What simple method can I use to debug an embedded processor without serial port or video?

Question

We have a small embedded system without any video or serial ports (i.e. we can't output text via printf).
We would like to track the progress of our code through the initialization sequence.
Is there some simple things we can do to help with this.
It is not running any OS, and the hardware platform is somewhat customizable.

Answer 1

The simplest most scalable solution are state LEDs. Toggle LEDs based on actions, either in binary form or when certain actions occur if you can narrow your focus.

The most powerful will be a hardware JTAG device. You don't even need to set breakpoints - simply being able to stop the application and inspect the state of memory may be enough. Note that some hardware platforms do not support "fancy" options such as memory watches or hardware breakpoints. The former is usually worked around with constantly stopping the processor and reading memory (turns your 10MHz system into a 1kHz system), while the latter is sometimes performed using code replacement (replace the targeted instruction with a different jump), which sometimes masks other problems. Be aware of these issues and which embedded processors they apply to.

Answer 2

There are a few strategies you can employ to help with debugging:

If you have Output Pins available, you can hook them up to LEDs (or an oscilloscope) and toggle the output pins high/low to indicate that certain points have been reached in the code.
For example, 1 blink might be program loaded, 2 blink is foozbar initialized, 3 blink is accepting inputs...

If you have multiple output lines available, you can use a 7 segment LED to convey more information (numbers/letters instead of blinks).

If you have the capabilities to read memory and have some RAM available, you can use the sprint function to do printf-like debugging, but instead of going to a screen/serial port, it is written in memory.