TWI function not working when Called from a Pin Interrupt Handler

Question

Hi, 
 The program I'm testing is mainly based on the example code Martin provided in
 devzone.nordicsemi.com/.../ 
 First test was that to read sensor data (MPU6050) by programming the nRF to request the data. This was successful. 
 Second test was to program MPU6050 to generate interrupts and then read the data. Monitoring a couple of pins and TWI pins showed that 
 
 MPU6050 was programmed correctly and the INT pulses could be seen 
 Code will go to the pin interrupt handler and the read acceleration function is called 
 TX part of the read operation (sending the slave address and the register address) is done, but RX part doesn't go through (no activity on SDA and SCL lines, and I think it eventually timeout) 
 
 In all these cases, read_accel() function was called from inside the int_pin_handler . When I tried calling the read function from main() the issue was solved and data can be read correctly. 
 Any thoughts on why is it acting like this? My SDK is 9, chip revision 3 and level 3 optimization is used. 
 main-2016-02-19.c 
 mpu9150.c

MartinBL · Accepted Answer

Hi 
 I believe you might be the victim of incorrect interrupt nesting. First of all, remember that it is good programming practice to keep the interrupt routines as short as possible. Preferably only a few lines of code. The way to achieve this is to set a flag in the IRQ and evaluate the flag in the main while loop, as you have already done in your code. And as you say this solves the problem. 
 The problem with calling mpu9150_read_accel() inside int_pin_handler() is that mpu9150_read_accel() takes a long time to complete. It includes two while() loops, which is very poor practice to have in an IRQ, and it triggers two TWI interrupts. So, i.e., you are nesting interrupts inside interrupts. 
 If you insist on nesting the interrupts this way you should be able to do so by setting the correct interrupt priorities. The TWI IRQ should have a higher priority than the GPIOTE IRQ. E.g. I was able to solve it by setting the GPIOTE priority to low and the TWI priority to high: 
 in nrf_drv_config.h search for this line 
 #define GPIOTE_CONFIG_IRQ_PRIORITY APP_IRQ_PRIORITY_LOW
 
 and make sure to use APP_IRQ_PRIORITY_LOW. 
 In main.c search for this line 
 .interrupt_priority = APP_IRQ_PRIORITY_HIGH
 
 and make sure to use APP_IRQ_PRIORITY_HIGH. 
 I thought this would be the default configuration though. Do you have it configured differently? 
 As a sidenote: The MPU drivers in the example code can certainly be improved. E.g. it should be possible make the transfers rely more on the interrupts and reduce the number of while() loops.

TWI function not working when Called from a Pin Interrupt Handler

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