nRF52832 LFCLK external signal and soft device

[apuret]

Hello,

I try to set up an external signal for the LFCLK on a nrf52832. From this question:

• I know the nrf52832 can do this (signal on XL1 pin, XL2 grounded).
• I know I must set the bits "bypass" and "external clock" in LFCLKSRC register.

But I don't know how to set these bits through the SDK functions, with a running soft device.

• Defines like CLOCK_LFCLKSRC_BYPASS_xxx and CLOCK_LFCLKSRC_EXTERNAL_xxx dont exist in the SDK 11.0.0.
• The function softdevice_handler_init() (through SOFTDEVICE_HANDLER_APPSH_INIT()) hasn't the required parameter in the nrf_clock_lf_cfg_t structure.
• The function softdevice_handler_init() forces LFCLK configuration and the LFCLKSRC register can't be set after the soft device initialization.

Can someone give me a valid code example?

Thanks !

[apuret]

With other answers and some studying, I have found the solution:

• external clock signal must be connected to XL1 pin,
• XL2 pin may be left floating or be grounded (if the signal voltage isn't rail-to-rail),
• LFCLK MUST be configured for external clock signal and started BEFORE enabling the softdevice,
• the softdevice may now be initialized with NRF_CLOCK_LF_SRC_XTAL for the LFCLK.

Here is a code example (for the nRF52832) :

/* Defines not available in the SDK 11.0.0 */
/* Register: CLOCK_LFCLKSRC */
/* Description: Clock source for the LFCLK */

/* Bits 16 : Enable or disable bypass of LFCLK crystal oscillator with external clock source */
#define CLOCK_LFCLKSRC_BYPASS_Pos (16UL) /*!< Position of BYPASS field. */
#define CLOCK_LFCLKSRC_BYPASS_Msk (0x1UL << CLOCK_LFCLKSRC_SRC_Pos) /*!< Bit mask of BYPASS field. */
#define CLOCK_LFCLKSRC_BYPASS_Disabled (0UL) /*!< Disabled bypass */
#define CLOCK_LFCLKSRC_BYPASS_Enabled (1UL) /*!< Enable bypass */

/* Bits 17 : Enable or disable external source for LFCLK */
#define CLOCK_LFCLKSRC_EXTERNAL_Pos (17UL) /*!< Position of EXTERNAL field. */
#define CLOCK_LFCLKSRC_EXTERNAL_Msk (0x1UL << CLOCK_LFCLKSRC_SRC_Pos) /*!< Bit mask of EXTERNAL field. */
#define CLOCK_LFCLKSRC_EXTERNAL_Disabled (0UL) /*!< Disabled external source */
#define CLOCK_LFCLKSRC_EXTERNAL_Enabled (1UL) /*!< Enable external source */

/* Configuring LFCLK to use an external signal */
NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos) |
	(CLOCK_LFCLKSRC_BYPASS_Enabled << CLOCK_LFCLKSRC_BYPASS_Pos ) |
	(CLOCK_LFCLKSRC_EXTERNAL_Enabled << CLOCK_LFCLKSRC_EXTERNAL_Pos);

/* Starting LFCLK */
NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
NRF_CLOCK->TASKS_LFCLKSTART = 1;
while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0)
{
	/* Nothing to do */
}

/* Initialising softdevice with a 'crystal' for LFCLK source (and no scheduler) */
nrf_clock_lf_cfg_t clockLfCfg = {
	.source        = NRF_CLOCK_LF_SRC_XTAL,
	.rc_ctiv       = 0,
	.rc_temp_ctiv  = 0,
	.xtal_accuracy = NRF_CLOCK_LF_XTAL_ACCURACY_20_PPM
};
SOFTDEVICE_HANDLER_INIT(&clockLfCfg, NULL);