Chapter 9

Resources

• Microcontrollers: From Assembly Language to C Using the PIC24 Family, chapter 9
  • Pages 317-332, seriously
  • Pages 337-348, Periodic Timer Interrupts, creatively
  • Pages 354-356, A Rotary Encoder Interface & A Keypad Interface, inquisitively
• Mississippi State ECE 3724 notes
  • Interrupts and a First Look at Times
• Mississippi State ECE 3724 video lectures
• Microchip documents
  • PIC32MX1XX/2XX datasheet, Chapters 7, 12 and 13 (pp. 87-94, 151-158)
  • PIC12F1572 datasheet, Chapters 7, 18, 19 and 20 (pp. 65-77, 143-159)
  • PIC32 Family Reference Manual (Documentation → Reference Manual)
  • PIC32 Family Reference Manual, Sect. 8 Interrupts
  • PIC32 Family Reference Manual, Sect. 14 Timers
  • PIC32 Family Reference Manual, Sect. 16 Output Compare
• MPLAB® XC32 C/C++ Compiler User’s Guide, Chapter 13 (pp. 187-198)

“Interrupts” you may have encountered

Processing has several event-driven functions that are a bit like interrupts.

• draw — timer-driven
• keyTyped
• mousePressed

JavaScript, with its many callback function is also event-driven.

• Understand JavaScript Callback Functions and Use Them
• Callback Hell
• Something for a future 255 lab

Arduino programmers sometimes use and overuse interrupts to make their program behave more predictably. Arduino PWM also uses timers.

• attachInterrupt()
• PWM Arduino

The Unix signal is a bit like an interrupt. The Unix command kill can deliver a signal to a process.

Interrupts

• ECE 3724 notes on Interrupts (pp. 1-7)
• UMassAmherstM5 tutorial on PIC32 interrupts

Properties of the PIC32 interrupt hardware

• INTCON — number and size of vectors
• PRISS — shadow registers will be used
• IFSx — interrupt has been requested
• IECx — interrupt is enabled
• IPCx — priority of interrupt

The PIC32 has a special set of CP0 registers that are used for operating systems functions such as interrupt handling. Notes from UC-Davis and UC-Berkeley provide detailed descriptions of how the MIPS32 architecture handles exceptions and interrupts.

The best thing to do when handling an interrupt

Defer.

The worse thing to do when handling an interrupt

Delay.

The ISR

• ECE 3724 notes on Interrupts (pp. 11-14)
• MPLAB® XC32 C/C++ Compiler User’s Guide, Chapter 13 (pp. 187-198)
  • __attribute__((interrupt(IPL3))) myISR(void) ;
  • __ISR(vector, IPL) myISR(void) ;
  • void __ISR (_TIMER_1_VECTOR, IPL7SRS) Timer1Handler (void);

Code generated by the compiler will save and restore registers. This includes the various callee-saved registers such as $tn.

Most PIC32 interrupt service routines are fairly simple.

Connecting interrupts to actions on pins

• On an embedded system
  • ECE 3724 notes on Interrupts (pp. 15-16, 20-21)
  • UMassAmherstM5 interrupt example
• On a multi-user operating system
  • userspace — GPIO Sysfs interface or How to use GPIO signals
    • Open /sys/class/gpio/export
    • Write pinNum to export file
    • Open /sys/class/gpio/gpiopinNum/direction
    • Write in to direction file
    • Open /sys/class/gpio/gpiopinNum/edge
    • Write both to edge file descriptor
    • Open /sys/class/gpio/gpiopinNum/value
    • Perform poll for POLLPRI event on value file descriptor
    • Perform lseek on value file descriptor
    • Perform read on value file descriptor
    • Close files as needed
    • Unexport as needed
  • kernelspace driver

Interesting things to do with interrupts

• Read a rotary encoder
• Scan a keypad

Timers

• ECE 3724 notes on Timers (pp. 22-28)
• PWM Arduino