NRF5 - make us ticker to be driven by high speed 1MHz timer

According to new ticker standards the following requirements for us ticker are not met on RRF5 boards:
- has a frequency between 250KHz and 8MHz (currently is driven by 32kHz clock)
- ticker increments by 1 each tick (currently is scaled to 1 MHz by incrementing counter by ~31)

Since BLE softdevice uses TIMER0 the proposition is to use high speed TIMER1 for us ticker configured as follows:
- TIMER counter width: 16 bits (max)
- TIMER frequency: 1MHz
This solution also uses Timer's capture/compare register 0 to specify interrupt time and Timer's capture/compare register 1 to read current timer value.

Author: mprse

targets/TARGET_NORDIC/TARGET_NRF5/common_rtc.c

@@ -0,0 +1,276 @@
+/*
+ * Copyright (c) 2013 Nordic Semiconductor ASA
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without modification,
+ * are permitted provided that the following conditions are met:
+ *
+ *   1. Redistributions of source code must retain the above copyright notice, this list
+ *      of conditions and the following disclaimer.
+ *
+ *   2. Redistributions in binary form, except as embedded into a Nordic Semiconductor ASA
+ *      integrated circuit in a product or a software update for such product, must reproduce
+ *      the above copyright notice, this list of conditions and the following disclaimer in
+ *      the documentation and/or other materials provided with the distribution.
+ *
+ *   3. Neither the name of Nordic Semiconductor ASA nor the names of its contributors may be
+ *      used to endorse or promote products derived from this software without specific prior
+ *      written permission.
+ *
+ *   4. This software, with or without modification, must only be used with a
+ *      Nordic Semiconductor ASA integrated circuit.
+ *
+ *   5. Any software provided in binary or object form under this license must not be reverse
+ *      engineered, decompiled, modified and/or disassembled.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include "us_ticker_api.h"
+#include "common_rtc.h"
+#include "app_util.h"
+#include "nrf_drv_common.h"
+#include "lp_ticker_api.h"
+#include "mbed_critical.h"
+
+#if defined(NRF52_ERRATA_20)
+    #include "softdevice_handler.h"
+#endif
+
+//------------------------------------------------------------------------------
+// Common stuff used also by lp_ticker and rtc_api (see "common_rtc.h").
+//
+#include "app_util_platform.h"
+
+bool              m_common_rtc_enabled = false;
+uint32_t volatile m_common_rtc_overflows = 0;
+
+__STATIC_INLINE void rtc_ovf_event_check(void)
+{
+    if (nrf_rtc_event_pending(COMMON_RTC_INSTANCE, NRF_RTC_EVENT_OVERFLOW)) {
+        nrf_rtc_event_clear(COMMON_RTC_INSTANCE, NRF_RTC_EVENT_OVERFLOW);
+        // Don't disable this event. It shall occur periodically.
+
+        ++m_common_rtc_overflows;
+    }
+}
+
+#if defined(TARGET_MCU_NRF51822)
+void common_rtc_irq_handler(void)
+#else
+void COMMON_RTC_IRQ_HANDLER(void)
+#endif
+{
+    rtc_ovf_event_check();
+
+#if DEVICE_LOWPOWERTIMER
+    if (nrf_rtc_event_pending(COMMON_RTC_INSTANCE, LP_TICKER_EVENT)) {
+
+        lp_ticker_irq_handler();
+    }
+#endif
+}
+
+// Function for fix errata 20: RTC Register values are invalid
+__STATIC_INLINE void errata_20(void)
+{
+#if defined(NRF52_ERRATA_20)
+    if (!softdevice_handler_is_enabled())
+    {
+        NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
+        NRF_CLOCK->TASKS_LFCLKSTART    = 1;
+
+        while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0)
+        {
+        }
+    }
+    NRF_RTC1->TASKS_STOP = 0;
+#endif
+}
+
+void RTC1_IRQHandler(void);
+
+void common_rtc_init(void)
+{
+    if (m_common_rtc_enabled) {
+        return;
+    }
+
+    errata_20();
+
+    NVIC_SetVector(RTC1_IRQn, (uint32_t)RTC1_IRQHandler);
+
+    // RTC is driven by the low frequency (32.768 kHz) clock, a proper request
+    // must be made to have it running.
+    // Currently this clock is started in 'SystemInit' (see "system_nrf51.c"
+    // or "system_nrf52.c", respectively).
+
+    nrf_rtc_prescaler_set(COMMON_RTC_INSTANCE, 0);
+
+#if defined(TARGET_MCU_NRF51822)
+    nrf_rtc_event_clear(COMMON_RTC_INSTANCE, OS_TICK_EVENT);
+#endif
+#if DEVICE_LOWPOWERTIMER
+    nrf_rtc_event_clear(COMMON_RTC_INSTANCE, LP_TICKER_EVENT);
+#endif
+    nrf_rtc_event_clear(COMMON_RTC_INSTANCE, NRF_RTC_EVENT_OVERFLOW);
+
+    // Interrupts on all related events are enabled permanently. Particular
+    // events will be enabled or disabled as needed (such approach is more
+    // energy efficient).
+    nrf_rtc_int_enable(COMMON_RTC_INSTANCE,
+#if DEVICE_LOWPOWERTIMER
+        LP_TICKER_INT_MASK |
+#endif
+        NRF_RTC_INT_OVERFLOW_MASK);
+
+    // This event is enabled permanently, since overflow indications are needed
+    // continuously.
+    nrf_rtc_event_enable(COMMON_RTC_INSTANCE, NRF_RTC_INT_OVERFLOW_MASK);
+    // All other relevant events are initially disabled.
+    nrf_rtc_event_disable(COMMON_RTC_INSTANCE, 0 |
+#if defined(TARGET_MCU_NRF51822)
+        OS_TICK_INT_MASK |
+#endif
+#if DEVICE_LOWPOWERTIMER
+        LP_TICKER_INT_MASK
+#endif
+        );
+
+    nrf_drv_common_irq_enable(nrf_drv_get_IRQn(COMMON_RTC_INSTANCE),
+#ifdef NRF51
+        APP_IRQ_PRIORITY_LOW
+#elif defined(NRF52) || defined(NRF52840_XXAA)
+        APP_IRQ_PRIORITY_LOWEST
+#endif
+        );
+
+    nrf_rtc_task_trigger(COMMON_RTC_INSTANCE, NRF_RTC_TASK_START);
+
+    m_common_rtc_enabled = true;
+}
+
+__STATIC_INLINE void rtc_ovf_event_safe_check(void)
+{
+    core_util_critical_section_enter();
+
+    rtc_ovf_event_check();
+
+    core_util_critical_section_exit();
+}
+
+
+uint32_t common_rtc_32bit_ticks_get(void)
+{
+    uint32_t ticks;
+    uint32_t prev_overflows;
+
+    do {
+        prev_overflows = m_common_rtc_overflows;
+
+        ticks = nrf_rtc_counter_get(COMMON_RTC_INSTANCE);
+        // The counter used for time measurements is less than 32 bit wide,
+        // so its value is complemented with the number of registered overflows
+        // of the counter.
+        ticks += (m_common_rtc_overflows << RTC_COUNTER_BITS);
+
+        // Check in case that OVF occurred during execution of a RTC handler (apply if call was from RTC handler)
+        // m_common_rtc_overflows might been updated in this call.
+        rtc_ovf_event_safe_check();
+
+        // If call was made from a low priority level m_common_rtc_overflows might have been updated in RTC handler.
+    } while (m_common_rtc_overflows != prev_overflows);
+
+    return ticks;
+}
+
+uint64_t common_rtc_64bit_us_get(void)
+{
+    uint32_t ticks = common_rtc_32bit_ticks_get();
+    // [ticks -> microseconds]
+    return ROUNDED_DIV(((uint64_t)ticks) * 1000000, RTC_INPUT_FREQ);
+}
+
+void common_rtc_set_interrupt(uint32_t us_timestamp, uint32_t cc_channel,
+                              uint32_t int_mask)
+{
+    // The internal counter is clocked with a frequency that cannot be easily
+    // multiplied to 1 MHz, therefore besides the translation of values
+    // (microsecond <-> ticks) a special care of overflows handling must be
+    // taken. Here the 32-bit timestamp value is complemented with information
+    // about current the system up time of (ticks + number of overflows of tick
+    // counter on upper bits, converted to microseconds), and such 64-bit value
+    // is then translated to counter ticks. Finally, the lower 24 bits of thus
+    // calculated value is written to the counter compare register to prepare
+    // the interrupt generation.
+    uint64_t current_time64 = common_rtc_64bit_us_get();
+    // [add upper 32 bits from the current time to the timestamp value]
+    uint64_t timestamp64 = us_timestamp +
+        (current_time64 & ~(uint64_t)0xFFFFFFFF);
+    // [if the original timestamp value happens to be after the 32 bit counter
+    //  of microsends overflows, correct the upper 32 bits accordingly]
+    if (us_timestamp < (uint32_t)(current_time64 & 0xFFFFFFFF)) {
+        timestamp64 += ((uint64_t)1 << 32);
+    }
+    // [microseconds -> ticks, always round the result up to avoid too early
+    //  interrupt generation]
+    uint32_t compare_value =
+        (uint32_t)CEIL_DIV((timestamp64) * RTC_INPUT_FREQ, 1000000);
+
+
+    core_util_critical_section_enter();
+    // The COMPARE event occurs when the value in compare register is N and
+    // the counter value changes from N-1 to N. Therefore, the minimal safe
+    // difference between the compare value to be set and the current counter
+    // value is 2 ticks. This guarantees that the compare trigger is properly
+    // setup before the compare condition occurs.
+    uint32_t closest_safe_compare = common_rtc_32bit_ticks_get() + 2;
+    if ((int)(compare_value - closest_safe_compare) <= 0) {
+        compare_value = closest_safe_compare;
+    }
+
+    nrf_rtc_cc_set(COMMON_RTC_INSTANCE, cc_channel, RTC_WRAP(compare_value));
+    nrf_rtc_event_enable(COMMON_RTC_INSTANCE, int_mask);
+
+    core_util_critical_section_exit();
+}
+
+// Since there is no SysTick on NRF51, the RTC1 channel 1 is used as an
+// alternative source of RTOS ticks.
+#if defined(TARGET_MCU_NRF51822)
+
+#include "mbed_toolchain.h"
+
+
+#define MAX_RTC_COUNTER_VAL     ((1uL << RTC_COUNTER_BITS) - 1)
+
+#ifndef RTC1_CONFIG_FREQUENCY
+    #define RTC1_CONFIG_FREQUENCY    32678 // [Hz]
+#endif
+
+
+
+void COMMON_RTC_IRQ_HANDLER(void)
+{
+    if(!nrf_rtc_event_pending(COMMON_RTC_INSTANCE, OS_TICK_EVENT)) {
+        common_rtc_irq_handler();
+    }
+}
+
+IRQn_Type mbed_get_m0_tick_irqn()
+{
+    return SWI3_IRQn;
+}
+
+
+#endif // defined(TARGET_MCU_NRF51822)

targets/TARGET_NORDIC/TARGET_NRF5/common_rtc.h

@@ -22,16 +22,12 @@
 #define RTC_COUNTER_BITS        24u
 
 // Instance 0 is reserved for SoftDevice.
-// Instance 1 is used as a common one for us_ticker, lp_ticker and (in case
+// Instance 1 is used as a common one for lp_ticker and (in case
 // of NRF51) as an alternative tick source for RTOS.
-// ["us_ticker.c" uses hard coded addresses of the 'NRF_RTC1->EVENT_COMPARE[1]'
-//  register in inline assembly implementations of COMMON_RTC_IRQ_HANDLER,
-//  please remember to update those in case of doing changes here]
 #define COMMON_RTC_INSTANCE     NRF_RTC1
 #define COMMON_RTC_IRQ_HANDLER  RTC1_IRQHandler
-#define US_TICKER_CC_CHANNEL    0
-#define OS_TICK_CC_CHANNEL      1
-#define LP_TICKER_CC_CHANNEL    2
+#define OS_TICK_CC_CHANNEL      0
+#define LP_TICKER_CC_CHANNEL    1
 
 #define US_TICKER_SW_IRQ_MASK 0x1
 #define LP_TICKER_SW_IRQ_MASK 0x2
@@ -41,8 +37,6 @@
 #define COMMON_RTC_INT_COMPARE_MASK(channel) \
     CONCAT_3(NRF_RTC_INT_COMPARE, channel, _MASK)
 
-#define US_TICKER_EVENT     COMMON_RTC_EVENT_COMPARE(US_TICKER_CC_CHANNEL)
-#define US_TICKER_INT_MASK  COMMON_RTC_INT_COMPARE_MASK(US_TICKER_CC_CHANNEL)
 #define OS_TICK_EVENT       COMMON_RTC_EVENT_COMPARE(OS_TICK_CC_CHANNEL)
 #define OS_TICK_INT_MASK    COMMON_RTC_INT_COMPARE_MASK(OS_TICK_CC_CHANNEL)
 #define LP_TICKER_EVENT     COMMON_RTC_EVENT_COMPARE(LP_TICKER_CC_CHANNEL)