General debugging methods


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Most of these debugging techniques can be applied to any microcontroller since they do not use any specific tools.

There are many ways to debug hardware:

  • ICE.
  • ICD.
  • Simulation.
  • Serial RS232.
  • LCD.
  • LED.
  • Hardware pins.
  • Logic Analyzer.

ICE

The In Circuit Emulator is the most expensive way to debug your hardware. You buy a special processor that physically takes the place of the normal processor. This special processor allows software access to the internal operation of the processor (you can set breakpoints on hardware modules).

ICD (PIC microcontroller)

The next best thing to ICE is ICD (In Circuit Debug) Sometimes known as (BDM - Background Debug Mode - for non PIC microcontrollers) let you single step through code actually running in the target processor.

For ICD the processor has a small amount of built in hardware that can halt the processor when the program reaches a specific address. The software can then read back all the registers and processor state. 

Simulation

With a source code simulator you can step through the high level language code and see its effect on memory and variables without having to look at the assembler code directly. This lets you focus on the high level language operation and lets you concentrate on the problem you are trying to solve.

One great advantage of the simulator is that you do not have to wait to download and program the target processor (you do have to re-compile the code after changing its source code though). So you can cut out the time consuming programming task just by using the simulator.

Note: Simulators are no good for debugging interrupts.

Debugging using hardware

Serial RS232

Newer microcontrollers have a built in UART giving you a virtually free debug tool that uses minimal resources and needs very little software coding.

For debug output you need to connect the UART output pin (TX) to a suitable level translator circuit e.g. a MAX232 chip. You may even get away with direct connection (via a resistor) to the input of your PC serial port (but this depends on the specifics of your PC hardware) - using a translator chip will always work.

Advantages

  • Minimal coding.
  • Simple to use.
  • Minimal extra hardware.

Disadvantages

  • Takes a long time to output a character (~1ms).
  • Takes even longer for blocks of characters (~10s of ms).
  • Needs extra hardware.

Even through it takes time to output a character it is a useful debug tool as you can output the value of a variable to see what the microcontroller is really doing.

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LCD

An LCD (Liquid Crystal Display) gives a convenient way of displaying debugging information. It is also useful for many different applications that need a text display output.

It is a module that displays text characters and a common screen size is 2 rows of 16 characters.

Most LCD modules use the HD44780 controller chip which is why LCD routines built into high level languages always work.

Advantages

  • Very quick update (40us 4 bit data bus).
  • Useful in many projects as the main display interface.
  • Simple to interface to an 8 bit port (only needs six of the 8 bits).

Disadvantages

  • Uses up an 8 bit port.
  • Hardware is more expensive (e.g. compared to a serial port chip).

LED

Using an LED as a microcontroller 'alive' indicator.

Even though it is such a simple thing to blink an LED on and off it is extremely useful as a debugging tool as you can tell at a glance whether the code you just downloaded is working.

Sometimes you can will incorrectly set parameters on the programming software or compiler which will stop the code dead.

The LED indicator gives a quick health check for your microcontroller which is easy to see.

Pin Debugging

This is the simplest and crudest debugging method; using any available port pin. Simply set or reset this pin at any point in the code that you want to monitor.

It has minimal impact on the code speed or size and can give you the following information:

  • You can tell if the code is active.
  • It gives you the repetition rate.
  • It gives you the routine time length (if you set the pin at the start and reset it at the end).

Note: To do testing you need an oscilloscope or a frequency counter and time interval measuring tool.

Logic Analyzer

This tool attaches to the pins you want to observe and captures the waveforms displaying multiple traces on a single display. It uses a trigger module that can be set to activate on combinations of the input signals or on their length. So you can trigger on specific patterns or on glitches or both.

For non-microcontroller based systems (e.g. 80486 based) where the data and address bus are exposed a logic analyzer can show the address and data organized into hex words i.e. readable. Some can disassemble the instructions showing what the processor was doing at the trigger point.

For a microcontroller based system the logic analyzer can be useful in examining peripheral operation e.g. for debugging the SPI or I2C busses some logic analyzers also have built in support for these protocols.

Another use for the logic analyzer is to capture output over a long period of time depending on the memory capacity of the logic analyzer.

Summary

In short there are many techniques to debug your hardware ranging from simple (an output pin) to complex (a logic analyzer). All of them can be useful depending on the problem you want to solve.

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Copyright © John Main 2006
Free to distribute if the article is kept complete.

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