sys/ztimer: expose ZTIMER_MSEC/USEC_BASE

ztimer_clock are meant to be chained. At the end of the chaine
there is always a timer device object (periph_rtt/rtc/timer).

Since ZTIMER_MSEC and ZTIMER_USEC can be the scaled/shifted with
respect to the base periph_rtt/rtc/timer it makes sense to chain
other ZTIMER_XXX on top of the base rtc/timer/rtt in order to avoid
chained convertions.

sys/include/ztimer.h

@@ -513,6 +513,41 @@ extern ztimer_clock_t *const ZTIMER_USEC;
  */
 extern ztimer_clock_t *const ZTIMER_MSEC;
 
+/**
+ * @brief   Base ztimer for the microsecond clock (ZTIMER_USEC)
+ *
+ * This ztimer will reference the counter device object at the end of the
+ * chain of ztimer_clock_t for ZTIMER_USEC.
+ *
+ * If the base counter device object's frequency (CONFIG_ZTIMER_USEC_BASE_FREQ)
+ * is not 1MHz then ZTIMER_USEC will be converted on top of this one. Otherwise
+ * they will reference the same ztimer_clock.
+ *
+ * To avoid chained conversions its better to base new ztimer_clock on top of
+ * ZTIMER_USEC_BASE running at CONFIG_ZTIMER_USEC_BASE_FREQ.
+ *
+ */
+extern ztimer_clock_t *const ZTIMER_USEC_BASE;
+
+/**
+ * @brief   Base ztimer for the millisecond clock (ZTIMER_MSEC)
+ *
+ * This ztimer will reference the counter device object at the end of the
+ * chain of ztimer_clock_t for ZTIMER_MSEC.
+ *
+ * If ztimer_periph_rtt is not used then ZTIMER_MSEC_BASE will reference the
+ * same base as ZTIMER_USEC_BASE.
+ *
+ * If the base counter device object's frequency (CONFIG_ZTIMER_MSEC_BASE_FREQ)
+ * is not 1KHz then ZTIMER_MSEC will be converted on top of this one. Otherwise
+ * they will reference the same ztimer_clock.
+ *
+ * To avoid chained conversions its better to base new ztimer_clock on top of
+ * ZTIMER_MSEC_BASE running at CONFIG_ZTIMER_MSEC_BASE_FREQ.
+ *
+ */
+extern ztimer_clock_t *const ZTIMER_MSEC_BASE;
+
 #ifdef __cplusplus
 extern "C" {
 #endif

sys/ztimer/auto_init.c

@@ -97,6 +97,7 @@
 #if MODULE_ZTIMER_USEC
 #  if CONFIG_ZTIMER_USEC_TYPE_PERIPH_TIMER
 static ztimer_periph_timer_t _ztimer_periph_timer_usec = { .min = CONFIG_ZTIMER_USEC_MIN };
+ztimer_clock_t *const ZTIMER_USEC_BASE = &_ztimer_periph_timer_usec.super;
 #    if CONFIG_ZTIMER_USEC_FREQ == FREQ_1MHZ
 ztimer_clock_t *const ZTIMER_USEC = &_ztimer_periph_timer_usec.super;
 #    elif CONFIG_ZTIMER_USEC_FREQ == 250000LU
@@ -113,20 +114,22 @@ ztimer_clock_t *const ZTIMER_USEC = &_ztimer_convert_frac_usec.super.super;
 #endif
 
 #if MODULE_ZTIMER_MSEC
-#  if MODULE_PERIPH_RTT
+#  if MODULE_ZTIMER_PERIPH_RTT
 static ztimer_periph_rtt_t _ztimer_periph_timer_rtt_msec;
-#  define ZTIMER_RTT_INIT (&_ztimer_periph_timer_rtt_msec)
+ztimer_clock_t *const ZTIMER_MSEC_BASE = &_ztimer_periph_timer_rtt_msec;
+#  define ZTIMER_RTT_INIT (ZTIMER_MSEC_BASE)
 #    if RTT_FREQUENCY!=FREQ_1KHZ
 static ztimer_convert_frac_t _ztimer_convert_frac_msec;
 ztimer_clock_t *const ZTIMER_MSEC = &_ztimer_convert_frac_msec.super.super;
 #  define ZTIMER_MSEC_CONVERT_LOWER_FREQ    RTT_FREQUENCY
 #  define ZTIMER_MSEC_CONVERT_LOWER         (&_ztimer_periph_timer_rtt_msec)
 #    else
-ztimer_clock_t *const ZTIMER_MSEC = &_ztimer_periph_timer_rtt_msec.super;
+ztimer_clock_t *const ZTIMER_MSEC = &_ztimer_periph_timer_rtt_msec;
 #    endif
 #  elif MODULE_ZTIMER_USEC
 static ztimer_convert_frac_t _ztimer_convert_frac_msec;
 ztimer_clock_t *const ZTIMER_MSEC = &_ztimer_convert_frac_msec.super.super;
+ztimer_clock_t *const ZTIMER_MSEC_BASE = &_ztimer_periph_timer_usec.super;
 #    if CONFIG_ZTIMER_USEC_FREQ < FREQ_1MHZ
 #      define ZTIMER_MSEC_CONVERT_LOWER         ZTIMER_USEC_CONVERT_LOWER
 #      define ZTIMER_MSEC_CONVERT_LOWER_FREQ    CONFIG_ZTIMER_USEC_FREQ