Not only calculating a point in time has to be relative, but also the comparison has to in relative time using a delta. The correct way to work with real-time counters is by consequently thinking about them as relative time.You can run the example program with any start value for the current time, and you will get the same results. With this knowledge, we can simplify the test by converting the unsigned integer into a signed one and just testing for a negative number: bool hasExpired(uint32_t timePoint)Īs you can see, the delta values are exactly the same as for the previous run. Let us test this with our previous example: #include Ĭt=0x10e0 tp=0x1064 tp-ct=0xff84 => -124 Simplify the Test It is just the representation of the value, which is different. There is no difference between the subtraction of signed or unsigned integers on the binary level. Therefore, if you convert an unsigned integer after a subtraction into a signed one, you get the correct negative results as you would expect. Highest bit? Isn’t this how signed integers work? If the highest bit in a signed integer is set, it indicates a negative value. You may also check for the highest bit in the value: bool hasExpired(uint32_t timePoint) You can easily verify this using the following code: #include What is Integer Overflow?Īn integer overflow happens, if you add to an integer value and the result exceeds the maximum number it can store. The last function is SysTick_DefaultHandler which just increases the variable by one, every millisecond. For some platforms, this can be a problem. after reading the first byte of the integer value. The interrupt will occur before or after reading the value, but never e.g. It means, it is not possible to process an interrupt in the middle of reading the integer. Interrupts do not need to be blocked while reading this value, because reading a single 32bit value is an atomic operation for the SAMD platform. You access the current value using the millis() function.
It is marked volatile to tell the compiler, this variable can be modified from an interrupt and reads can not be optimised away. There is the variable _ulTickCount which is an unsigned 32bit integer.
#Vba integer overflow code
The following code is a summary of the relevant parts of the simpler implementation for the SAMD platform: static volatile uint32_t _ulTickCount=0 If you can find most of this code in the file wired.c ( AVR) or delay.c (SAMD). For the Arduino platform, a hardware timer is set to create an interrupt for each millisecond. These counters are usually implemented using a hardware timer and an interrupt.
How is the Real-Time Counter Implemented? Therefore, even if the main code stops at one point and waits for a specific condition, the real-time counter will get increased in the “background” at the given interval. It just states the fact this counter ideally does count independently of any other parts of the firmware. What if I Need to Measure very Long Periods?Ī real-time counter is a variable or register which increases at a given time interval.Is there an Issue if the Real-Time Counter Overflows?.How is the Real-Time Counter Implemented?.
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