Probably one of the key features in any microcontroller is interrupt system. ARM Cortex-M3 microcontrollers may have up to 256 interrupts sources. First 15 interrupt sources are called system exceptions. These exceptions rise within Cortex core like reset, NMI, hard fault and error, debug and SystTick timer interrupt. In the exception table they start from address 0×00000004 and are numbered from 1 to 15. There is no 0 number exception (FYI – the very top of exception table address is used to store starting point of stack pointer):



Each exception vector holds four byte wide address of service routine that is called when exception occurs. Exception table usually is located in startup code like this:


It is located in .vectors memory section which is described in linker script and usually resides at very beginning of flash memory.

Inside NVIC of Cortex-M3

Nested Vectored Interrupt Controller (NVIC) is an essential part of Cortex processor. It is pretty complex module that takes care of interrupt processing logic. Additionally it holds control registers for SysTick timer and debug control. Generally speaking NVIC can support up to 240 external interrupts. Speaking of STM32F100RB microcontroller that resides in discovery board NVIC supports 41 maskable interrupt channel and 16 priority levels. As NVIC are closely coupled with processor core it is assured that interrupts are processed with low latency. NVIC supports some advanced interrupt handling modes including Interrupt preemption, tail chaining, late arrival. These features allow to reach low latency and more robust response. These features allow to avoid stack overhead when processing multiple interrupts that arrive pretty much at same time. For instance tail chaining mechanism allows skipping stack pop if there is another pending interrupt once current is completed. Refer to Cortex-M3 programming manual for more info.

Fe words about priority. Generally speaking every interrupt has associated an 8-bit priority level register. But not all bits are used to set priorities. STM32F100RB microcontroller has 16 priority levels what means that 4 MSB bits are used to set priorities. The lower priority number the higher priority. If needed these bits can be split in to two groups where you can create sub-priority levels for each preemptive priority. Subpriority levels are handy when two or more same priority level interrupts occur. Then one with higher subpriority will be handled first:

Handling external interrupts

External interrupts are connected to NVIC through special external interrupt controller (EXTI). It allows to map multiple GPIO lines to NVIC external interrupt inputs. STM32F100RB interrupt controller consists of 18 edge detector lines. Each line can be configured to trigger on different event like rising edge, falling edge or both. As ports are 16-bit wide then 16 lines are dedicated to map port pins. For instance EXTI0 line can be mapped to pins 0 of all ports or any other combination.

Additional 3 EXTI lines are dedicated to RTC alarm interrupt, power voltage detect (PVD). EXTI lines then connects to NVIC controller. EXTI lines 0..4 connects individually to NVIC while lines 5 to 16 are grouped in to two lines that connect to NVIC. RTC alarm interrupt, power voltage detect lines also connect to NVIC individually.

It is hardly possible to cover all exception handling functionality. More should come during practice as each individual case has it’s own scent. When pushing projects to RTOS based applications these things start really matter. But for now lets make another code example where we will implement interrupt based routines.

Writing interrupt based code example

We are going to improve our previous code example where button were checked and LEDs lit in while(1) super-loop. This time we are going to configure so that button press would generate interrupt. Within interrupt service routine we will process button function and then return to loop. Also we are going to implement SysTick timer which would blink LED.

First of all we need to initialize EXTI module that would map button pin to EXTI0 line and then configure this line on NVIC controller where we will set up a priority and enable interrupt. To do so in button.c source file we create:



To use button as EXTI we need external interrupt to port pin.
This simply sets pin input for EXTI in AFIO_EXTCR register. As we map pin0 of GPIOA port this command sets least bit of AFIO_EXTICR1 register. Once mapping is done we need to configure EXTI module itself:
Here we select which EXTI line we need to configure. Then select Interrupt mode for this line (also EXTI lines can be configured to generate software interrupts). Then we select rising edge triggering source as our button pin is going high when pressed. And lastly we simply enable this line so it could output signals on EXTI line that goes in to NVIC.

NVIC also has to be set in order to process upcoming interrupts:


Here is same situation: we select which interrupt source to configure (EXT0_IRQn), then set priority and subpriority – both to 0 and lastly we enable that interrupt. After NVIC is initialized it can start processing incoming interrupts from EXTI0 line that is mapped to button pin.

In our template we are using separate source file (stm32f10x_it.c) where interrupt service routines are implemented. Here we write our EXTI0 handler:


It toggles LED every time we press a button. One important thing about EXTI lines!. Once line have been set it won’t reset automatically so after single button press it stays high constantly and this leads chained interrupts as NVIC “thinks” that there are new interrupt requests incoming. To avoid this we need to reset EXTI line manually after we process interrupt. For this we use function:




Additionally we wanted to run SysTickTimer which would blink a led every 1s. CMSIS core_cm3.c source has a nice function that initializes and starts Systick timer with single line:




We only need to pass how many ticks to count between SysTick interrupts. Also we need to write our handler in stm32f10x_it.c file:


We just toggle led on every SysTick interrupt.

After everything is set up we can write our main program:

All we do is just initialize button, leds and start SysTick timer. Other work is done by interrupt routines. This saves processing power significantly. In battery operated devices processor is sent to one of power safe modes between interrupts or simply do other tasks while interrupts automatically work in background.

Download project file:
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