Lab 13 Department of Computer Science and Information Engineering National Taiwan University Lab13 – Interrupt + Timer 2014/12/23 1 /16

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Understand the Linux interrupt handling, the hardware timer and Linux timer, and also learn how to control 8-bit LED lamps, 4-Digits 7 segment LED and keypad. 2014/12/23/16 2

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Host System  Windows XP  Build System  VirtualBox + Ubuntu 8.04  Target System  Creator XScale PXA270  Software  Creator PXA270 LCD driver, please refer to Lab12  You can download all software from RSWiki CSL Course Software.RSWiki CSL Course Software 2014/12/23/16 3

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  An interrupt is a signal to the processor.  The processor responds by suspending its current activities, saving its state, and executing a small program called an interrupt handler (interrupt service routine, ISR) to deal with the event.  The act of initiating a hardware interrupt is referred to as an interrupt request (IRQ).  Interrupts are a commonly used technique for computer multitasking. 2014/12/23/16 4 Reference: wiki info: interrupt,

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Asynchronous interrupt: caused by an external event.  Maskable interrupt (IRQ)  E.g., I/O request  Non-maskable interrupt  E.g., watchdog timer  Synchronous interrupt (Exception)  Faults  E.g., page fault  Traps  E.g., divide by zero  Aborts  E.g., machine check  Software interrupt  E.g., system call 2014/12/23/16 5 Reference: Daniel P. Bovert & Marco Cesati, “Understanding the Linux Kernel 3 rd ”, chapter 4, O’Reilly

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Systems often use a programmable interrupt controller (PIC) to group the device interrupts together before passing on the signal to a single interrupt pin on the CPU. 2014/12/23/16 6 Reference: David Rusling, “The Linux Kernel”, chapter 7, New Riders Pub

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Linux uses a table ( irq_action ) of pointers to data structures ( irqaction ) containing the addresses of the interrupt service routine (ISR).  When interrupt happens, the kernel will execute the corresponding ISR to handle it. 2014/12/23/16 7 Reference: David Rusling, “The Linux Kernel”, chapter 7, New Riders Pub

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  In Linux, there is a related API, request_irq(), to register an interrupt handler, so developers can set their own handler with a specific IRQ. int request_irq( unsigned int irq, void (*handler) (int, void *, struct pt_regs *), unsigned long irqflags, const char *devname, void *dev_id );  In the Creator PXA270 LCD driver, you can find this API which registers a timer interrupt. 2014/12/23/16 8

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Computer systems usually have at least one timer. These are typically digital counters that decrement at a fixed frequency, which are often configurable and interrupt the processor when reaching zero.  As the number of hardware timers in a computer system or processor is finite, OSes often use a single hardware timer to implement an extensible set of software timers.  Some timer devices count up instead of down and have a register whose value is compared with the counter to determine when to interrupt and restart the count at zero. 2014/12/23/16 9 Reference: wiki info: timer,

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  The oscillator provides a fixed input frequency to the timer device, and the counter counts down one unit for each cycle of the oscillator.  When it reaches zero, it generates an interrupt signal.  There might also be a counter input register whose value is loaded into the counter when it reaches zero. 2014/12/23/16 10 Reference: VMWare, “Timekeeping in VMware Virtual Machines”,

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  PXA270 provides a set of timers that allows software to generate timer interrupts.  The related registers on PXA270 are as follows:  OS Timer Count Registers ( OSCRx ): counter  Count up one unit for each cycle of the oscillator.  OS Timer Match Registers ( OSMRx ): counter input  When OSCR reaches the value of OSMR, it will generate an interrupt.  OS Match Control Registers ( OMCRx ): control OS timers  Set the frequency of oscillator, set periodic timer, automatically reset OSCRx after it reaches the value of OSMRx, etc.  For more information about these registers, please refer to Intel PXA27x Processor Family Developer’s Manual, Chapter 22.Intel PXA27x Processor Family Developer’s Manual 2014/12/23/16 11

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Interrupt handlers need to finish up quickly and do not keep interrupts blocked for long.  Therefore, Linux splits an interrupt handler into two halves.  Top-half  Perform time critical tasks, and schedule its bottom half.  Bottom-half  Awake waiting processes, and start up another I/O operation and so on. 2014/12/23/16 12 Reference: Jonathan Corbet et al., “Linux Device Drivers 3 rd ”, chapter 10, O’Reilly

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  You can see /include/linux/jiffies.h which defines the timer interrupt frequency.  #define ACTHZ  And see /kernel/timer.c which defines both top-half and bottom-half of interrupt handlers.  Top-half: do_timer(…)  Bottom-half: run_timer_softirq(…) 2014/12/23/16 13

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Trace the driver’s code for 8-bit LED lamps, 4-Digits 7 segment LED and keypad, and see how they work.  Modify the driver codes such that the 8-bit LED lamps can sparkle in your own pattern.  Hints  See creator_pxa270_lcdtxt_ioctl for these operations.  See _7segment_timer_irq which is the timer interrupt handler.  Trace ScanTimer in the driver to know how the keypad works.  For more information about hardware registers, please refer to Create Creator PreSOCes Development Kit User’s Guide, 3.7.2, and /12/23/16 14

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Implement a stopwatch.  The resolution should be in 0.1 second and 0.01 second which can be switched by the keypad.  The value of the counter should be displayed on the 4-digit 7-segment LED in decimal form.  It can be started, suspended, resumed, and reset by pressing the buttons of the keypad.  Define these actions of the keypad by yourselves. 2014/12/23/16 15

Lab 13 Department of Computer Science and Information Engineering National Taiwan University  Show your sparkling 8-bit LED lamps in PXA270.  Show your stopwatch.  Display the time in 4-Digits 7 segment LED.  Control the stopwatch by the keypad in PXA /12/23/16 16