AN183 - Cirrus Logic, Inc.

AN183

2AN183REV1

1. INTRODUCTION

This application note explains how to install a

interrupt service routine (ISR) in the Angel

debugging environment. This application note

assumes you are using a Cirrus Logic 72xx

development board that is running Angel,

Version 1.20, and the ARM® Tools, Version 2.5.

The ARM project file (LED_ISR.APJ) and two

source files (ISRSHELL.S and LED_ISR.C) are

the example files for this application note. Treat the

source code in the example files as an important

reference source for understanding the concepts

presented in this application note.

2. BACKGROUND

There are two traditional methods for installing an

interrupt service routine:

nApplication software installs the ISR

nUse Angel Debugger to Install ISR

2.1 Method One: Application Software

Implementation of ISR

The first method assumes that the interrupt vectors

reside in RAM starting at location 0x0. In this

case, the application software saves off the existing

interrupt vector and replaces it with a branch to the

new ISR. The new ISR then exits by chaining to the

previous ISR. This approach is explained in detail

in the C hapter 9 of the ARM Software Development

Toolkit User Guide.

This method is useful when Angel is compiled to

run entirely in upper memory and RAM exists at

the bottom of memory where the exce ption vectors

reside (including IRQ). This method can also be

used if you are using a JTAG-based debugger (such

as Multi-ICE™, JEENI™, or Wiggler) and have

programmed the Memory Management Unit

(MMU) to map DRAM to the first 16M bytes of

memory (see the application note, AN177, Using

JTAG for Debugging EP72xx Microcontrollers).

2.2 Method Two: Use Angel Functionality

The Angel debug monitor resides in ROM

(FLASH) memory starting at 0x0. Angel is used to

call a function to install, save, and restore the

existing ISR vectors.

These functions are typical of a BIOS or an

operating system, and are also found in target

debuggers such as Angel. The Angel debugger

contains two special SWI (software interrupt) calls

that allow for the installation of an ISR. This is

done using the Angel semi-hosting SWI (by default

this is 0x123456 in ARM state).

3. SOFTWARE

The example program installs an interrupt that

flashes the heartbeat LED once a second. It also

uses the printf semi-hosting functions of Angel to

print the status to the console screen. The C code

defines two special Angel SWI’s:

nAngel_GetIRQ, which gets the existing inter-

rupt vector

nAngel_SetIRQ, which installs the location for

the new ISR

The procedure for installing the ISR follows these

steps:

1) Get the old vector using A ngel_GetIREQ, and

stash it away in a global location.

2) Install the ISR routine vector by calling

Angel_SetIRQ.

3) Program the peripherals and the interrupt mask

registers.

4. SAMPLE EXERCISE

Change the interrupt rate from 1 second to 10ms.

(Hint: See source code comments.) Then change

the program so that the LED blinks at 1 second

intervals. (Hint: you will need to modify the

interrupt routine to blink only on every 100th

occurrence.)