AD7794 long delay

[Hugo]

Hi,

I'm trying to write a quick driver for the AD7794 on nrf51822. This works on a special SPI: the MISO line is alse the "data ready" line.
So when I want to read a value, I need first to ask to read the right channel, then to wait for a value, and at last to read value itself. This is not so easy with the spi_master library, but I dealed with it... out of the softdevice section.

this it my function:

int read_data(int address)
{
    uint8_t resp[5]={1,2,3,4,5};
    uint8_t config_data[2]={0b01001010,0x90};
    uint8_t mode_data[2]={0x30,0x11};
    uint32_t value;
    config_data[1]+=address;
    

    //configure the reading
    printf("read\r\n");
    spi_write(BSP_SPI_CS,0b10,config_data,2);

    //use manual slave select, and set it low
    nrf_gpio_pin_clear(BSP_SPI_CS);
    nrf_gpio_cfg_output(BSP_SPI_CS);
    nrf_gpio_pin_clear(BSP_SPI_CS);

    //configure the channel, and ast for single read
    spi_write(-1,0b1,mode_data,2);


    //set the MOSI line up
    nrf_gpio_pin_set(BSP_SPI_SDI);
    nrf_gpio_cfg_output(BSP_SPI_SDI);
    nrf_gpio_pin_set(BSP_SPI_SDI);

    //set the MISO line as input
    nrf_gpio_cfg_input(BSP_SPI_SD0, NRF_GPIO_PIN_NOPULL  );
    //wait fot the line to be down (! ! ! )
    while(nrf_gpio_pin_read(BSP_SPI_SD0)!=0);
    //MISO and MOSI back to norma
    nrf_gpio_cfg_default (BSP_SPI_SD0);
    nrf_gpio_cfg_default (BSP_SPI_SDI);

    //read the value
    spi_read(-1,0b11,resp,3);
    value=(uint32_t)(((uint32_t)resp[1]<<16)+((uint32_t)resp[2]<<8)+(uint32_t)resp[3]);
    //set the CS back to high
    nrf_gpio_pin_set(BSP_SPI_CS);
    return value;
}

this is what it looks like:

The problem is when I'm using the softdevice, I'm locked into the "while GPIO not 0" loop. The MISO line still works fine and go down (but so not read correctly by the app). Afer few mode ms, the MISO line get crazy, and afer a long while (1 or 2 sec), the chip reboot.

I tried many things such as add delay in the loop, add app_sched_execute() in it as I thought it might get bored waiting(stupid, I know), etc... but I don't find a solution, even a bad one.

Does someone has any idea? thanks a lot !


 

[Susheel Nuguru]

Hi,

I see atleast one deadlock scenario here. At the below line in your code

    //set the MISO line as input
    nrf_gpio_cfg_input(BSP_SPI_SD0, NRF_GPIO_PIN_NOPULL  );
    //wait fot the line to be down (! ! ! )
    while(nrf_gpio_pin_read(BSP_SPI_SD0)!=0);

You are assuming that your application will be active and able to catch the pin toggle to low in that while loop. But on a multi context system (when using softdevice or scheduler or any other RTOS) it is possible that some other context is running other than your app. I think this is what is happening in your application 

1) you set MISO as input

2) wait for pin to go low.

3) something interrupts your app (softdevice or interrupt) 

4) When something else is running other than your app, then the MISO pin toggles to 0 and toggles back to 1

5) your application gets control back from (softdevice or interrupt) and it has missed the toggle it waits for and sees that the pin is still at 1 and continues the while loop.

5) Since your app is stuck in this loop it is not sending read commands to the sensor and hence the MISO pin will not toggle anymore.

This is clearly a deadlock in your app.

You need to make sure that when your app is in the background and something else is running, then the pin toggle is captured somehow. It is wise to configure the pin to generate interrupt at this time when the pin goes low and then set a flag in the interrupt. You can wait for this flag in the while loop instead of waiting for pin to toggle. If you set the flag the the interrupt then you make sure that this toggle is not lost and captured as a flag value change in ISR.

[Hugo]

Hin thanks, and congratulation, you found the bug.
Actually, as you see in the 1st screenshot, asking and waiting for a measurement takes 4ms. 
When the while loop explodes, the 4ms are not done yet, wich means that if I was waiting for an interrupt, the interrupt woudn't have been called yet.
Maybe you are right, and this is cleaner, but I'm quite sure it won't work, would it?

In an other hand, I fully agree that the while loop is really ugly and is a great way to block the CPU and everything including the softdevice. I'm looking for a softer function, such as:
void no_breaking_delay_ms( int delay_time);
or
void wait_quietly_for_an_event_enjoying_a_mojito_on_the_beach_watching_all_the_other_thread_are_still_running(event_t event_name);
If both functions exist, it would be even better.

As Stian, and I said, there is an other way: cut the function into 3 function, the beginning, the interrup, the end. And of course, cut any function that need to read the adc in two too. Let'ts try to avoid this, please. Lingchi is a programming technic which leads the code to death.

In linux, there is "sleep" and "mutex"
In chibios there is "chThdSleep (systime_t time)" and chEvtWaitOne (eventmask_t mask)
In freeRTOS there is "vTaskDealy" and eventGroups
etc...

Maybe, there is just no solution, is it?

[Susheel Nuguru]

Hugo said:

In an other hand, I fully agree that the while loop is really ugly and is a great way to block the CPU and everything including the softdevice

The while loop does not block softdevice from interrupting your app. That is what we believe is the source of this problem. 

There is a solution as I wrote earlier, instead of waiting for the pin state change, your app needs to wait/block/suspend on a flag/mutex/semaphore. Make sure that the flag/mutex/semaphore change their state in the gpio ISR which is making sure that you do not loose this info. I do not see any other way to do this a failproof way.

[Hugo]

Great, it tooks me, 1mninute on google to deal with it on 3 different scheduler, I still don't know how to do it with the nordic softdevice scheduler. Can you name a function to "wait/block/suspend on a flag/mutex/semaphore"? can you link me an exemple? or the best can you ask me to read the manual, but with a link to the right page of the manual? this would be lovely, and thank you in advance :)

[Stian Røed Hafskjold]

The scheduler is very simple, it does not provide this functionality. It's basically just for transferring execution from interrupt context to main context. You put the event handler in a queue in the ISR, and you execute the handler in main context. That's all. FreeRTOS, linux etc which you mentioned earlier are OS's. If you want similar functionality you can use freeRTOS on the nRF chip, but I think this is a bit too much just for an SPI driver.

A clean and nice solution is to use flag in the GPIOTE interrupt handler and wait for it in the SPI function. Did you try the following?

bool data_ready = 0;

void gpiote_interrupt(){
    //got a MISO falling edge interrupt
    data_ready = 1;
}

void SPI_read_sensor(){
    spi_send_command();
    deinit_spi(); // SPI lines cannot be configured while SPI is enabled
    configure_gpiote_interrupt_on_MISO();
    while(!data_ready){
        __WFE(); //wait for event. CPU sleeps until event
    }
    data_ready = 0;
    deinit_gpiote();
    init_spi();
    spi_read_data();
}

Note that you cannot reconfigure the GPIO pins while SPI is enabled.

Another thing you should be careful with is whether or not the spi_read/write calls are blocking. For example: If the "spi_read_data()" call above is non-blocking, the function will return instantly and the scheduler might continue execute the next function in the queue before the SPI data has actually been read. Or similarly, if the "spi_read_command()" call is non-blocking it will enter the waiting loop before the spi read command is sent.

[Hugo]

Thank you Stian,

I'm actually  in vacation. My turn ;)
I'll try this WFE() function within a month.

But atbout OS and scheduler, not every OS has a scheduler, and a scheduler might not be an OS. This is because there is no precise definition of OS. An OS a is just a toolset that can handle hardware. . So saying that the softdevice, which is an OS in my opinion, is not an OS, is not an excuse of providing a nice scheduler or a less good one.
I'm happy to know that the scheduler cannot do what I need, the way I need, because it's a step further. I'm quite scared to use both softdevice which has a scheduler used by bluetooth runtime, and an other scheduler, as they need "to share" interupts from a timer, and they might interract.  I'll check if it's possible, when I'll come back.
Thanks for your answer.