Friday, January 16, 2009

Dirty little bugs - a pin/macro approach

Program testing can be used to show the presence of bugs, but never to show their absence!

Edsger W. Dijkstra


Measuring and observing usually (if not always) change what one is measuring. I've never seen Schrödinger's cat, at least not when leading with embedded systems nor in my garden, but the former implication causes many problems when trying to debug time-critical functions/operations in embedded software and hardware.

Usually a printf-like UART output works but that would slow down or even hang code which can't wait for the UART to stream its data out. Even though with a substantial buffer it will come a time where it will get stuck.

There is a nice, well known techinque, that uses output pins to reflect some state, such as which function or interrupt handler the microcontroller is doing at the time. Since setting the output value requires only a few instructions, this approach suits better the time-critical scenarios. This technique is helpful in many ways: one may measure from interrupt latency and cpu usage to interrupt or thread deadlock.

That's how the following code came up to mind, here is a simple header file which is based on the macros I posted some time ago here.

#ifndef _dbgpin_h_
#define _dbgpin_h_

#include "portutil.h"

/* Enables or disables debugging */

/* Debug pins */
#define DBGPIN_01 0,2
#define DBGPIN_02 0,3
//---- etc

/* Initialization */
#define DBGPIN_INIT( pin ) do { FPIN_AS_OUTPUT(pin); FPIN_CLR(pin); } while(0);

* Beware of returns, exclude them!
#define DBGPIN_BLOCK( pin, block ) \
FPIN_SET_( pin ); \
do { \
block \
} while(0); \
FPIN_CLR_( pin );
#define DBGPIN_BLOCK( pin, block ) block

#endif /* _dbgpin_h_ */

Here is an example:

a = b;

There is a small drawback: the do-while structure won't let any variable declared inside 'block' to be visible outside. That can be solved by removing the do-while, if you need it that way just go ahead and delete those two lines.

Also notice that a return in 'block' won't allow the macro to clear the corresponding pin, so applying the macro to a function with many return points is messy. The alternative is to change the original function's name and call it through a sort of wrapper which does it inside a DBGPIN_BLOCK macro. It's not pure beauty but it works nice, and once you've done the debugging it can be disabled by changing DBGPIN_ENABLED to 0.

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