8051 and a C switch statement

On Sun, 30 Dec 2007 19:18:32 +0200, in comp.arch.embedded Anton
Erasmus <nobody@spam.prevent.net> wrote:

>On Fri, 28 Dec 2007 23:39:04 +0100, Martin Griffith
><mart_in_medina@ya___.es> wrote:
>
>>Got  a 4k8 serial biphase signal being decoded by a at85c52 with an
>>external non retrig monostable (hc221), and I'm running out of time
>>occasionally waiting for the generic 2*8  LCD to clear it's busy flag.
>>The LCD is fed with the info from the incoming biphase 
>>
>>To reduce the "reaction time" I could use  a long switch statement on
>>the signal's clock interrupt bitcount from the sync word, 80 per
>>frame, do just a little bit each interrupt.
>>
>>The real question is
>>
>>" how big can I make the switch statement?"
>>Whats the limit?
>>I'm using the rasionance C compiler.
>
>There can be another problem, depending on your compiler.
>The older IAR C51 Compilers generated thread unsafe code for
>a switch statement for more than about 3 items, hence if one used
>big switch statements inside an interrupt routine and outside, the
>code crashed. This was for IAR C51 V4 and V5. I do not know whether 
>this has been fixed, or if an option had been added to force the
>compiler to generate thread safe code. 
>
>You can always split a big switch statement into multiple smaller
>ones. This is a bit slower than one big one.
>
>i.e.
>  switch (var)	/* Check bottom bits */
>  {
>     case 0:
>     break;
>
>
>     case maxsize:
>     break;
>     default:
>     switch(var)
>     {
>         case maxsize+1:
>          break;
>     }
>
>
>  }
>
>
>Regards
>  Anton Erasmus
Ah, there was I happily writing a 16 case switch.

Generally speaking, should you keep code in a case statement short?

I was thinking that it should be like a interrupt, short and sweet

case 7:
{
temp=temp<<1;
temp=temp&0xff;
*LCDdata=temp+47;
break;
}

sort of thing

Sorry if the question seems a bit 101ish, but they didn't have
computers for humans when I was a lad.


martin

On Sun, 30 Dec 2007 18:45:20 +0100, Martin Griffith
<mart_in_medina@ya___.es> wrote:

>On Sun, 30 Dec 2007 19:18:32 +0200, in comp.arch.embedded Anton
>Erasmus <nobody@spam.prevent.net> wrote:
>
>>On Fri, 28 Dec 2007 23:39:04 +0100, Martin Griffith
>><mart_in_medina@ya___.es> wrote:
>>
>>>Got  a 4k8 serial biphase signal being decoded by a at85c52 with an
>>>external non retrig monostable (hc221), and I'm running out of time
>>>occasionally waiting for the generic 2*8  LCD to clear it's busy flag.
>>>The LCD is fed with the info from the incoming biphase 
>>>
>>>To reduce the "reaction time" I could use  a long switch statement on
>>>the signal's clock interrupt bitcount from the sync word, 80 per
>>>frame, do just a little bit each interrupt.
>>>
>>>The real question is
>>>
>>>" how big can I make the switch statement?"
>>>Whats the limit?
>>>I'm using the rasionance C compiler.
>>
>>There can be another problem, depending on your compiler.
>>The older IAR C51 Compilers generated thread unsafe code for
>>a switch statement for more than about 3 items, hence if one used
>>big switch statements inside an interrupt routine and outside, the
>>code crashed. This was for IAR C51 V4 and V5. I do not know whether 
>>this has been fixed, or if an option had been added to force the
>>compiler to generate thread safe code. 
>>
>>You can always split a big switch statement into multiple smaller
>>ones. This is a bit slower than one big one.
>>
>>i.e.
>>  switch (var)	/* Check bottom bits */
>>  {
>>     case 0:
>>     break;
>>
>>
>>     case maxsize:
>>     break;
>>     default:
>>     switch(var)
>>     {
>>         case maxsize+1:
>>          break;
>>     }
>>
>>
>>  }
>>
>>
>>Regards
>>  Anton Erasmus
>Ah, there was I happily writing a 16 case switch.
>
>Generally speaking, should you keep code in a case statement short?

If it is in an interrupt routine, then it normally should be short,
not because it is in a switch statement, but because it is executed
in an interrupt routine.
I personally keep the segments in a switch statement reasonably short.
I find that the code becomes difficult to maintain if one has huge
sections in each case statement, especially if it is a switch
statement with many cases. Also the switch statement is normally
for decision making, while the contents of the case blocks are for
controlling/action. I find it good practice to split these concepts.

[snipped]

Regards
  Anton Erasmus