CC3235MODASF: Atomic increment,decrement

Mauro Fantina

Genius 3800 points

Part Number: CC3235MODASF

Hello ...

is there a way to perform atomic increment or decrement of uint8_t variable ?

Is necessary to use mutex ?

over 4 years ago

0 Robert Chen over 4 years ago

Mastermind 9125 points

In addition to mutex provided by OS, you can just disable/enable interrupts (set/clear global interrupt mask).

If only one task will write the variable and other tasks will only read the variable then you don't need to write it atomically. So, sometimes, you can use two variables to prevent mutex. Only task-A will write to variable-A and only task-B will write to variable-B. Both tasks A and B will read variables A and B.

0 Mauro Fantina over 4 years ago in reply to Robert Chen

Genius 3800 points

so you are tell me TI-OS is not preemptive

0 Sarah P over 4 years ago in reply to Mauro Fantina

TI__Mastermind 38980 points

Hi Mauro,

TI-RTOS is a preemptive scheduler: https://dev.ti.com/tirex/explore/node?node=AEwahlHnEy6Wj4XMYzQTrQ__fc2e6sr__LATEST

Best regards,

Sarah

0 Mauro Fantina over 4 years ago in reply to Sarah P

Genius 3800 points

OK, so, if I increment a uint8_t variable in a sw task and I decrement this uint8_t variable in another sw task I have to mutex this operations

0 Robert Chen over 4 years ago in reply to Mauro Fantina

Mastermind 9125 points

There is nothing to do with "preemptive". If task-A has higher priority than task-B then you can probably remove the atomic protection code (mutex or disable/enable interrupts) in task-A. But it is dangerous to do it this way since the you may change the priority of task A/B in the future. So, it is always safer to use mutex in all tasks which will modify this shared variable.

The CPU of CC3235 is Cortex-M4 which supports LDREX/STREX instructions to do atomic operation. TI-RTOS should utilize these instructions to implement mutex. So, the overhead should be very small to use mutex. But you can also use LDREX/STREX instructions to atomically modify the variable by yourself. You can use intrinsic __ldrexb() and __strexb() in C code.

0 Mauro Fantina over 4 years ago in reply to Robert Chen

Genius 3800 points

I have two task:

one put some data in a FIFO and counts how many data are in FIFO

    for(;;)
    {
        if(numData < AVAILABLE_SPACE)
        {
            /* Write Data */

            numData ++;
        }
    }

the other pop data

    for(;;)
    {
        if(numData > 0)
        {
            /* Read Data */

            numData --;
        }
    }

how can I use LDREX/STREX ??

0 Robert Chen over 4 years ago in reply to Mauro Fantina

Mastermind 9125 points

I believe you can google a lot of LDREX/STREX examples.

Replace

numData ++;

while (1) {
	uint8_t count = __ldrexb(&numData);
	count++;
	if (__strexb(count, &numData) == 0) {
		asm(" DMB");
		break;
	}
}

Replace "numData--" by above code with "count--".

I didn't test the code but it should work. The concept is simple. You should goole & study associated documents to understand how it work.

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CC3235MODASF: Atomic increment,decrement