Optimization: If vs CASE, which one?

Altera_Forum · ‎05-23-2014

Hello people,

I was recently reading the ALTERA synthesis manual and I saw that the IF statement is more resource demanding than the CASE statement. In cases where we use a lone IF as in the following: (reseting a counter)


...........
SIGNAL DATAREF : SIGNED(15 DOWNTO 0);
.........
IF(DATAREF >= TO_SIGNED(1000,16)) THEN
   DATAREF<= TO_SIGNED(0,16);
END IF;

Then, does it matter if we use an IF or a CASE statement?

Thanks in advance

Altera_Forum · ‎05-23-2014

I would challenge you to use a case statement in that code.

It really referes to muxes. an if/elsif/else tree builds a priority encoder, whereas a similar casestatement will build a simple mux. But synthesisors are getting better now, so they can work out when a load of if/elseifs are mutually exclusive and therefore build a mux instead of a priority encoder.

But unless you really care about logic usage (you probably dont with modern devices - ram and DSP usages are usually the problem areas) you should write readible code, rather than code that is really bady written but saves you a couple of LUTs.

Altera_Forum · ‎05-23-2014

Hey! Thanks for the answer!

The thing is that this block of code is not the only one. For example I have a protection block that takes some 32 bit values from an ADC and checks each one individually for values over a limit. So I use maybe 10 or 12 IF statements with comparison for 32 bit values. How do I optimize that?

IF(VOLTAGE >= TO_SIGNED(2400,32)) THEN

ERROR<= ERROR OR X"0A"

END IF;

IF(CURRENT >= TO_SIGNED(1400,32)) THEN

ERROR<= ERROR OR X"10"

END IF;

and so on..

Thanks

Altera_Forum · ‎05-23-2014

I have seen somewhere that I should save the result of the comparison to a signal and then make the IF or CASE check. So as pipeling the comparison with the decision. Is it the way to go?

Altera_Forum · ‎05-23-2014

Thess are parallel statements, no priority involved if the results are ORed. A case construct won't represent the function.

If pipelining is necessary depends on the timing constraints, you should try.