EET 2261 Unit 9Interrupts

Read Almy, Chapters 17 – 19.

Homework #9 and Lab #9 due next week.

Quiz next week.

• An interrupt is a mechanism for causing a program to temporarily suspend what it’s doing, and do something else instead.

• Interrupts are commonly used with I/O devices (keyboards, etc.). By using interrupt-driven I/O, a program can efficiently service the I/O devices only when they need attention.

• The alternative is polled I/O, in which the program periodically stops what it’s doing and checks to see whether any I/O device needs service.

Interrupts

• Suppose you’re at home working on a demanding task (writing a paper), and at the same time you’re waiting for FedEx to deliver an important package.

• Two possible approaches:

1. Get up from your desk every 30 seconds to look out the window for the FedEx truck.

2. Focus on writing your paper, and let the FedEx driver ring the doorbell to announce her arrival.

An Analogy

• On this analogy:

• Your paper-writing task is like the processor’s main program.

• The FedEx delivery is like an I/O device.

• The first approach (getting up from your desk every 30 seconds) is like polled I/O. Not a good way to work, is it?

• The second approach (relying on the doorbell) is like interrupt-driven I/O. Makes better use of your time!

An Analogy (Continued)

• In real life, there may be many different events that could interrupt you from your primary task:

Doorbell Telephone Tornado siren Dog scratching at the door to be let out

• Similarly, the HCS12 has many (about 50) different kinds of interrupts that may require the CPU to temporarily set aside its main task.

• Some of these interrupts come from outside the HCS12 chip, and some come from circuits on the HCS12 chip itself.

An Analogy (Continued)

• For complete list, see pages 75-76 of the Device User Guide (or the similar list on page 157 of the book).

• The list orders interrupts from highest priority to lowest. (Priority comes into play if two interrupts occur at the same time.)

List of Interrupts

• Of the 50-plus kinds of “interrupts,” the three highest-priority ones are more correctly called resets:• Reset• Clock Monitor Fail Reset• Computer Operating Properly (COP) Reset

• The next two highest-priority ones are more correctly called traps:• Unimplemented Instruction Trap• Software Interrupt (SWI) Trap

• The rest are just called interrupts. These are the ones we’ll focus on.

Resets, Traps, and Interrupts

• The programmer can choose whether to allow or block interrupts. (But resets and traps cannot be blocked.) If an interrupt is blocked, it will be ignored by the CPU.

• We’ll see later that many special function registers hold bits that let the user allow or block specific kinds of interrupts.

• Two bits in the Condition Code Register (CCR) also play a key role in allowing or blocking interrupts….

Allowing or Blocking Interrupts

Review: Condition Code Register• The CCR is an 8-bit register that contains:

• 5 flag bits (H, N, Z, V, C)• 2 interrupt mask bits (X and I)• STOP instruction control bit (S)

CCR Bits X and I

• The X bit blocks (if X=1) or allows (if X=0) interrupts from the chip’s XIRQ pin. See page 25 of CPU Reference Manual.

• The I bit blocks (if I=1) or allows (if I=0) all other interrupts, including interrupts from the chip’s IRQ pin. See pages 25-26 of CPU Reference Manual.

• Assuming that interrupts are allowed, when the CPU receives an interrupt signal, it sets aside whatever it is doing and runs code called an interrupt service routine (ISR) or interrupt handler for that specific interrupt.

• The programmer must write these ISRs and place them in memory. An ISR is basically a subroutine that gets called automatically when a interrupt occurs.

Interrupt Service Routine

• For each type of interrupt, there is a fixed location in memory into which the programmer must load the starting address of the interrupt service routine (ISR).

• The group of memory locations set aside to hold the starting addresses of the ISRs is called the interrupt vector table.

Interrupt Vector Table

• Memory locations $FF00 through $FFFF are reserved for the interrupt vector table.

• Figure from page 26 of Device User Guide.

Location of Interrupt Vector Table

• See Table 5-1 on pages 75-76 of the Device User Guide.

• For each type of interrupt, this table tells us:

Information in Table 5-1

• The source (name) of the interrupt.• The locations in memory where the programmer must store the starting address of the interrupt’s service routine.

• Whether the interrupt can be masked (blocked) by a bit in the Condition Code Register (CCR).

• Whether the interrupt can be allowed/blocked somewhere other than the CCR.

• Whether the interrupt can be elevated to a higher priority.

• When the HCS12 receives an interrupt, it follows these steps:• It finishes executing the current instruction.• It pushes the contents of PC, Y, X, A, B, and

CCR onto the stack, thus saving a snapshot of exactly what the CPU was doing before it was interrupted.

• It fetches the address of the appropriate interrupt service routine (ISR) from the interrupt vector table and places this address in the Program Counter (PC).

• Continues on next slide….

Steps in Executing an Interrupt

• …It sets the I bit in CCR high to ensure that no other interrupt can interrupt the CPU while it’s serving the current one.

• It fetches and executes instructions belonging to the interrupt service routine.

• The last instruction of the interrupt service routine must be an RTI (Return from Interrupt).

• RTI causes the CPU to pull the original PC, Y, X, A, B, and CCR values from the stack, thus restoring the CPU to the state it was in before it serviced the interrupt.

• It then continues to run the code from where itleft off before the interrupt.

Steps in Executing an Interrupt (Continued)

• Many of the bits in the chip’s special function registers are devoted to configuring and controlling interrupts.

• Most of the bits in these registers are either

• enable bits, which we set or reset to decide whether we’re allowing interrupts

• or flag bits, which the hardware sets to indicate that an interrupt has occurred. Your interrupt service routine should reset these flags to “clear” the interrupt.

Interrupts and Special Function Registers

• Your program named Lab07SwitchesToLEDs sat in a loop, reading the switches and sending the switch values to the LEDs:

Back: LDAA PTH STAA PORTB BRA Back

• A better way is to use an interrupt to tell us when the switch settings have changed, instead of repeatedly polling the switches.

Polling Versus Interrupts for Port H

• Port H has its own interrupt, which can be caused by any of the bits in Port H:

• Enabled or disabled by the bits in PIEH:

• Flag bits are in PIFH:

Port H Interrupts

• Programming an interrupt involves several steps on the programmer’s part:

1. Enable the interrupt (usually by clearing the I bit in the CCR and setting one or more local enable bits).

2. Write the interrupt service routine (ISR), which is much like a subroutine except it must end with RTI, not RTS.

3. Place the ISR’s starting address in the appropriate location within the interrupt vector table.

Programming an Interrupt

Example: Programming an Interrupt

Skeleton of code for using Port H interrupt:

Note that this example enables interrupts on only one of the bits of Port H. You might instead need to enable interrupts for several of the Port H bits, or for all of them.

• As part of your interrupt service routine (ISR), be sure to clear the flag bit that caused the interrupt. Otherwise, when the HCS12 exits your ISR, the flag bit will still be set and will immediately cause another interrupt.

• Strangely, to clear an interrupt flag bit you must write a 1 to it after having read it while it was a 1. (You don’t write a 0, which is what you might expect).

• Next slide extends the previous example to include this step.

Clearing the Interrupt Flag Bit

Previous Example With Additional Line to Clear the Interrupt Flag

Skeleton of code for using Port H interrupt:

• We’ve been looking at the Port H interrupt. Ports J and P have similar interrupts. • These Port H, J, and P interrupts provide one way to

interrupt the HCS12 when an external event occurs.

• The textbook refers to these interrupts as “key wakeups.”

• None of the other I/O ports (A, B, E, K, M, S, T) can generate interrupts.

• But two of the HCS12’s other pins, named XIRQ and IRQ, can also generate interrupts. (IRQ stands for Interrupt Request.)

Other External Interrupts

Review: CCR Bits X and I

• Recall from above that IRQ interrupts, like almost all other interrupts, are blocked if the Condition Code Register’s I bit is equal to 1.

• And XIRQ interrupts are blocked if the Condition Code Register’s X bit is equal to 1.

• The XIRQ and IRQ pins are active-low. They’re normally held high, and they generate an interrupt (assuming they’re not masked by the X or I bits in the CCR) when they are pulled low.

XIRQ and IRQ Pins Are Active-Low

• That’s why the pin diagram (on page 52 of the Device User Guide) calls them and , and why our book calls them *XIRQ and *IRQ.

• Note also that pin 56 serves double-duty as both the pin and as Port E’s bit 0.

• Also, pin 55 serves double-duty as both the pin and as Port E’s bit 1.

Pins Are Shared with Port E

• So if you want to use XIRQ and IRQ interrupts, you can’t also use bits 0 and 1 of Port E for general-purpose I/O.

• We’ve ben looking at ways in which external events can interrupt the HCS12.

• As we’ll see in the weeks ahead, many of the functional blocks within the HCS12 chip itself can also generate interrupts. This lets these blocks force the CPU to temporarily suspend execution of its current program and service their needs.

Internal Interrupts