When is a Cortex write to a device realised - arm

When writing to device registers on a Cortex M0 (in my case, on an STM32L073), a question arises as to how careful one should be in a) ordering accesses to device memory and b) deciding that a change to a peripheral configuration has actually completed to the point that any dependencies become valid.
Taking a specific example to change the internal voltage regulator to a different voltage. You write the change to PWR->CR and read the status from PWR->CSR. I see code that does something like this:
Write to PWR->CR to set the voltage range
Spin until (PWR->CSR & voltage flag) becomes zero
In my mind there are three issues here:
Access ordering. This is Device Memory so transaction order is preserved relative to other Device access transactions. I would assume this means a DSB is not required between the write to CR and the read from CSR. A linked question and the answer to this is: [ARM CortexA]Difference between Strongly-ordered and Device Memory Type
Device memory can be buffered. Is there a possibility that a write to CR could still be in process when the read from CSR occurs. This would mean that the voltage flag would be clear and the code would proceed. In actual fact the flag hasn't gone high yet!
Hardware response time. Is there a latency between the write and the effects becoming final? In actuality this should always be documented - for the STM32 the docs definitively say that the flag is set when the CR register changes.
Are there any race condition possibilities here? It's really the buffering that worries me - that a peripheral write is still in progress when a peripheral read takes place.

Access ordering.
Accesses are strongly ordered and you do not need barrier instructions to read back the same register.
Device memory can be buffered. Is there a possibility that a write to CR
Yes, it is possible. But it is not because of buffering but because of the bus propagation time. It may take several clocks before a particular operation will go through all bridges.
Hardware response time. Is there a latency between the write and the
effects becoming final
Even if there is a latency it is not important from your point of view. If you set bit in the CR register and wait for the result in the status register. Simply wait for the status bit to have the expected value.

Related

How to know whether an IRQ was served immediately on ARM Cortex M0+ (or any other MCU)

For my application (running on an STM32L082) I need accurate (relative) timestamping of a few types of interrupts. I do this by running a timer at 1 MHz and taking its count as soon as the ISR is run. They are all given the highest priority so they pre-empt less important interrupts. The problem I'm facing is that they may still be delayed by other interrupts at the same priority and by code that disables interrupts, and there seems to be no easy way to know this happened. It is no problem that the ISR was delayed, as long as I know that the particular timestamp is not accurate because of this.
My current approach is to let each ISR and each block of code with interrupts disabled check whether interrupts are pending using NVIC->ISPR[0] and flagging this for the pending ISR. Each ISR checks this flag and, if needed, flags the timestamp taken as not accurate.
Although this works, it feels like it's the wrong way around. So my question is: is there another way to know whether an IRQ was served immediately?
The IRQs in question are EXTI4-15 for a GPIO pin change and RTC for the wakeup timer. Unfortunately I'm not in the position to change the PCB layout and use TIM input capture on the input pin, nor to change the MCU used.
update
The fundamental limit to accuracy in the current setup is determined by the nature of the internal RTC calibration, which periodically adds/removes 32kHz ticks, leading to ~31 µs jitter. My goal is to eliminate (or at least detect) additional timestamping inaccuracies where possible. Having interrupts blocked incidentally for, say, 50+ µs is hard to avoid and influences measurements, hence the need to at least know when this occurs.
update 2
To clarify, I think this is a software question, asking if a particular feature exists and if so, how to use it. The answer I am looking for is one of: "yes it is possible, just check bit X of register Y", or "no it is not possible, but MCU ... does have such a feature, called ..." or "no, such a feature is generally not available on any platform (but the common workaround is ...)". This information will guide me (and future readers) towards a solution in software, and/or requirements for better hardware design.
In general
The ideal solution for accurate timestamping is to use timer capture hardware (built-in to the microcontroller, or an external implementation). Aside from that, using a CPU with enough priority levels to make your ISR always the highest priority could work, or you might be able to hack something together by making the DMA engine sample the GPIO pins (specifics below).
Some microcontrollers have connections between built-in peripherals that allow one peripheral to trigger another (like a GPIO pin triggering timer capture even though it isn't a dedicated timer capture input pin). Manufacturers have different names for this type of interconnection, but a general overview can be found on Wikipedia, along with a list of the various names. Exact capabilities vary by manufacturer.
I've never come across a feature in a microcontroller for indicating if an ISR was delayed by a higher priority ISR. I don't think it would be a commonly-used feature, because your ISR can be interrupted by a higher priority ISR at any moment, even after you check the hypothetical was_delayed flag. A higher priority ISR can often check if a lower priority interrupt is pending though.
For your specific situation
A possible approach is to use a timer and DMA (similar to audio streaming, double-buffered/circular modes are preferred) to continuously sample your GPIO pins to a buffer, and then you scan the buffer to determine when the pins changed. Note that this means the CPU must scan the buffer before it is overwritten again by DMA, which means the CPU can only sleep in short intervals and must keep the timer and DMA clocks running. ST's AN4666 is a relevant document, and has example code here (account required to download example code). They're using a different microcontroller, but they claim the approach can be adapted to others in their lineup.
Otherwise, with your current setup, I don't think there is a better solution than the one you're using (the flag that's set when you detect a delay). The ARM Cortex-M0+ NVIC does not have a feature to indicate if an ISR was delayed.
A refinement to your current approach might be making the ISRs as short as possible, so they only do the timestamp collection and then put any other work into a queue for processing by the main application at a lower priority (only applicable if the work is more complex than the enqueue operation, and if the work isn't time-sensitive). Eliminating or making the interrupts-disabled regions short should also help.

DSB on ARM Cortex M4 processors

I have read the ARM documentation and it appears that they say in some places that the Cortex M4 can reorder memory writes, while in other places it indicates that M4 will not.
Specifically I am wondering if the DBM instruction is needed like:
volatile int flag=0;
char buffer[10];
void foo(char c)
{
__ASM volatile ("dbm" : : : "memory");
__disable_irq(); //disable IRQ as we use flag in ISR
buffer[0]=c;
flag=1;
__ASM volatile ("dbm" : : : "memory");
__enable_irq();
}
Uh, it depends on what your flag is, and it also varies from chip to chip.
In case that flag is stored in memory:
DSB is not needed here. An interrupt handler that would access flag would have to load it from memory first. Even if your previous write is still in progress the CPU will make sure that the load following the store will happen in the correct order.
If your flag is stored in peripheral memory:
Now it gets interesting. Lets assume flag is in some hardware peripheral. A write to it may make an interrupt pending or acknowledge an interrupt (aka clear a pending interrupt). Contrary to the memory example above this effect happens without the CPU having to read the flag first. So the automatic ordering of stores and loads won't help you. Also writes to flag may take effect with a surprisingly long delay due to different clock domains between the CPU and the peripheral.
So the following szenario can happen:
you write flag=1 to clear an handled interrupt.
you enable interrupts by calling __enable_irq()
interrupts get enabled, write to flag=1 is still pending.
wheee, an interrupt is pending and the CPU jumps to the interrupt handler.
flag=1 takes effect. You're now in an interrupt handler without anything to do.
Executing a DSB in front of __enable_irq() will prevent this problem because whatever is triggered by flag=1 will be in effect before __enable_irq() executes.
If you think that this case is purely academic: Nope, it's real.
Just think about a real-time clock. These usually runs at 32khz. If you write into it's peripheral space from a CPU running at 64Mhz it can take a whopping 2000 cycles before the write takes effect. Now for real-time clocks the data-sheet usually shows specific sequences that make sure you don't run into this problem.
The same thing can however happen with slow peripherals.
My personal anecdote happened when implementing power-saving late in a project. Everything was working fine. Then we reduced the peripheral clock speed of I²C and SPI peripherals to the lowest possible speed we could get away with. This can save lots of power and extend battery live. What we found out was that suddenly interrupts started to do unexpected things. They seem to fire twice each time wrecking havoc. Putting a DSB at the end of each affected interrupt handler fixed this because - you can guess - the lower clock speed caused us to leave the interrupt handlers before clearing the interrupt source was in effect due to the slow peripheral clock.
This section of the Cortex M4 generic device user guide enumerates the factors which can affect reordering.
the processor can reorder some memory accesses to improve efficiency, providing this does not affect the behavior of the instruction sequence.
the processor has multiple bus interfaces
memory or devices in the memory map have different wait states
some memory accesses are buffered or speculative.
You should also bear in mind that both DSB and ISB are often required (in that order), and that C does not make any guarantees about the ordering (except in-thread volatile accesses).
You will often observe that the short pipeline and instruction sequences can combine in such a way that the race conditions seem unreachable with a specific compiled image, but this isn't something you can rely on. Either the timing conditions might be rare (but possible), or subsequent code changes might change the resulting instruction sequence.

Issue with global variable while making 32-bit counter

I am trying to do quadrature decoding using atmel xmega avr microcontroller. Xmega has only 16-bit counters. And in addition I have used up all the available timers.
Now to make 32-bit counter I have used one 16-bit counter and in its over/under flow interrupt I have increment/decrement a 16-bit global variable, so that by combining them we can make 32-bit counter.
ISR(timer_16bit)
{
if(quad_enc_mov_forward)
{
timer_over_flow++;
}
else if (quad_enc_mov_backward)
{
timer_over_flow--;
}
}
so far it is working fine. But I need to use this 32-bit value in various tasks running parallel. I'm trying to read 32-bit values as below
uint32_t current_count = timer_over_flow;
current_count = current_count << 16;
current_count = current_count + timer_16bit_count;
`timer_16_bit_count` is a hardware register.
Now the problem I am facing is when I read the read timer_over_flow to current_count in the first statement and by the time I add the timer_16bit_count there may be overflow and the 16bit timer may have become zero. This may result in taking total wrong value.
And I am trying to read this 32-bit value in multiple tasks .
Is there a way to prevent this data corruption and get the working model of 32-bit value.
Details sought by different members:
My motor can move forward or backward and accordingly counter increments/decrements.
In case of ISR, before starting my motor I'm making the global variables(quad_enc_mov_forward & quad_enc_mov_backward) set so that if there is a overflow/underflow timer_over_flow will get changed accordingly.
Variables that are modified in the ISR are declared as volatile.
Multiple tasks means that I'm using RTOS Kernel with about 6 tasks (mostly 3 tasks running parallel).
In the XMEGA I'm directly reading TCCO_CNT register for the lower byte.
One solution is:
uint16_t a, b, c;
do {
a = timer_over_flow;
b = timer_16bit_count;
c = timer_over_flow;
} while (a != c);
uint32_t counter = (uint32_t) a << 16 | b;
Per comment from user5329483, this must not be used with interrupts disabled, since the hardware counter fetched into b may be changing while the interrupt service routine (ISR) that modifies timer_over_flow would not run if interrupts are disabled. It is necessary that the ISR interrupt this code if a wrap occurs during it.
This gets the counters and checks whether the high word changed. If it did, this code tries again. When the loop exits, we know the low word did not wrap during the reads. (Unless there is a possibility we read the high word, then the low word wrapped, then we read the low word, then it wrapped the other way, then we read the high word. If that can happen in your system, an alternative is to add a flag that the ISR sets when the high word changes. The reader would clear the flag, read the timer words, and read the flag. If the flag is set, it tries again.)
Note that timer_over_flow, timer_16bit_count, and the flag, if used, must be volatile.
If the wrap-two-times scenario cannot happen, then you can eliminate the loop:
Read a, b, and c as above.
Compare b to 0x8000.
If b has a high value, either there was no wrap, it was read before a wrap upward (0xffff to 0), or it was read after a wrap downward. Use the lower of a or c.
Otherwise, either there was no wrap, b was read after a wrap upward, or it was read before a wrap downward. Use the larger of a or c.
The #1 fundamental embedded systems programming FAQ:
Any variable shared between the caller and an ISR, or between different ISRs, must be protected against race conditions. To prevent some compilers from doing incorrect optimizations, such variables should also be declared as volatile.
Those who don't understand the above are not qualified to write code containing ISRs. Or programs containing multiple processes or threads for that matter. Programmers who don't realize the above will always write very subtle, very hard-to-catch bugs.
Some means to protect against race conditions could be one of these:
Temporary disabling the specific interrupt during access.
Temporary disabling all maskable interrupts during access (crude way).
Atomic access, verified in the machine code.
A mutex or semaphore. On single-core MCU:s where interrupts cannot be interrupted in turn, you can use a bool as "poor man's mutex".
Just reading TCCO_CNT in multithreaded code is race condition if you do not handle it correctly. Check the section on reading 16bit registers in XMega manual. You should read lower byte first (this will be probably handled transparently by compiler for you). When lower byte is read, higher byte is (atomically) copied into the TEMP register. Then, reading high byte does read the TEMP register, not the counter. In this way atomic reading of 16bit value is ensured, but only if there is no access to TEMP register between low and high byte read.
Note that this TEMP register is shared between all counters, so context switch in right (wrong) moment will probably trash its content and therefore your high byte. You need to disable interrupts for this 16bit read. Because XMega will execute one instruction after the sei with interrupts disabled, the best way is probably:
cli
ld [low_byte]
sei
ld [high byte]
It disables interrupts for four CPU cycles (if I counted it correctly).
An alternative would to save shared TEMP register(s) on each context switch. It is possible (not sure if likely) that your OS already does this, but be sure to check. Even so, you need to make sure colliding access does not occur from an ISR.
This precaution should be applied to any 16bit register read in your code. Either make sure TEMP register is correctly saved/restored (or not used by multiple threads at all) or disable interrupts when reading/writing 16bit value.
This problem is indeed a very common and very hard one. All solutions will toit will have a caveat regarding timing constraints in the lower priority layers. To clarify this: the highest priority function in your system is the hardware counter - it's response time defines the maximum frequency that you can eventually sample. The next lower priority in your solution is the interrupt routine which tries to keep track of bit 2^16 and the lowest is your application level code which tries to read the 32-bit value. The question now is, if you can quantify the shortest time between two level changes on the A- and B- inputs of your encoder. The shortest time usually does occur not at the highest speed that your real world axis is rotating but when halting at a position: through minimal vibrations the encoder can double swing between two increments, thereby producing e.g. a falling and a rising edge on the same encoder output in short succession. Iff (if and only if) you can guarantee that your interrupt processing time is shorter (by a margin) than this minmal time you can use such a method to virtually extend the coordinate range of your encoder.

Relaying UART1 with UART2 within an ISR() (PIC24H)

I am programming a microcontroller of the PIC24H family and using xc16 compiler.
I am relaying U1RX-data to U2TX within main(), but when I try that in an ISR it does not work.
I am sending commands to the U1RX and the ISR() is down below. At U2RX, there are databytes coming in constantly and I want to relay 500 of them with the U1TX. The results of this is that U1TX is relaying the first 4 databytes from U2RX but then re-sending the 4th byte over and over again.
When I copy the for loop below into my main() it all works properly. In the ISR(), its like that U2RX's corresponding FIFObuffer is not clearing when read so the buffer overflows and stops reading further incoming data to U2RX. I would really appreciate if someone could show me how to approach the problem here. The variables tmp and command are globally declared.
void __attribute__((__interrupt__, auto_psv, shadow)) _U1RXInterrupt(void)
{
command = U1RXREG;
if(command=='d'){
for(i=0;i<500;i++){
while(U2STAbits.URXDA==0);
tmp=U2RXREG;
while(U1STAbits.UTXBF==1); //
U1TXREG=tmp;
}
}
}
Edit: I added the first line in the ISR().
Trying to draw an answer from the various comments.
If the main() has nothing else to do, and there are no other interrupts, you might be able to "get away with" patching all 500 chars from one UART to another under interrupt, once the first interrupt has ocurred, and perhaps it would be a useful exercise to get that working.
But that's not how you should use an interrupt. If you have other tasks in main(), and equal or lower priority interrupts, the relatively huge time that this interrupt will take (500 chars at 9600 baud = half a second) will make the processor what is known as "interrupt-bound", that is, the other processes are frozen out.
As your project gains complexity, you won't want to restrict main() to this task, and there is no need to for it be involved at all, after setting up the UARTs and IRQs. After that it can calculate π ad infinitum if you want.
I am a bit perplexed as to your sequence of operations. A command 'd' is received from U1 which tells you to patch 500 chars from U2 to U1.
I suggest one way to tackle this (and there are many) seeing as you really want to use interrupts, is to wait until the command is received from U1 - in main(). You then configure, and enable, interrupts for RXD on U2.
Then the job of the ISR will be to receive data from U2 and transmit it thru U1. If both UARTS have the same clock and the same baud rate, there should not be a synchronisation problem, since a UART is typically buffered internally: once it begins to transmit, the TXD register is available to hold another character, so any stagnation in the ISR should be minimal.
I can't write the actual code for you, since it would be supposed to work, but here is some very pseudo code, and I don't have a PIC handy (or wish to research its operational details).
ISR
has been invoked because U2 has a char RXD
you *might* need to check RXD status as a required sequence to clear the interrupt
read the RXD register, which also might clear the interrupt status
if not, specifically clear the interrupt status
while (U1 TXD busy);
write char to U1
if (chars received == 500)
disable U2 RXD interrupt
return from interrupt
ISR's must be kept lean and mean and the code made hyper-efficient if there is any hope of keeping up with the buffer on a UART. Experiment with the BAUD rate just to find the point at which your code can keep up, to help discover the right heuristic and see how far away you are from achieving your goal.
Success could depend on how fast your micro controller is, as well, and how many tasks it is running. If the microcontroller has a built in UART theoretically you should be able to manage keeping the FIFO from overflowing. On the other hand, if you paired up a UART with an insufficiently-powered micro controller, you might not be able to optimize your way out of the problem.
Besides the suggestion to offload the lower-priority work to the main thread and keep the ISR fast (that someone made in the comments), you will want to carefully look at the timing of all of the lines of code and try every trick in the book to get them to run faster. One expensive instruction can ruin your whole day, so get real creative in finding ways to save time.
EDIT: Another thing to consider - look at the assembly language your C compiler creates. A good compiler should let you inline assembly language instructions to allow you to hyper-optimize for your particular case. Generally in an ISR it would just be a small number of instructions that you have to find and implement.
EDIT 2: A PIC 24 series should be fast enough if you code it right and select a fast oscillator or crystal and run the chip at a good clock rate. Also consider the divisor the UART might be using to achieve its rate vs. the PIC clock rate. It is conceivable (to me) that an even division that could be accomplished internally via shifting would be better than one where math was required.

Interfacing a slow device to a MCF5270 Microcontroller

I'm looking for ways to interface a slow device (an ADC - ~1.2MHz) to my mcf5270 microcontroller, which is a rather fast device (~150MHz). I am using ADC0817, and have tried the following: GPIO read off the data lines, memory mapped access (since the adc is uP compatible).
When mapped as a location in memory, I am using the maximum wait states (15ws, which is about 107ns). The mcf5270 does not support any further waiting without using their bus model (which the ADCs do not support).
Using GPIO, only junk values are read.
Note: The ADC is actually working, and other than reading data the interface to it is functional. I am using C/C++ and assembly.
In short, I am looking for suggestions for ways in which to try to read the data off the ADC0817.
Comments and responses are greatly appreciated.
You could trigger the ADC through some GPIO, do an appropriate number of NOPs and then read the value. Also, you'd need to disable interrupts while doing this.
I think memory mapped should work - normally I would wait for the interrupt from the ADC, then read the byte at the specified address. Since you get an interrupt, there is no wait state needed. Am I missing something? GPIO should also work. How do you know the ADC is working? Can you put a logic analyzer on the data & interrupt to prove the ADC is producing the correct output?
I think from what he's saying MMIO wont' work because he would need more than the maximum number of wait states on the bus to get the glue logic to respond - ie, the A/D won't set its bus pins soon enough for the uC to be happy. That's a problem sometimes.
But the GPIO approach should work. If junk values are being read I assume that you aren't following the timing diagram provided. You'll have to be able to wait certain amounts of time after you signal a read before the data is valid. Check out the datasheet for specifics.

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