samd/clock_config: Document the #defines use in init_clocks().
Which may be set in the respective mpconfigboard.h files.
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@ -96,6 +96,40 @@ void check_usb_recovery_mode(void) {
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#endif // MICROPY_HW_XOSC32K
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}
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// Purpose of the #defines for the clock configuration.
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//
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// Both CPU and periperal devices are clocked by the DFLL48M clock.
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// DFLL48M is either free running, or controlled by the 32kHz crystal, or
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// Synchronized with the USB clock.
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//
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// #define MICROPY_HW_XOSC32K (0 | 1)
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//
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// If MICROPY_HW_XOSC32K = 1, the 32kHz crystal is used as input for GCLK 1, which
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// serves as refernce clock source for the DFLL48M oscillator,
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// The crystal is used, unless MICROPY_HW_MCU_OSC32KULP is set.
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// In that case GCLK1 (and the CPU clock) is driven by the 32K Low power oscillator.
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// The reason for offering this option is a design flaw of the Adafruit
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// Feather boards, where the RGB Led and Debug signals interfere with the
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// crystal, causing the CPU to fail if it is driven by the crystal.
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//
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// If MICROPY_HW_XOSC32K = 0, the 32kHz signal for GCLK1 (and the CPU) is
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// created by dividing the 48MHz clock of DFLL48M, but not used otherwise.
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//
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// If MICROPY_HW_DFLL_USB_SYNC = 0, the DFLL48M oscillator is free running using
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// the pre-configured trim values. In that mode, the peripheral clock is
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// not exactly 48Mhz and has a substantional temperature drift.
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//
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// If MICROPY_HW_DFLL_USB_SYNC = 1, the DFLL48 is synchronized with the 1 kHz USB sync
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// signal. If after boot there is no USB sync withing 500ms, the configuratuion falls
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// back to a free running 48Mhz oscillator.
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//
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// In all modes, the 48MHz signal has a substantial jitter, largest when
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// MICROPY_HW_DFLL_USB_SYNC is active. That is caused by the repective
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// reference frequencies of 32kHz or 1 kHz being low. That affects most
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// PWM. Std Dev at 1kHz 0.156Hz (w. Crystal) up to 0.4 Hz (with USB sync).
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//
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// If none of the mentioned defines is set, the device uses the internal oscillators.
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void init_clocks(uint32_t cpu_freq) {
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dfll48m_calibration = 0; // please the compiler
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@ -142,6 +142,46 @@ void check_usb_recovery_mode(void) {
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#endif // MICROPY_HW_XOSC32K
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}
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// Purpose of the #defines for the clock configuration.
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//
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// The CPU clock is generated by DPLL0, which takes 32768 Hz as reference frequency,
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// supplied through GCLK1.
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//
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// DFLL48M is used for the peripheral clock, e.g. for PWM, UART, SPI, I2C.
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// DFLL48M is either free running, or controlled by the 32kHz crystal, or
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// Synchronized with the USB clock.
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//
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// GCLK1 takes it's input either from the 32kHz crystal, the internal low power
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// RC oscillator or from DFLL48M.
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//
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// #define MICROPY_HW_XOSC32K (0 | 1)
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//
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// If MICROPY_HW_XOSC32K = 1, the 32kHz crystal is used for the DFLL48M oscillator
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// and for GCLK1, feeding the CPU, unless MICROPY_HW_MCU_OSC32KULP is set.
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// In that case GCLK1 (and the CPU clock) is driven by the 32K Low power oscillator.
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// The reason for offering this option is a design flaw of the Adafruit
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// Feather boards, where the RGB Led and Debug signals interfere with the
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// crystal, causing the CPU to fail if it is driven by the crystal. The
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// peripheral devices are affected as well, but continue it's operation.
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//
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// If MICROPY_HW_XOSC32K = 0, the 32kHz signal for GCLK1 (and the CPU) is
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// created by dividing the 48MHz clock of DFLL48M.
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//
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// If MICROPY_HW_DFLL_USB_SYNC = 0, the DFLL48M oscillator is free running using
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// the pre-configured trim values. In that mode, the peripheral clock is
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// not exactly 48Mhz and has a substantional temperature drift.
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//
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// If MICROPY_HW_DFLL_USB_SYNC = 1, the DFLL48 is synchronized with the 1 kHz USB sync
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// signal. If after boot there is no USB sync withing 500ms, the configuratuion falls
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// back to a free running 48Mhz oscillator.
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//
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// In all modes, the 48MHz signal has a substantial jitter, largest when
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// MICROPY_HW_DFLL_USB_SYNC is active. That is caused by the repective
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// reference frequencies of 32kHz or 1 kHz being low. That affects most
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// PWM. Std Dev at 1kHz 0.156Hz (w. Crystal) up to 0.4 Hz (with USB sync).
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//
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// If none of the mentioned defines is set, the device uses the internal oscillators.
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void init_clocks(uint32_t cpu_freq) {
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dfll48m_calibration = 0; // please the compiler
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