Interrupts

The Nested Vectored Interrupt Controller (NVIC) is an integral part of any Cortex-M microcontroller. On the LPC18xx, the NVIC supports 53 vectored interrupts. 

Each of the LPC43xx cores has its own nested verctor interrupt controller. The ARM Cortex-M4 core supports 53 vectored interrupts. The NVIC in ARM Cortex-M0 core supports 32 interrupts.

Most of the peripheral interrupts are shared between the Cortex-M0 and Cortex-M4 NVICs. You can handle peripheral interupts with Cortex-M0 core and use Cortex-M4 core for complex calculations (like digital signal processing).

The  LPC43xx interrupts are also used for inter-process communication. We will use these interrupts later in the tutorial, when we discuss the communication between Cortex-M0 and Cortex-M4 cores.

The CMSIS library contains the assembly language source file startup_LPC43xx.s that declare the interrupt vector table (it is almost identical to the startup_LPC18xx.s  file for LPC43xx microcontrollers). You can find it at the lpc43xx\Core\Device\NXP\LPC43xx\Source\Templates\IAR folder.

Let's add the startup_LPC43xx.s file to our project using menu [Project | Add Files...]. In the Add Files dialog box navigate to the lpc43xx\Core\Device\NXP\LPC43xx\Source\Templates\IAR folder and open the startup_LPC43xx.s file:

The first ten interrupts are fixed by ARM. You will find the same interrupts in any other Cortex-M microcontroller. We will implicitly use the first two elements of the interrupt table array:

__vector_table
        DCD     sfe(CSTACK)
        DCD     Reset_Handler

The first element is initialized with the initial stack pointer - CSTACK. It is defined in the linker script file.

The second element contains the address of the reset handler, which is implemented in the same startup_LPC43xx.s file. It simply calls the __iar_program_start() function, which initializes the device RAM and calls our main() function.

Reset_Handler
        LDR     R0, =__iar_program_start
        BX      R0

All interrupt handlers in startup_LPC43xx.s file are defined with the PUBWEAK directive:

        PUBWEAK GPIO0_IRQHandler
        SECTION .text:CODE:REORDER(1)
GPIO0_IRQHandler
        B GPIO0_IRQHandler

The PUBWEAK directive allows the same symbol to be defined in more than one module. We will use this feature to override the GPIO interrupt.