[GitHub] ccollins476ad commented on a change in pull request #667: Apollo2 - Tickless idle and HAL timer implementation

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ccollins476ad commented on a change in pull request #667: Apollo2 - Tickless idle and HAL timer implementation
URL: https://github.com/apache/mynewt-core/pull/667#discussion_r152096024
 
 

 ##########
 File path: hw/mcu/ambiq/apollo2/src/hal_os_tick.c
 ##########
 @@ -17,31 +17,118 @@
  * under the License.
  */
 
+/**
+ * NOTE: Unlike other MCUs, this one does not use SysTick to implement the
+ * tickless idle state.  The SysTick timer uses HCLK as its source, and HCLK is
+ * gated while this MCU is in deep sleep (e.g., during a `wfi` instruction).
+ *
+ * To enable a wake up from deep sleep, the idle state is instead implemented
+ * using this MCU's system timer (STIMER) with LFRC as the source.
+ */
+
 #include <assert.h>
 #include <os/os.h>
-#include <mcu/system_apollo2.h>
-#include <hal/hal_os_tick.h>
+#include "mcu/system_apollo2.h"
+#include "hal/hal_os_tick.h"
+#include "bsp/cmsis_nvic.h"
+
+#include "am_mcu_apollo.h"
+
+#define APOLLO2_OS_TICK_FREQ    1024 /* Hz */
+#define APOLLO2_OS_TICK_IRQ     STIMER_CMPR0_IRQn
+
+/*** Value of timer when the ISR last executed. */
+static uint32_t apollo2_os_tick_prev;
+
+/*** Number of system ticks per single OS tick. */
+static uint32_t apollo2_os_tick_dur;
+
+static void
+apollo2_os_tick_set_timer(int os_ticks)
+{
+    uint32_t sys_ticks;
+    uint32_t cfg;
+
+    OS_ASSERT_CRITICAL();
+
+    sys_ticks = os_ticks * apollo2_os_tick_dur;
+
+    /* Freeze time, set timer expiry, then unfreeze time. */
+    cfg = am_hal_stimer_config(AM_HAL_STIMER_CFG_FREEZE);
+    am_hal_stimer_compare_delta_set(0, sys_ticks);
+    am_hal_stimer_config(cfg);
+}
+
+static void
+apollo2_os_tick_handler(void)
+{
+    uint32_t cur;
+    int os_ticks;
+    int delta;
+    os_sr_t sr;
+
+    OS_ENTER_CRITICAL(sr);
+
+    /* Calculate elapsed ticks and advance OS time. */
+    cur = am_hal_stimer_counter_get();
+    delta = cur - apollo2_os_tick_prev;
+    os_ticks = delta / apollo2_os_tick_dur;
+    os_time_advance(os_ticks);
+
+    /* Clear timer interrupt. */
+    am_hal_stimer_int_clear(AM_HAL_STIMER_INT_COMPAREA);
+
+    /* Update the time associated with the most recent tick. */
+    apollo2_os_tick_prev += os_ticks * apollo2_os_tick_dur;
+
+    /* Schedule timer to interrupt at the next tick. */
+    apollo2_os_tick_set_timer(apollo2_os_tick_dur);
+
+    OS_EXIT_CRITICAL(sr);
+}
 
-/*
- * XXX implement tickless mode.
- */
 void
 os_tick_idle(os_time_t ticks)
 {
     OS_ASSERT_CRITICAL();
+
+    /* Since the STIMER only uses relative scheduling, all ticks values are
+     * valid.  There is no need to check for wrap around.
+     */
+
+    /* Only set the timer for nonzero tick values.  For values of 0, just let
+     * the timer expire on the next tick, as scheduled earlier.
+     */
+    if (ticks > 0) {
+        apollo2_os_tick_set_timer(ticks);
 
 Review comment:
   I could be wrong, but I don't think so.  `apollo2_os_tick_set_timer()` multiplies its parameter by `apollo2_os_tick_dur` before setting the output compare.

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