I'm using a PIC18 with Fosc = 10MHz. So if I use Delay10KTCYx(250), I get 10,000 x 250 x 4 x (1/10e6) = 1 second.
How do I use the delay functions in the C18 for very long delays, say 20 seconds? I was thinking of just using twenty lines of Delay10KTCYx(250). Is there another more efficient and elegant way?
Thanks in advance!
It is strongly recommended that you avoid using the built-in delay functions such as Delay10KTCYx()
Why you might ask?
These delay functions are very inaccurate, and they may cause your code to be compiled in unexpected ways. Here's one such example where using the Delay10KTCYx() function can cause problems.
Let's say that you have a PIC18 microprocessor that has only two hardware timer interrupts. (Usually they have more but let's just say there are only two).
Now let's say you manually set up the first hardware timer interrupt to blink once per second exactly, to drive a heartbeat monitor LED. And let's say you set up the second hardware timer interrupt to interrupt every 50 milliseconds because you want to take some sort of digital or analog reading at exactly 50 milliseconds.
Now, lastly, let's say that in your main program you want to delay 100,000 clock cycles. So you put a call to Delay10KTCYx(10) in your main program. What happenes do you suppose? How does the PIC18 magically count off 100,000 clock cycles?
One of two things will happen. It may "hijack" one of your other hardware timer interrupts to get exactly 100,000 clock cycles. This would either cause your heartbeat sensor to not clock at exactly 1 second, or, cause your digital or analog readings to happen at some time other than every 50 milliseconds.
Or, the delay function will just call a bunch of Nop() and claim that 1 Nop() = 1 clock cycle. What isn't accounted for is "overheads" within the Delay10KTCYx(10) function itself. It has to increment a counter to keep track of things, and surely it takes more than 1 clock cycle to increment the timer. As the Delay10KTCYx(10) loops around and around it is just not capable of giving you exactly 100,000 clock cycles. Depending on a lot of factors you may get way more, or way less, clock cycles than you expected.
The Delay10KTCYx(10) should only be used if you need an "approximate" amount of time. And pre-canned delay functions shouldn't be used if you are already using the hardware timer interrupts for other purposes. The compiler may not even successfully compile when using Delay10KTCYx(10) for very long delays.
I would highly recommend that you set up one of your timer interrupts to interrupt your hardware at a known interval. Say 50,000 clock cycles. Then, each time the hardware interrupts, within your ISR code for that timer interrupt, increment a counter and reset the timer over again to 0 cycles. When enough 50,000 clock cycles have expired to equal 20 seconds (or in other words in your example, 200 timer interrupts at 50,000 cycles per interrupt), reset your counter. Basically my advice is that you should always manually handle time in a PIC and not rely on pre-canned Delay functions - rather build your own delay functions that integrate into the hardware timer of the chip. Yes, it's going to be extra work - "but why can't I just use this easy and nifty built-in delay function, why would they even put it there if it's gonna muck up my program?" - but this should become second nature. Just like you should be manually configuring EVERY SINGLE REGISTER in your PIC18 upon boot-up, whether you are using it or not, to prevent unexpected things from happening.
You'll get way more accurate timing - and way more predictable behavior from your PIC18. Using pre-canned Delay functions is a recipe for disaster... it may work... it may work on several projects... but sooner or later your code will go all buggy on you and you'll be left wondering why and I guarantee the culprit will be the pre-canned delay function.
To create very long time use an internal timer. This can helpful to avoid block in your application and you can check the running time. Please refer to PIC data sheet on how to setup a timer and its interrupt.
If you want a very high precision 1S time I suggest also to consider an external RTC device or an internal RTC if the micro has one.
Related
I am working on 8051 MCU from si labs. I want to generate exact 1ms delay using timer. For this I want to know what is the machine cycle time of a given MCU. The time taken by the MCU to complete one machine instruction. Then I can calculate how many machine cycles to complete 1ms delay.
Creating a time delay by counting MCU cycles is a poor method - especially if you are coding in C where you have no control over the machine instructions the compiler will generate - your loop will likely change depending on compiler options such as optimisation level.
Moreover the MCU has no means of measuring its own clock; its only concept of time passing is in clock-cycle units - asking it how long a cycle is is rather like asking a human how long a second is. The answer to the question of how long a clock-cycle is from the point of view of the MCU is always 1.
As the programmer of the system, it is your responsibility to know the clock speed. Typically the hardware defines the speed by its crystal or oscillator rate, and the MCU PLL settings determine the multiplier. Most often you will embed this speed as a constant in the start-up code; your code might access this constant.
Even then, you are better off creating delays using an on-chip timer unit rather than software-based instruction counting (and not all 8051 instructions are single cycle). In that case, you still need to know the clock speed; then the timer clock may be further divided from that.
To use the timer you need to know what is the frequency of the timer clock. Then you just need to : timer_clocks=delay*frequency;
Instruction timings you need to know only if you want blocking delay. There are two sources: uC documentation or experiment. To know how many loops you need just connect the oscilloscope to the pin and loop as many times as needed to archive the required impulse length
I've used PICs before and now I'm using with STM32F415.
On a time-critical part of my code I need to put a very exact delay to adjust the period of the DAC-DMA that are working together to create a periodic analog signal.
The delay I want to add goes from 0 to 63 clock cycles (If I were able to do 10-63 clock cycles it would be OK aswell). In PIC24F assembly, there's the instruction "REPEAT" which allows me to repeat the next instruction a certain number of times. That would work great for me as I'd be able to just do:
REPEAT #0xNUMBER
NOP
I'm trying to find something similar with the STM32F4, but I had no luck searching in the instruction set, Reference Manual, and on the Internet in general.
I've already tried to use for/while loops in C and a timer dedicated to it, but the extra instructiosn required consume too much time (40-50 cycles depending on the way I program it).
If someone has an idea or knows how to do it, It would be very useful for me.
Thanks a lot.
English is not my mother-tongue language so I'm sorry for any possible mistakes. Let me know and I'll try to improve it :)
EDIT 1 (23-jul-17)
Thanks to everyone answering, I've been very busy and couldn't answer every one of you individually.
I'll try #berendi solution of gated clocks, it seems as the best fit for my application.
I'm learning a lot of things about the STM32 I didn't know, thank you everyone!
I stop the timer that makes de DMA-DAC work, do the delay and then
enable it again.
So, if I'm understanding it correctly, you have Timer A controlling your DAC, triggering a conversion at each counter overflow, and you'd like to delay it for a variable number of clock cycles.
Most (if not all) timers of the STM32F4 support gated mode slave operation, where you can select another timer (Timer B) as a master, and Timer A counts only as long as the trigger output of Timer B is low. In other words, Timer A will stop counting on a rising edge from Timer B, and resume counting on a falling edge. Now, configure Timer B to output a single pulse when enabled, where the pulse width is the delay you want, then Timer A will be delayed for the exact duration of the pulse.
See the chapters on One-pulse mode, Timers and external trigger synchronization, and the description of the CR1, CR2, and SMCR registers in the reference manual.
NOP is not a very good solution for the delay. Use barrier instructions instead as the execution time is exactly as stated in the ARM documentation (3, 4 or 5 cycles depending what instruction and the core version). You can place n consecutive barriers to archive the delay you need
On a PIC you can do this and it is a very common solution, the execution time was deterministic. Outside architectures like that, and older chips that were also deterministic (before clones came out) that would be okay as well. But in general that is not how you do a delay, it is not deterministic, you can get an "at least this long" for a tuned loop, but you cant get "exactly this long" even tuned, or should never expect to. That is why there are timers, multiple usually, in mcu designs and that is what you use for measuring time. For the problem you are trying to solve that is the solution here, one timer or cascaded timers if you really need that.
Arm does not have an x86 like repeat instruction your smallest loop is going to be two instructions and I have countless times demonstrated that on the same chip this loop can vary in speed, so tune it, add a line of code and the delay properties of this loop change
here:
sub r0,#1
bne here
for classic (gas) syntax, for unified syntax use subs instead of sub.
You are also on an stm32 where they have a buried cache on the instruction side that you cannot turn off nor control, it generally gives you no wait state performance, certainly for things like this but obviously they dont have a cache the size of the flash so pre-fetch cycles have to happen somewhere, and you have to expect that sometimes you are going to have to feel that prefetch when you jump into this loop.
I'm working on an embedded project that's running on an ARM Cortex M3 based microcontroller. Some code provided by our vendor uses a delay function that sets up built-in hardware timer and then spins until the timer expires. Typically this is used to wait between 1 and a couple hundred microseconds. These delays are almost because they are waiting on some register, chip or bus to complete an action and need to wait at least the given number of microseconds. The hardware timer also appears to cost at least 6 microseconds in overhead to setup.
In a multithreaded environment this is a problem because there are N threads but only 1 hardware timer. I could disable interrupts while the timer is being used to prevent context switches and thus race conditions but it seems a bit ugly. I am thinking of replacing the function that uses the hardware timer with a function that uses the ARM CPU Cycle Counter (CCNT). Are there are pitfalls I am missing or other alternatives? Obviously the cycle counter function requires it be tuned to the proper CPU frequency which will never change for our system, but I suppose could be detected at boot programmatically using the hardware timer.
Setup the timer once at startup and let the counter run continuously. When you want to start a delay, read the counter value and remember this start value. Then in the delay loop read the counter value again and loop until the counter value minus the start value is greater than or equal to the requested delay ticks. (If you do the subtraction correctly then rollovers will wash out and you don't need special handling to check for them.)
You could multiplex your timer such that you have a table of when each thread wants to fire off and a function pointer / vector for execution. When the timer interrupt occurs, fire off that thread's interrupt and then set the timer to the next one in the list, minus elapsed time. This is what I see many *nix operating systems do in their kernel code, so there should be code to pull from as example.
A bigger concern is the fact that you are spin locking the thread waiting for the timer. Besides CPU usage, and depending on what OS you have (or if you have an OS) you could easily introduce thread inversion issues or even full on lock ups. It might be better to use thread primitives instead so that any OS can actually sleep your threads and wake them when needed.
Short question: How to get seconds since reset in STM32L051T6 microcontroller?
My effort and detailed issue:
I am using an STM32L051T6 series microcontroller. I need to count seconds since power on. I am also using low power mode. So I wrote code to use wakeup timer interrupt functionality of internal RTC of microcontroller. I used 1 second interval wake up timer with external LSE clock of 32768 Hz. I observed the accumulated seconds since power on (SSPO) after 3 days and found that it is falling behind by 115 seconds compared to actual time elapsed. My guess for this drift is interrupt latency in executing wakeup timer interrupt. How can I remove drift of this 115 seconds? Or is there any other better method than using wakeup interrupt to count seconds since power on?
UPDATE:
I tried to use Systick with HAL_GetTick() function as seconds since power on. But even systick is also getting delayed over time.
If you want to measure time with accuracy over a longer period, an RTC is the way to go. As you mentioned that you have an RTC, you can use the method below.
At startup, load the RTC with zero.
Then you can read the seconds elapsed when required without the errors above.
Edit: As per comment, the RTC can be changed by user. In that case,
If you can modify the RTC write function called by the user, then when the user calls the RTC write function, you update a global variable VarA = time set by user. The elapsed time will be Time read by RTC - VarA.
If the RTC is accurate, you should use the RTC by storing its value at boot time and later comparing to that saved value. But you said that the RTC can be reset by user so I can see two ways to cope with it:
if you have enough control on the system, replace the command or IHM that a user can use to reset the clock with a wrapper that inform you module and allows to read the RTC before and after it has been reset
if you have not enough control or cannot wrap the user's reset (because it uses a direct system call, etc.) use a timer to control the RTC value on every second
But you should define a threshold on the delta on RTC clock. If it is small, it is likely to be an adjustment because unless your system uses an atomic clock, even RTC can derive over time. In that case I would not care because you can hardly know whether it derived since last reboot or not. If you want a more clever algorythm, you can make the threshold dependant on the current time since last reboot: the longer the system is up, the higher the probability it has derived since then.
On the opposite, a large delta is likely to be a correction because RTC was blatantly erroneous, the saving battery is out of use, or what else. In that case you should compute the new start RTC time that gives same duration with the new RTC value.
As a rule of thumb, I would use a threshold of about 1 or 2 seconds per uptime day without RTC clock adjustement (ref) - meaning I would also store the time of last RTC adjustement, initially boot time.
I know QueryPerformanceCounter() can be used for timing functions. I want to know:
1-Can I increase the resolution of the timer by over-clocking the CPU (so it ticks faster)?
2-Basically what makes some timers more precise than others, (e.g, QueryPerformanceCounter() is more precise as compared to GetTickCount())? If there is single crystal oscillator on the motherboard , why some timers are slower as compared to others?
QueryPerformanceCounter has very high resolution - normally less than one nanosecond. I don't see why you'd like to increase it. Overclocking will increase it, but it seems like a very weak reason for overclocking.
QueryPerformanceCounter is very accurate, but somewhat expensive and not very convenient.
a. It's expensive because it uses the expensive rdtsc instruction. Faster timers can just read an integer from memory. This integer needs to be updated, and we don't want to do it too often (1000 times a second is reasonable), so we get a very cheap timer, with low precision. That's basically GetTickCount.
b. It's inconvenient because it uses units which change between computers. Sometimes it will be nanoseconds, sometimes half-nano, or other values. It makes it harder to calculate with.
a. Another source of inconvenience is that it returns very large numbers, which may overflow when you try to do math with them, so you need to be careful.
The timing source for QPC is machine dependent. It is typically picked up from a frequency available somewhere in the chipset. Whether overclocking the cpu is going to affect it is highly dependent on your motherboard design. The simplest way is to just try it, use QueryPerformanceFrequency to see the effect.
GetTickCount is driven from an entirely different timer source, the signal that also generates the clock interrupt. It is not very precise, 1/64 of second normally, but it is highly accurate. Your machine contacts a time server from time to time to recalibrate the clock and adjust the clock correction factor. Which makes it accurate to about a second over an entire year. QPC is very precise, but not nearly as accurate. Use it only to time short intervals.
1 - Yes, Internally, one of the better timers is rdtsc, which does give you the clock value. Combining this with information from cpuid instruction, gives you time.
2 - The other timers rely upon various timing sources, such as the 8253 timer, for instance.
QPF is a wrapper added by Microsoft on and over what rdtsc provides. Read this article for more info:
http://www.strchr.com/performance_measurements_with_rdtsc