLOCK_OSC16M_ON_DEMAND true
# define CONF_CLOCK_OSC16M_RUN_IN_STANDBY false
/* SYSTEM_CLOCK_SOURCE_XOSC configuration - External clock/oscillator */
# define CONF_CLOCK_XOSC_ENABLE false
# define CONF_CLOCK_XOSC_EXTERNAL_CRYSTAL SYSTEM_CLOCK_EXTERNAL_CRYSTAL
# define CONF_CLOCK_XOSC_EXTERNAL_FREQUENCY 12000000UL
# define CONF_CLOCK_XOSC_STARTUP_TIME SYSTEM_XOSC_STARTUP_32768
# define CONF_CLOCK_XOSC_AUTO_GAIN_CONTROL true
# define CONF_CLOCK_XOSC_ON_DEMAND true
# define CONF_CLOCK_XOSC_RUN_IN_STANDBY false
/* SYSTEM_CLOCK_SOURCE_XOSC32K configuration - External 32KHz crystal/clock oscillator */
# define CONF_CLOCK_XOSC32K_ENABLE false /* previously was true */
# define CONF_CLOCK_XOSC32K_EXTERNAL_CRYSTAL SYSTEM_CLOCK_EXTERNAL_CRYSTAL
# define CONF_CLOCK_XOSC32K_STARTUP_TIME SYSTEM_XOSC32K_STARTUP_65536
# define CONF_CLOCK_XOSC32K_ENABLE_1KHZ_OUPUT false
# define CONF_CLOCK_XOSC32K_ENABLE_32KHZ_OUTPUT true
# define CONF_CLOCK_XOSC32K_ON_DEMAND false
# define CONF_CLOCK_XOSC32K_RUN_IN_STANDBY true
/* SYSTEM_CLOCK_SOURCE_OSC32K configuration - Internal 32KHz oscillator */
# define CONF_CLOCK_OSC32K_ENABLE true /* previously was false */
# define CONF_CLOCK_OSC32K_STARTUP_TIME SYSTEM_OSC32K_STARTUP_130
# define CONF_CLOCK_OSC32K_ENABLE_1KHZ_OUTPUT false
# define CONF_CLOCK_OSC32K_ENABLE_32KHZ_OUTPUT true
# define CONF_CLOCK_OSC32K_ON_DEMAND true
# define CONF_CLOCK_OSC32K_RUN_IN_STANDBY false
/* SYSTEM_CLOCK_SOURCE_OSCULP32K configuration - Internal Ultra Low Power 32KHz oscillator */
/* SYSTEM_CLOCK_SOURCE_DFLL configuration - Digital Frequency Locked Loop */
# define CONF_CLOCK_DFLL_ENABLE true
# define CONF_CLOCK_DFLL_LOOP_MODE SYSTEM_CLOCK_DFLL_LOOP_MODE_CLOSED
# define CONF_CLOCK_DFLL_ON_DEMAND false
# define CONF_CLOCK_DFLL_RUN_IN_STANDBY false
/* DFLL open loop mode configuration */
# define CONF_CLOCK_DFLL_FINE_VALUE (512)
/* DFLL closed loop mode configuration */
# define CONF_CLOCK_DFLL_SOURCE_GCLK_GENERATOR GCLK_GENERATOR_1
# define CONF_CLOCK_DFLL_MULTIPLY_FACTOR (48000000/32768)
# define CONF_CLOCK_DFLL_QUICK_LOCK true
# define CONF_CLOCK_DFLL_TRACK_AFTER_FINE_LOCK true
# define CONF_CLOCK_DFLL_KEEP_LOCK_ON_WAKEUP true
# define CONF_CLOCK_DFLL_ENABLE_CHILL_CYCLE true
# define CONF_CLOCK_DFLL_MAX_COARSE_STEP_SIZE (0x1f / 8)
# define CONF_CLOCK_DFLL_MAX_FINE_STEP_SIZE (0xff / 8)
/* SYSTEM_CLOCK_SOURCE_DPLL configuration - Digital Phase-Locked Loop */
# define CONF_CLOCK_DPLL_ENABLE false
# define CONF_CLOCK_DPLL_ON_DEMAND true
# define CONF_CLOCK_DPLL_RUN_IN_STANDBY false
# define CONF_CLOCK_DPLL_LOCK_BYPASS false
# define CONF_CLOCK_DPLL_WAKE_UP_FAST false
# define CONF_CLOCK_DPLL_LOW_POWER_ENABLE false
# define CONF_CLOCK_DPLL_LOCK_TIME SYSTEM_CLOCK_SOURCE_DPLL_LOCK_TIME_DEFAULT
# define CONF_CLOCK_DPLL_REFERENCE_CLOCK SYSTEM_CLOCK_SOURCE_DPLL_REFERENCE_CLOCK_XOSC32K
# define CONF_CLOCK_DPLL_FILTER SYSTEM_CLOCK_SOURCE_DPLL_FILTER_DEFAULT
# define CONF_CLOCK_DPLL_PRESCALER SYSTEM_CLOCK_SOURCE_DPLL_DIV_1
# define CONF_CLOCK_DPLL_REFERENCE_FREQUENCY 32768
# define CONF_CLOCK_DPLL_REFERENCE_DIVIDER 1
# define CONF_CLOCK_DPLL_OUTPUT_FREQUENCY 48000000
/* DPLL GCLK reference configuration */
# define CONF_CLOCK_DPLL_REFERENCE_GCLK_GENERATOR GCLK_GENERATOR_1
/* DPLL GCLK lock timer configuration */
# define CONF_CLOCK_DPLL_LOCK_GCLK_GENERATOR GCLK_GENERATOR_1
/* Set this to true to configure the GCLK when running clocks_init. If set to
* false, none of the GCLK generators will be configured in clocks_init(). */
# define CONF_CLOCK_CONFIGURE_GCLK true
/* Configure GCLK generator 0 (Main Clock) */
# define CONF_CLOCK_GCLK_0_ENABLE true
# define CONF_CLOCK_GCLK_0_RUN_IN_STANDBY true
# define CONF_CLOCK_GCLK_0_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_DFLL
# define CONF_CLOCK_GCLK_0_PRESCALER 1
# define CONF_CLOCK_GCLK_0_OUTPUT_ENABLE false
/* Configure GCLK generator 1 */
# define CONF_CLOCK_GCLK_1_ENABLE true
# define CONF_CLOCK_GCLK_1_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_1_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC32K /* Previously was SYSTEM_CLOCK_SOURCE_XOSC32K */
# define CONF_CLOCK_GCLK_1_PRESCALER 1
# define CONF_CLOCK_GCLK_1_OUTPUT_ENABLE false
/* Configure GCLK generator 2 */
# define CONF_CLOCK_GCLK_2_ENABLE false
# define CONF_CLOCK_GCLK_2_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_2_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC16M
# define CONF_CLOCK_GCLK_2_PRESCALER 1
# define CONF_CLOCK_GCLK_2_OUTPUT_ENABLE false
/* Configure GCLK generator 3 */
# define CONF_CLOCK_GCLK_3_ENABLE true
# define CONF_CLOCK_GCLK_3_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_3_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_DFLL
# define CONF_CLOCK_GCLK_3_PRESCALER 1
# define CONF_CLOCK_GCLK_3_OUTPUT_ENABLE false
/* Configure GCLK generator 4 */
# define CONF_CLOCK_GCLK_4_ENABLE false
# define CONF_CLOCK_GCLK_4_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_4_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC16M
# define CONF_CLOCK_GCLK_4_PRESCALER 1
# define CONF_CLOCK_GCLK_4_OUTPUT_ENABLE false
/* Configure GCLK generator 5 */
# define CONF_CLOCK_GCLK_5_ENABLE false
# define CONF_CLOCK_GCLK_5_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_5_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC16M
# define CONF_CLOCK_GCLK_5_PRESCALER 1
# define CONF_CLOCK_GCLK_5_OUTPUT_ENABLE false
/* Configure GCLK generator 6 */
# define CONF_CLOCK_GCLK_6_ENABLE false
# define CONF_CLOCK_GCLK_6_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_6_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC16M
# define CONF_CLOCK_GCLK_6_PRESCALER 1
# define CONF_CLOCK_GCLK_6_OUTPUT_ENABLE false
/* Configure GCLK generator 7 */
# define CONF_CLOCK_GCLK_7_ENABLE false
# define CONF_CLOCK_GCLK_7_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_7_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC16M
# define CONF_CLOCK_GCLK_7_PRESCALER 1
# define CONF_CLOCK_GCLK_7_OUTPUT_ENABLE false
/* Configure GCLK generator 8 */
# define CONF_CLOCK_GCLK_8_ENABLE false
# define CONF_CLOCK_GCLK_8_RUN_IN_STANDBY false
# define CONF_CLOCK_GCLK_8_CLOCK_SOURCE SYSTEM_CLOCK_SOURCE_OSC16M
# define CONF_CLOCK_GCLK_8_PRESCALER 1
# define CONF_CLOCK_GCLK_8_OUTPUT_ENABLE false
#endif /* CONF_CLOCKS_H_INCLUDED */
此修改在编译和运行阶段不会导致任何错误。但是，用于通过USB CDC Virtual COM发送A字符的udi_cdc_putc（＆＃39; A＆＃39;）似乎不起作用。我无法观察到主机端（PC）收到的任何字符。同样，没有从主机端（PC）接收到设备端（Atmel SAM）的字符。
我认为这个虚拟COM失败可能有两个主要原因：
请让我知道你的意见，并就如何解决这个问题提出想法。
非常感谢，期待很快收到社区的回复。