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STM32 16bits tickers: consider all corner cases in us_ticker_set_interrupt 

The present commit comes from monkiineko mbed contributor.
The comments in code explains in details all the possible case and
how they are handled.
pull/4424/head
Laurent MEUNIER 2017-06-02 13:10:41 +02:00
parent fbfbb5e982
commit ea2cc1d0e1
1 changed files with 102 additions and 18 deletions
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120
targets/TARGET_STM/us_ticker_16b.c
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@ -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.
*/
}
}
}