STSW-ST25R-LIB using SPI while in IRQ context

[afr_e2]

Hello everyone,

in a project we are using the ST25R3916 chip to read RFID cards. To do this we are using the sources of the STSW-ST25R-LIB library. If the ST25R3916 sets its IRQ output, the routine st25r3916isr() is called. In this ISR the interrupt registers are read via SPI.

void st25r3916Isr( void )
{
 st25r3916CheckForReceivedInterrupts();
 ...
}

void st25r3916CheckForReceivedInterrupts( void )
{
 ...
 /* In case the IRQ is Edge (not Level) triggered read IRQs until done */
 while( platformGpioIsHigh( ST25R_INT_PORT, ST25R_INT_PIN ) )
 {
 st25r3916ReadMultipleRegisters( ST25R3916_REG_IRQ_MAIN, iregs, ST25R3916_INT_REGS_LEN );
 ...
 }
 ...
}

As other components can be connected to this SPI bus, no SPI communication is alowed in the context of an interrupt, as otherwise 2 SPI operations can overlap and this can lead to undesirable effects.

How can I get around this problem? Is there some kind of polling mode or how can I move the SPI operation into the context of the main loop?

Thank you for your support!

Best regards,
Andy

[Ulysses HERNIOSUS]

Hi Andy,

 

IMO the freertos demo of the  STSW-ST25R-LIB shows a deferred interrupt, also the Linux port (stsw-st25r013) defers the interrupt reading.

BR, Ulysses

 

 

[afr_e2]

Hi Ulysses,

thank you for the reply. I checked both implementations and have seen, that both are using a mulithreading OS and spend a seperate thread to handle the ISR. In that case you can use a semaphore or mutex.

In my project I'm not using a RTOS, its running completelly baremetal, so I cannot synchronize the ISR with a semaphore.

Regards,
Andy

[TDK]

The standard way of solving this problem is to set a flag in the interrupt which you check for in the main loop to do the thing you want.

// global
volatile int flag = 0;

// in interrupt
void IRQHandler() {
 flag = 1;
}

// in main loop
if (flag == 1) {
 flag = 0;
 // SPI operations here
}

 

[afr_e2]

Hi TDK,

thank you for your reply. I already tried to do so, but then I'm not able to read the card anymore. Maybe it takes too much time between irq rising and handling?!? So, is there any other possibility to synchronize the SPI access with the main loop?

Greetz, Andy

[Ulysses HERNIOSUS]

Hi Andy,

not sure how you have actually done this.

I recommend for your case to change functions st25r3916WaitForInterruptsTimed() and st25r3916GetInterrupt() to add at the beginning and inside the while a check for the interrupt pin or another interrupt flag as described by TDK to execute then the st25r3916Isr(). 

It should also work without the flag but I don't recommend it as it will create lots of traffic on the SPI bus and makes debugging using Logic Analyzer difficult.

BR, Ulysses