Contribution description

@BOZHENG001 encountered the issue where all NimBLE-based examples would fail to establish connection while advertising works normally on feather-nrf52840-sense, while they work properly on nrf52840dk. As suggested by @andrzej-kaczmarek in apache/mynewt-nimble#1777 (comment), this is an issue with the internal RC low-frequency clock which needs to be calibrated.

While RIOT already tries to do so, it currently ignores the fact that the high-frequency clock needs to be manually started for the calibration to succeed. This is documented for nRF51 here on page 53 and for nRF52-series for example here for nRF52832 and here for nRF52840. The documentation of nRF5240 does not explicitly mention the necessity to manually start the HFCLK, but it shouldn't harm there either.

Testing procedure

make -C examples/nimble_heart_rate_sensor BOARD=feather-nrf52840-sense flash term fails to establish a connection (e.g. to the Android app "nRF Connect"). This is also possible to reproduce with BOARD=nrf52840dk after switching to the internal RC oscillator:

diff --git a/boards/common/nrf52xxxdk/include/periph_conf_common.h b/boards/common/nrf52xxxdk/include/periph_conf_common.h
index 0b7bbdd019..af35d672ff 100644
--- a/boards/common/nrf52xxxdk/include/periph_conf_common.h
+++ b/boards/common/nrf52xxxdk/include/periph_conf_common.h
@@ -22,7 +22,7 @@
 #define PERIPH_CONF_COMMON_H
 
 #include "periph_cpu.h"
-#include "cfg_clock_32_1.h"
+#include "cfg_clock_32_0.h"
 #include "cfg_i2c_default.h"
 #include "cfg_rtt_default.h"
 #include "cfg_timer_default.h"

With this PR, connection can be reliably established.

Issues/PRs references

The issue has also been discussed on Matrix with @maribu, thanks for your input!