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Merge pull request #1276 from jpbrucker/nrf51_clock_fallback 

Nordic: fallback to the internal LF clock in case Xtal doesn't start
pull/1144/merge
Martin Kojtal 2015-08-13 10:17:26 +01:00
parent ab6a668ec1 15019ef882
commit 99877b836d
1 changed files with 33 additions and 2 deletions
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35
libraries/mbed/targets/cmsis/TARGET_NORDIC/TARGET_MCU_NRF51822/system_nrf51.c
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@ -34,6 +34,7 @@
#include <stdint.h>
#include <stdbool.h>
#include "nrf.h"
#include "nrf_delay.h"
#include "system_nrf51.h"
/*lint ++flb "Enter library region" */
@ -42,6 +43,7 @@
static bool is_manual_peripheral_setup_needed(void);
static bool is_disabled_in_debug_needed(void);
static void init_clock(void);
#if defined ( __CC_ARM )
@ -82,8 +84,18 @@ void SystemInit(void)
}
// Start the external 32khz crystal oscillator.
init_clock();
}
void init_clock(void)
{
/* For compatibility purpose, the default behaviour is to first attempt to initialise an
external clock, and after a timeout, use the internal RC one. To avoid this wait, boards that
don't have an external oscillator can set TARGET_NRF_LFCLK_RC directly. */
int i = 0;
const uint32_t polling_period = 200;
const uint32_t timeout = 1000000;
/* for Nordic devices without an external LF crystal */
#if defined(TARGET_NRF_LFCLK_RC)
NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_RC << CLOCK_LFCLKSRC_SRC_Pos);
#else
@ -92,7 +104,26 @@ void SystemInit(void)
NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
NRF_CLOCK->TASKS_LFCLKSTART = 1;
// Wait for the external oscillator to start up.
/* Wait for the external oscillator to start up.
nRF51822 product specification (8.1.5) gives a typical value of 300ms for external clock
startup duration, and a maximum value of 1s. When using the internal RC source, typical delay
will be 390µs, so we use a polling period of 200µs.
We can't use us_ticker at this point, so we have to rely on a less precise method for
measuring our timeout. Because of this, the actual timeout will be slightly longer than 1
second, which isn't an issue at all, since this fallback should only be used as a safety net.
*/
for (i = 0; i < (timeout / polling_period); i++) {
if (NRF_CLOCK->EVENTS_LFCLKSTARTED != 0)
return;
nrf_delay_us(polling_period);
}
/* Fallback to internal clock. Belt and braces, since the internal clock is used by default
whilst no external source is running. This is not only a sanity check, but it also allows
code down the road (e.g. ble initialisation) to directly know which clock is used. */
NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_RC << CLOCK_LFCLKSRC_SRC_Pos);
NRF_CLOCK->TASKS_LFCLKSTART = 1;
while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {
// Do nothing.
}