Computer System Laboratory

1. Computer System Laboratory Lab13 – Interrupt + Timer /16

2. Experimental Goal • 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. /16

3. Environment • 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 CSL Course Software. /16

4. Introduction to Interrupt • 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. Reference: wiki info: interrupt, http://en.wikipedia.org/wiki/Interrupt /16

5. Two Types of Interrupt • 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 Reference: Daniel P. Bovert & Marco Cesati, “Understanding the Linux Kernel 3rd”, chapter 4, O’Reilly /16

6. Interrupt Routing • 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. Reference: David Rusling, “The Linux Kernel”, chapter 7, New Riders Pub /16

7. Linux Interrupt Handling • 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. Reference: David Rusling, “The Linux Kernel”, chapter 7, New Riders Pub /16

8. Register an Interrupt Handler • 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. intrequest_irq( unsigned intirq, void (*handler) (int, void *, structpt_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. /16

9. Introduction to Timer • 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. Reference: wiki info: timer, http://en.wikipedia.org/wiki/Timer /16

10. Timer • 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. Reference: VMWare, “Timekeeping in VMware Virtual Machines”, http://www.vmware.com/vmtn/resources/238 /16

11. Timer Registers on PXA270 • 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. /16

12. Top and Bottom Halves • Interrupt handlers need to finish up quickly anddo 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. Reference: Jonathan Corbet et al., “Linux Device Drivers 3rd”, chapter 10, O’Reilly /16

13. Linux Timer Interrupt • You can see <Linux src>/include/linux/jiffies.hwhich defines the timer interrupt frequency. • #define ACTHZ • And see <Linux src>/kernel/timer.c which defines both top-half and bottom-halfof interrupt handlers. • Top-half: do_timer(…) • Bottom-half: run_timer_softirq(…) /16

14. Lab Steps (1/2) • 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. /16

15. Lab Steps (2/2) • 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. /16

16. Lab Requirement • 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 PXA270. /16