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All,
I am trying to track down a problem of some code that was written by an 'overseas' 3rd party (I will be nice and not state the country of origin).
This code uses a single timer set at a 1mS interrupt rate in order to determine if the SPI is still communicating externally with it's master. If no communications are detected the timer resets the SPI port, clears the interrupt, and jumps to the reset vector. The problem is: the SPI port remains dead until a power cycle is accomplished.
The obvious fix is to use the watchdog (which is what I will eventually do), but I would like to understand the why of why this does not work (yes, bad coding practice is the real reason)...
Since the code jumps to the reset vector this is what I have been able to analyze:
(1) Since this is not a true reset (ie: via watchdog) all hardware registers are not reset - problem potential here. (2) The jump to the reset vector was accomplished while in supervisor mode, so the privleged registers (ie: SP,etc) can be written. (3) The timer interrupt was cleared prior to making the jump to the reset vector, so all interrupts are still enabled. (4) Since this is not a true reset, all resident code can still execute (ie: interrupt handlers). (5) The startup code will reset all initialized data, registers, etc prior to jumping to program main(), effectively returning data to a power up state.
One reason I can currently come up with as to why the SPI is never functional after this occurs is that maybe an interrupt occurs while in the startup code (clearing a tracking variable or resetting the processor registers). But the interrupt would also inhibit the startup code until it was serviced. This potential cause is (probably) not the only reason for this issue, and why I am asking for your input(s).
Unfortunately, this board has no JTAG to connect so stepping through the code is not an option. I could write to the serial port - if it was connected, but it isnt. Right now I am trying to analyze my way through this code before using a 'hammer' approach to solving this problem.
What else am I missing in this analysis? Thanks.
The compiler is fully allowed to move any and all writes to buffer below the write to buffer_ready.
That kind of interpretation makes it diffult to ascribe meaning to the word 'previous':
"At certain specified points in the execution sequence called sequence points, all side effects of previous evaluations shall be complete"
That kind of interpretation makes it diffult to ascribe meaning to the word 'previous': [...]
Well yes, that's the kind of difficulty that will result from ripping a single sentence out of a text written in densely worded Legalese, and trying to take the result strictly at face value.
If your referring to this line posted by Jack - it is part of the Utah CS paper - not the spec
Well, ok, here's the complete paragraph from the standard:
Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment. Evaluation of an expression may produce side effects. At certain specified points in the execution sequence called sequence points, all side effects of previous evaluations shall be complete and no side effects of subsequent evaluations shall have taken place.
Now, consider this:
buffer_ready=1;
The code above modifies an object and therefore generates a side effect. The for loop contains several sequence points. If the position of 'buffer_ready=1' is moved relative to the for loop the condition that previous evaluation side effects must be complete and subsequent evaluation side effects must not have happened is breached.
Ridiculous. But makes the point nonetheless:
volatile int launch_missle; char default_coordinates[BUF_SIZE]; void LoadNLaunch() { int i; for (i=0; i<BUF_SIZE; i++) default_coordinates[i] = 0; launch_missle = 1; }