diff --git a/ports/samd/mcu/samd21/clock_config.c b/ports/samd/mcu/samd21/clock_config.c
index 4c05dd7cc7..d84ee52676 100644
--- a/ports/samd/mcu/samd21/clock_config.c
+++ b/ports/samd/mcu/samd21/clock_config.c
@@ -52,7 +52,7 @@ uint32_t get_peripheral_freq(void) {
 
 void set_cpu_freq(uint32_t cpu_freq_arg) {
 
-    // Set 1 waitstate to be safe
+    // Set 1 wait state to be safe
     NVMCTRL->CTRLB.reg = NVMCTRL_CTRLB_MANW | NVMCTRL_CTRLB_RWS(1);
 
     int div = MAX(DFLL48M_FREQ / cpu_freq_arg, 1);
@@ -75,7 +75,7 @@ void set_cpu_freq(uint32_t cpu_freq_arg) {
         while (GCLK->STATUS.bit.SYNCBUSY) {
         }
         // configure the FDPLL96
-        // CtrlB: Set the ref ource to GCLK, set the Wakup-Fast Flag.
+        // CtrlB: Set the ref source to GCLK, set the Wakeup-Fast Flag.
         SYSCTRL->DPLLCTRLB.reg = SYSCTRL_DPLLCTRLB_REFCLK_GCLK | SYSCTRL_DPLLCTRLB_WUF;
         // Set the FDPLL ratio and enable the DPLL.
         int ldr = cpu_freq / FDPLL_REF_FREQ;
@@ -106,25 +106,36 @@ void set_cpu_freq(uint32_t cpu_freq_arg) {
         while (GCLK->STATUS.bit.SYNCBUSY) {
         }
     }
-    // Set 0 waitstates for slower CPU clock
+    // Set 0 wait states for slower CPU clock
     NVMCTRL->CTRLB.reg = NVMCTRL_CTRLB_MANW | NVMCTRL_CTRLB_RWS(cpu_freq > 24000000 ? 1 : 0);
     SysTick_Config(cpu_freq / 1000);
 }
 
 void check_usb_recovery_mode(void) {
     #if !MICROPY_HW_XOSC32K
-    mp_hal_delay_ms(500);
-    // Check USB status. If not connected, switch DFLL48M back to open loop
-    if (USB->DEVICE.DeviceEndpoint[0].EPCFG.reg == 0) {
-        // Set/keep the open loop mode of the device.
-        SYSCTRL->DFLLVAL.reg = dfll48m_calibration;
-        SYSCTRL->DFLLCTRL.reg = SYSCTRL_DFLLCTRL_CCDIS | SYSCTRL_DFLLCTRL_ENABLE;
+    // Check USB status for up to 1 second. If not connected,
+    // switch DFLL48M back to open loop
+    for (int i = 0; i < 100; i++) {
+        if (USB->DEVICE.DeviceEndpoint[0].EPCFG.reg != 0) {
+            return;
+        }
+        mp_hal_delay_ms(10);
     }
+    // Set/keep the open loop mode of the device.
+    SYSCTRL->DFLLVAL.reg = dfll48m_calibration;
+    SYSCTRL->DFLLCTRL.reg = SYSCTRL_DFLLCTRL_CCDIS | SYSCTRL_DFLLCTRL_ENABLE;
     #endif // MICROPY_HW_XOSC32K
 }
 
 // Purpose of the #defines for the clock configuration.
 //
+// The CPU is either driven by the FDPLL96 oscillator for f >= 48MHz,
+// or by the DFLL48M for lower frequencies. The FDPLL96 takes 32768 Hz
+// as reference frequency, supplied through GCLK1.
+//
+// DFLL48M is used for the peripheral clock, e.g. for PWM, UART, SPI, I2C.
+// DFLL48M is either free running, or controlled by the 32kHz crystal, or
+// Synchronized with the USB clock.
 // Both CPU and peripheral devices are clocked by the DFLL48M clock.
 // DFLL48M is either free running, or controlled by the 32kHz crystal, or
 // Synchronized with the USB clock.
@@ -136,7 +147,7 @@ void check_usb_recovery_mode(void) {
 // The crystal is used, unless MICROPY_HW_MCU_OSC32KULP is set.
 // In that case GCLK1 (and the CPU clock) is driven by the 32K Low power oscillator.
 // The reason for offering this option is a design flaw of the Adafruit
-// Feather boards, where the RGB Led and Debug signals interfere with the
+// Feather boards, where the RGB LED and Debug signals interfere with the
 // crystal, causing the CPU to fail if it is driven by the crystal.
 //
 // If MICROPY_HW_XOSC32K = 0, the 32kHz signal for GCLK1 (and the CPU) is
@@ -144,10 +155,10 @@ void check_usb_recovery_mode(void) {
 //
 // If MICROPY_HW_DFLL_USB_SYNC = 0, the DFLL48M oscillator is free running using
 // the pre-configured trim values. In that mode, the peripheral clock is
-// not exactly 48Mhz and has a substantional temperature drift.
+// not exactly 48Mhz and has a substitutional temperature drift.
 //
 // If MICROPY_HW_DFLL_USB_SYNC = 1, the DFLL48 is synchronized with the 1 kHz USB sync
-// signal. If after boot there is no USB sync within 500ms, the configuration falls
+// signal. If after boot there is no USB sync within 1000 ms, the configuration falls
 // back to a free running 48Mhz oscillator.
 //
 // In all modes, the 48MHz signal has a substantial jitter, largest when
@@ -181,7 +192,7 @@ void init_clocks(uint32_t cpu_freq) {
     NVMCTRL->CTRLB.bit.RWS = 1; // 1 read wait state for 48MHz
 
     #if MICROPY_HW_XOSC32K
-    // Set up OSC32K according datasheet 17.6.3
+    // Set up OSC32K according data sheet 17.6.3
     SYSCTRL->XOSC32K.reg = SYSCTRL_XOSC32K_STARTUP(0x3) | SYSCTRL_XOSC32K_EN32K |
         SYSCTRL_XOSC32K_XTALEN;
     SYSCTRL->XOSC32K.bit.ENABLE = 1;
@@ -211,12 +222,12 @@ void init_clocks(uint32_t cpu_freq) {
     while (GCLK->STATUS.bit.SYNCBUSY) {
     }
 
-    // Enable access to the DFLLCTRL reg acc. to Errata 1.2.1
+    // Enable access to the DFLLCTRL register according to Errata 1.2.1
     SYSCTRL->DFLLCTRL.reg = SYSCTRL_DFLLCTRL_ENABLE;
     while (SYSCTRL->PCLKSR.bit.DFLLRDY == 0) {
     }
     // Step 2: Set the coarse and fine values.
-    // Get the coarse value from the calib data. In case it is not set,
+    // Get the coarse value from the calibration data. In case it is not set,
     // set a midrange value.
     uint32_t coarse = (*((uint32_t *)FUSES_DFLL48M_COARSE_CAL_ADDR) & FUSES_DFLL48M_COARSE_CAL_Msk)
         >> FUSES_DFLL48M_COARSE_CAL_Pos;
diff --git a/ports/samd/mcu/samd51/clock_config.c b/ports/samd/mcu/samd51/clock_config.c
index 50912b77ed..b30cc46f24 100644
--- a/ports/samd/mcu/samd51/clock_config.c
+++ b/ports/samd/mcu/samd51/clock_config.c
@@ -117,27 +117,32 @@ void set_cpu_freq(uint32_t cpu_freq_arg) {
 
 void check_usb_recovery_mode(void) {
     #if !MICROPY_HW_XOSC32K
-    mp_hal_delay_ms(500);
-    // Check USB status. If not connected, switch DFLL48M back to open loop
-    if (USB->DEVICE.DeviceEndpoint[0].EPCFG.reg == 0) {
-        // as per Errata 2.8.3
-        OSCCTRL->DFLLMUL.reg = 0;
-        while (OSCCTRL->DFLLSYNC.bit.DFLLMUL == 1) {
-        }
-        // Set the mode to open loop mode
-        OSCCTRL->DFLLCTRLB.reg = 0;
-        while (OSCCTRL->DFLLSYNC.bit.DFLLCTRLB == 1) {
-        }
-        OSCCTRL->DFLLCTRLA.reg = OSCCTRL_DFLLCTRLA_RUNSTDBY | OSCCTRL_DFLLCTRLA_ENABLE;
-        while (OSCCTRL->DFLLSYNC.bit.ENABLE == 1) {
-        }
-        OSCCTRL->DFLLVAL.reg = dfll48m_calibration; // Reload DFLLVAL register
-        while (OSCCTRL->DFLLSYNC.bit.DFLLVAL == 1) {
-        }
-        // Set the mode to open loop mode
-        OSCCTRL->DFLLCTRLB.reg = 0;
-        while (OSCCTRL->DFLLSYNC.bit.DFLLCTRLB == 1) {
+    // Check USB status for up to 1 second. If not connected,
+    // switch DFLL48M back to open loop
+    for (int i = 0; i < 100; i++) {
+        if (USB->DEVICE.DeviceEndpoint[0].EPCFG.reg != 0) {
+            return;
         }
+        mp_hal_delay_ms(10);
+    }
+    // No connection. Switch back to open loop mode.
+    // as per Errata 2.8.3
+    OSCCTRL->DFLLMUL.reg = 0;
+    while (OSCCTRL->DFLLSYNC.bit.DFLLMUL == 1) {
+    }
+    // Set the mode to open loop mode
+    OSCCTRL->DFLLCTRLB.reg = 0;
+    while (OSCCTRL->DFLLSYNC.bit.DFLLCTRLB == 1) {
+    }
+    OSCCTRL->DFLLCTRLA.reg = OSCCTRL_DFLLCTRLA_RUNSTDBY | OSCCTRL_DFLLCTRLA_ENABLE;
+    while (OSCCTRL->DFLLSYNC.bit.ENABLE == 1) {
+    }
+    OSCCTRL->DFLLVAL.reg = dfll48m_calibration; // Reload DFLLVAL register
+    while (OSCCTRL->DFLLSYNC.bit.DFLLVAL == 1) {
+    }
+    // Set the mode to open loop mode
+    OSCCTRL->DFLLCTRLB.reg = 0;
+    while (OSCCTRL->DFLLSYNC.bit.DFLLCTRLB == 1) {
     }
     #endif // MICROPY_HW_XOSC32K
 }
@@ -169,10 +174,10 @@ void check_usb_recovery_mode(void) {
 //
 // If MICROPY_HW_DFLL_USB_SYNC = 0, the DFLL48M oscillator is free running using
 // the pre-configured trim values. In that mode, the peripheral clock is
-// not exactly 48Mhz and has a substantional temperature drift.
+// not exactly 48Mhz and has a substitutional temperature drift.
 //
 // If MICROPY_HW_DFLL_USB_SYNC = 1, the DFLL48 is synchronized with the 1 kHz USB sync
-// signal. If after boot there is no USB sync within 500ms, the configuration falls
+// signal. If after boot there is no USB sync within 1000 ms, the configuration falls
 // back to a free running 48Mhz oscillator.
 //
 // In all modes, the 48MHz signal has a substantial jitter, largest when
@@ -223,16 +228,16 @@ void init_clocks(uint32_t cpu_freq) {
     OSC32KCTRL->RTCCTRL.reg = OSC32KCTRL_RTCCTRL_RTCSEL_XOSC1K;
     // Setup XOSC32K
     OSC32KCTRL->INTFLAG.reg = OSC32KCTRL_INTFLAG_XOSC32KRDY | OSC32KCTRL_INTFLAG_XOSC32KFAIL;
-    OSC32KCTRL->CFDCTRL.bit.CFDEN = 1; // Fall back to internal Osc on crystal fail
+    OSC32KCTRL->CFDCTRL.bit.CFDEN = 1; // Fall back to internal oscillator on crystal fail
     OSC32KCTRL->XOSC32K.reg =
         OSC32KCTRL_XOSC32K_CGM_HS |
         OSC32KCTRL_XOSC32K_XTALEN |
         OSC32KCTRL_XOSC32K_EN32K |
         OSC32KCTRL_XOSC32K_EN1K |
         OSC32KCTRL_XOSC32K_RUNSTDBY |
-        OSC32KCTRL_XOSC32K_STARTUP(4) |
+        OSC32KCTRL_XOSC32K_STARTUP(3) |
         OSC32KCTRL_XOSC32K_ENABLE;
-    // make sure osc32kcrtl is ready
+    // Wait until the oscillator is running and stable
     while (OSC32KCTRL->STATUS.bit.XOSC32KRDY == 0) {
     }