diff --git a/targets/TARGET_STM/us_ticker_16b.c b/targets/TARGET_STM/us_ticker_16b.c index 6523922655..cea80ff99b 100644 --- a/targets/TARGET_STM/us_ticker_16b.c +++ b/targets/TARGET_STM/us_ticker_16b.c @@ -28,15 +28,6 @@ volatile uint32_t oc_int_part = 0; static int us_ticker_inited = 0; -void set_compare(uint16_t count) -{ - TimMasterHandle.Instance = TIM_MST; - // Set new output compare value - __HAL_TIM_SET_COMPARE(&TimMasterHandle, TIM_CHANNEL_1, count); - // Enable IT - __HAL_TIM_ENABLE_IT(&TimMasterHandle, TIM_IT_CC1); -} - void us_ticker_init(void) { if (us_ticker_inited) return; @@ -80,25 +71,118 @@ uint32_t us_ticker_read() void us_ticker_set_interrupt(timestamp_t timestamp) { + // NOTE: This function must be called with interrupts disabled to keep our + // timer interrupt setup atomic + TimMasterHandle.Instance = TIM_MST; + // Set new output compare value + __HAL_TIM_SET_COMPARE(&TimMasterHandle, TIM_CHANNEL_1, timestamp & 0xFFFF); + // Ensure the compare event starts clear + __HAL_TIM_CLEAR_FLAG(&TimMasterHandle, TIM_FLAG_CC1); + // Enable IT + __HAL_TIM_ENABLE_IT(&TimMasterHandle, TIM_IT_CC1); + int current_time = us_ticker_read(); int delta = (int)(timestamp - current_time); if (delta <= 0) { // This event was in the past - /* Force the event to be handled in next interrupt context - * This prevents calling interrupt handlers in loops as - * us_ticker_set_interrupt might called again from the + /* Immediately set the compare event to cause the event to be handled in + * the next interrupt context. This prevents calling interrupt handlers + * recursively as us_ticker_set_interrupt might be called again from the * application handler */ oc_int_part = 0; - TimMasterHandle.Instance = TIM_MST; HAL_TIM_GenerateEvent(&TimMasterHandle, TIM_EVENTSOURCE_CC1); } else { - /* set the comparator at the timestamp lower 16 bits - * and count the number of wrap-around loops to do with - * the upper 16 bits + /* Set the number of timer wrap-around loops before the actual timestamp + * is reached. If the calculated delta time is more than halfway to the + * next compare event, check to see if a compare event has already been + * set, and if so, add one to the wrap-around count. This is done to + * ensure the correct wrap count is used in the corner cases where the + * 16 bit counter passes the compare value during the process of + * configuring this interrupt. + * + * Assumption: The time to execute this function is less than 32ms + * (otherwise incorrect behaviour could result) + * + * Consider the following corner cases: + * 1) timestamp is 1 us in the future: + * oc_int_part = 0 initially + * oc_int_part left at 0 because ((delta - 1) & 0xFFFF) < 0x8000 + * Compare event should happen in 1 us and us_ticker_irq_handler() + * called + * 2) timestamp is 0x8000 us in the future: + * oc_int_part = 0 initially + * oc_int_part left at 0 because ((delta - 1) & 0xFFFF) < 0x8000 + * There should be no possibility of the CC1 flag being set yet + * (see assumption above). When the compare event does occur in + * 32768 us, us_ticker_irq_handler() will be called + * 3) timestamp is 0x8001 us in the future: + * oc_int_part = 0 initially + * ((delta - 1) & 0xFFFF) >= 0x8000 but there should be no + * possibility of the CC1 flag being set yet (see assumption above), + * so oc_int_part will be left at 0, and when the compare event + * does occur in 32769 us, us_ticker_irq_handler() will be called + * 4) timestamp is 0x10000 us in the future: + * oc_int_part = 0 initially + * ((delta - 1) & 0xFFFF) >= 0x8000 + * There are two subcases: + * a) The timer counter has not incremented past the compare + * value while setting up the interrupt. In this case, the + * CC1 flag will not be set, so oc_int_part will be + * left at 0, and when the compare event occurs in 65536 us, + * us_ticker_irq_handler() will be called + * b) The timer counter has JUST incremented past the compare + * value. In this case, the CC1 flag will be set, so + * oc_int_part will be incremented to 1, and the interrupt will + * occur immediately after this function returns, where + * oc_int_part will decrement to 0 without calling + * us_ticker_irq_handler(). Then about 65536 us later, the + * compare event will occur again, and us_ticker_irq_handler() + * will be called + * 5) timestamp is 0x10001 us in the future: + * oc_int_part = 1 initially + * oc_int_part left at 1 because ((delta - 1) & 0xFFFF) < 0x8000 + * CC1 flag will not be set (see assumption above). In 1 us the + * compare event will cause an interrupt, where oc_int_part will be + * decremented to 0 without calling us_ticker_irq_handler(). Then + * about 65536 us later, the compare event will occur again, and + * us_ticker_irq_handler() will be called + * 6) timestamp is 0x18000 us in the future: + * oc_int_part = 1 initially + * oc_int_part left at 1 because ((delta - 1) & 0xFFFF) < 0x8000 + * There should be no possibility of the CC1 flag being set yet + * (see assumption above). When the compare event does occur in + * 32768 us, oc_int_part will be decremented to 0 without calling + * us_ticker_irq_handler(). Then about 65536 us later, the + * compare event will occur again, and us_ticker_irq_handler() will + * be called + * 7) timestamp is 0x18001 us in the future: + * oc_int_part = 1 initially + * ((delta - 1) & 0xFFFF) >= 0x8000 but there should be no + * possibility of the CC1 flag being set yet (see assumption above), + * so oc_int_part will be left at 1, and when the compare event + * does occur in 32769 us, oc_int_part will be decremented to 0 + * without calling us_ticker_irq_handler(). Then about 65536 us + * later, the compare event will occur again, and + * us_ticker_irq_handler() will be called + * + * delta - 1 is used because the timer compare event happens on the + * counter incrementing to match the compare value, and it won't occur + * immediately when the compare value is set to the current counter + * value. */ - oc_int_part = (uint32_t)(delta >> 16); - set_compare(timestamp & 0xFFFF); + oc_int_part = ((uint32_t)delta - 1) >> 16; + if ( ((delta - 1) & 0xFFFF) >= 0x8000 && + __HAL_TIM_GET_FLAG(&TimMasterHandle, TIM_FLAG_CC1) == SET ) { + ++oc_int_part; + /* NOTE: Instead of incrementing oc_int_part here, we could clear + * the CC1 flag, but then you'd have to wait to ensure the + * interrupt is knocked down before returning and reenabling + * interrupts. Since this is a rare case, it's not worth it + * to try and optimize it, and it keeps the code simpler and + * safer to just do this increment instead. + */ + } } }