cpu/nrf5x: allow to request/release HFXO clk src

cpu/nrf5x_common/clock.c

@@ -18,6 +18,9 @@
  */
 
 #include "cpu.h"
+#include "irq.h"
+#include "assert.h"
+#include "nrf_clock.h"
 #include "periph_conf.h"
 
 /* make sure both clocks are configured */
@@ -28,6 +31,8 @@
 #error "Clock init: CLOCK_LFCLK is not defined by your board!"
 #endif
 
+static unsigned _hfxo_requests = 0;
+
 void clock_init_hf(void)
 {
     /* for the nRF51 we can chose the XTAL frequency */
@@ -39,12 +44,36 @@ void clock_init_hf(void)
 #endif
 #endif
 
-#if CLOCK_HFCLK
+    /* allow to always enable the HFXO as non-default option */
-    /* start the HF clock */
+#if CLOCK_HFXO_ONBOOT
+    clock_hfxo_request();
+#endif
+}
+
+void clock_hfxo_request(void)
+{
+    unsigned state = irq_disable();
+    ++_hfxo_requests;
+    if (_hfxo_requests == 1) {
         NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
         NRF_CLOCK->TASKS_HFCLKSTART = 1;
         while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0) {}
-#endif
+    }
+    irq_restore(state);
+}
+
+void clock_hfxo_release(void)
+{
+    /* if this function is called while the counter is zero the state in a
+     * driver requesting the HFXO is broken! */
+    assert(_hfxo_requests);
+
+    unsigned state = irq_disable();
+    --_hfxo_requests;
+    if (_hfxo_requests == 0) {
+        NRF_CLOCK->TASKS_HFCLKSTOP = 1;
+    }
+    irq_restore(state);
 }
 
 void clock_start_lf(void)

cpu/nrf5x_common/include/nrf_clock.h

@@ -23,12 +23,33 @@
 extern "C" {
 #endif
 
+/**
+ * @brief   The high frequency clock (HFCLK) uses the internal oscillator per
+ *          default. Setting this define to 1 will enable the HFXO clock source
+ *          on boot so it will always be active.
+ */
+#ifndef CLOCK_HFXO_ONBOOT
+#define CLOCK_HFXO_ONBOOT       0
+#endif
+
 /**
  * @brief   Initialize the high frequency clock (HFCLK) as configured in the
  *          board's periph_conf.h
  */
 void clock_init_hf(void);
 
+/**
+ * @brief   Request the external high frequency crystal (HFXO) as HF clock
+ *          source. If this is the first request, the HFXO will be enabled.
+ */
+void clock_hfxo_request(void);
+
+/**
+ * @brief   Release the use of the HFXO. If this was the last active request,
+ *          the HFXO will be disabled
+ */
+void clock_hfxo_release(void);
+
 /**
  * @brief   Start the low frequency clock (LFCLK) as configured in the board's
  *          periph_conf.