Interrupts are unexpected events in a sequence of execution of instructions causing an interruption of the normal programme flow. The causes of interrupts can be different. For the family of dsPIC30F microcotrollers, in the transducer or general purpose applications, 41 interrupt sources and 4 types of traps, ranked according to the priority schematic, have been specified.

For each specified interrupt or trap the microcontroller (MCU) has a clearly defined further programme sequence specified by the interrupt vector table (IVT). The IVT contains the initial addresses of interrupt routines for each interrupt source or trap. IVT is located in the programme memory starting at location 0x000004 up to location 0x00007E. In addition to the interrupt vestor table, in the family of dsPIC30F microcontrollers, the alterante interrupt vector table (AIVT) is also specified. The purpose of the AIVT is to enable the realization of alternate interrupt routines. The AIVT is also located in the programme memory starting at location 0x000084 up to location 0x0000FE. On the basis of the priorities specified by the special function registers, the interrupt controller is responsible for the preparation and preprocessing of interrupts:

• IFS0<15:0>, IFS1<15:0>, and IFS2<15:0> are the registers containing all the interrupt request flags. Each source of interrupt has a status bit set by the respective peripherals or external signals. These flags are cleared by the user software.
• IEC0<15:0>, IEC1<15:0>, and IEC2<15:0> are the registers containing all the interrupt enable control bits. These control bits are used to individually enable interrupts from the peripherals or external signals.
• IPC0<15:0>, IPC1<15:0>, ... IPC10<7:0> are the registers used to set the interrupt priority level for each of the 41 sources of interrupt.
• IPL<3:0> are the bits containing the current CPU priority level. IPL<3> bit is located in the register CORCON and the remaining three bits IPL<2:0> are in the STATUS register (SR) of the microcontroller.
• INTCON1<15:0> and INTCON2<15:0> are the registers containing global interrupt control functions. INTCON1contains the control and status bits for the processor trap sources; INTCON2 controls the external interrupt requests behaviour and the use of the alternate vector table by setting the ALTIVT bit (INTCON2<15:0>).

During processor initialization it is necessary to enable interrupts which will be used and assign the corresponding priority levels.   Interrupt nesting is enabled by the NSTDIS bit of the control register INTCON1. Nesting of interrupt routines may be optionally disabled by setting this bit. This may be significant if it is necessary to carry out a part of the programme without an interrupt that could change the state which is the basis for this part of the programme or if the interrupt routine changes some variable which are of significance for further execution of the programme. The priority level of each interrupt is assigned by setting the interrupt priority bits IP<2:0> for each source of interrupt. The bits IP<2:0> are the least significant 3 bits of each nibble (4 bits) within the IPCx register. Bit no. 3 of each nibble iz always zero. The user can assign 7 priority leveles, from 1to 7. Level 7 is the highest and level 1 the lowest priority level for the maskable interrupts, i.e. for the interrupts that could be enabled by the control bits of IECx. Natural order priority is specified by the position of an interrupt in the vector table (IVT). It is used only to resolve conflicts between simultaneous pending interrupts with the same user assigned priority level. Then, the interrupt of the higher natural level of priority is executed first. As an example, table 3-1 shows for the microcontroller dsPIC30F4013 the interrupt vector table (IVT) with all sources of interrupts, interrupt number in the vector table, and the number which defines the natural order priority.

INT NUM	VECTOR NUM	IVT ADDRESS	AIVT ADDRESS	INTERRUPT SOURCE
Highest Natural Order Priority
0	8	0x000014	0x000094	INT0 - External Interrupt 0
1	9	0x000016	0x000096	IC1 - Input Capture 1
2	10	0x000018	0x000098	OC1 - Output Compare 1
3	11	0x00001A	0x00009A	T1 - Timer 1
4	12	0x00001C	0x00009C	IC2 - Input Capture 2
5	13	0x00001E	0x00009E	OC2 - Output Compare 2
6	14	0x000020	0x0000A0	T2 - Timer 2
7	15	0x000022	0x0000A2	T3 - Timer 3
8	16	0x000024	0x0000A4	SPI1
9	17	0x000026	0x0000A6	U1RX - UART1 Receiver
10	18	0x000028	0x0000A8	U1TX - UART1 Transmitter
11	19	0x00002A	0x0000AA	ADC - ADC Convert Done
12	20	0x00002C	0x0000AC	NVM - NVM Write Complete
13	21	0x00002E	0x0000AE	SI2C - I2C Slave Interrupt
14	22	0x000030	0x0000B0	MI2C - I2C Master Interrupt
15	23	0x000032	0x0000B2	Input Change Interrupt
16	24	0x000034	0x0000B4	INT1 - External Interrupt 1
17	25	0x000036	0x0000B6	IC7 - Input Capture 7
18	26	0x000038	0x0000B8	IC8 - Input Capture 8
19	27	0x00003A	0x0000BA	OC3 - Output Compare 3
20	28	0x00003C	0x0000BC	OC3 - Output Compare 4
21	29	0x00003E	0x0000BE	T4 - Timer 4
22	30	0x000040	0x0000C0	T5 - Timer 5
23	31	0x000042	0x0000C2	INT2 - External Interrupt 2
24	32	0x000044	0x0000C4	U2RX - UART2 Receiver
25	33	0x000046	0x0000C6	U2TX - UART2 Transmitter
26	34	0x000048	0x0000C8	Reserved
27	35	0x00004A	0x0000CA	C1 - Combined IRQ for CAN1
28-40	36-48	0x00004C – 0x000064	0x0000CC – 0x0000E4	Reserved
41	49	0x000066	0x0000E6	DCI - CODEC Transfer Done
42	50	0x000068	0x0000E8	LVD - Low Voltage Detect
43-53	51-61	0x00006A – 0x00007E	0x0000EA – 0x0000FE	Reserved
Lowest Natural Order Priority

Table 3-1. Interrupt vector table of microcontroller dsPIC30F4013

Example:

The example shows how dsPIC reacts to a rising signal edge at the pin RF6(INT0). For each rising edge the value at port D is incremented by 1.

program ExtInt1;
procedure IntDetection; org $0014; //Interrupt on INT0
begin
  INC(LATD);
  IFS0.0:=0;        //interrupt flag cleared
end;
begin
  TRISD:=0;     //portd is output
  TRISF:=$FFFF; //portf is input
  IFS0:=0;      //interrupt flag cleared
  IEC0:=1;      //Interrupt is set on a rising edge at INT0 (RF6)
  WHILE TRUE do nop;
end.

Let us summarize the algorithm how dsPIC processes interrupts in this example. Two ports are used, PORTD as the output to display the number of interrupt events and PORTF as the input; this means that an interrupt will occur when at INT0 (RF6) logic 0 changes to logic 1. In the register IEC0 the least significant bit (IEC0.0) is set for allowing reaction to interrupt INT0. The meanings of other bits are shown in table 3-7. When an interrupt occurs, the procedure IntDetection is called. How does dsPIC “know” to call exactly this procedure? By instructionorg in the interrupt vector table (see table 3-1) at the memory location 0x000014 is written the procedure IntDetection. What does dsPIC do when an interrupt occurs, i.e. when at RF6 logic 1 appears after logic 0? At first, it writes logic 1 in the least significant bit of the register IFS0. Then, it tests if the interrupt INT0 is enabled (the least significant bit of IEC0). If yes, it reads from the interrupt vector table which part of the programme should be executed. MicroPascal compiler will, at position 0x000014, write the code where procedure IntDetection starts and dsPIC will execute this procedure and return to the main program. Two operations are carried out in the interrupt procedure. At first, the interrupt flag is cleared (dsPIC does not do that automatically, but leaves this to the user software). Then, the value at port D is incremented by 1 with INC(LATD). What would happen if the interrupt flag in the IFS register was not cleared? If this line is omitted, dsPIC will wait until first interrupt, call and execute the procedure IntDetection. What is the difference? The register IFS0 will still indicate that an interrupt occured. As soon as the microcontroller returns to the main programme and executes one instruction, it would understand that an interrupt occured again and theIntDetection procedure would be called again. This means that IntDetection procedure would be executed after each instruction of the main programme. The reader is encouraged to erase the line IFS0.0:=0; and see what happens. What is the purpose of WHILE TRUE DO NOP;? When dsPIC comes to the end of a programme, it starts from the beginning as if reset. However, at the end of the programme the compiler inserts an endless loop to prevent this of happening. This has been inserted here to make the reader aware of the existence of this loop and that reseting dsPIC is not possible.