How to handle varied length of output signal

I am doing compression algorithm using VHDL. I need to output different
length of codes according to different intermediate computation results.
In this way, I compress the original information.

For example, if intermediate computation result is less than certain
threshold, I will output 5 bits, but if it is greater than the
threshold, I only need to output 3 bits.

How could I handle the definition of the output signal?
I thougth about using 2 'Z's to concatenate with 3 bits. It didn't work,
turned my output to "ZX". Sad

Any suggestions or hints? TIA

systolic

systolic wrote:
>
> I am doing compression algorithm using VHDL. I need to output different
> length of codes according to different intermediate computation results.
> In this way, I compress the original information.
>
> For example, if intermediate computation result is less than certain
> threshold, I will output 5 bits, but if it is greater than the
> threshold, I only need to output 3 bits.
>
> How could I handle the definition of the output signal?
> I thougth about using 2 'Z's to concatenate with 3 bits. It didn't work,
> turned my output to "ZX". Sad
>
> Any suggestions or hints? TIA

I am curios about how you will use this result with dynamically changing
size. You can compress the data, but a compression algorithm can't
change the number of wires in hardware. You still need enough wires to
carry the maximum width signal.

Think about what you are doing in terms of the hardware you will be
building. VHDL is not a programming language, it is a Hardware
Description Language V*HDL*.

Hint: just set the unused bits to zeros...

--

Rick "rickman" Collins


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rickman

On Thu, 2004-10-21 at 01:50 -0500, systolic wrote:
> I am doing compression algorithm using VHDL. I need to output different
> length of codes according to different intermediate computation results.
> In this way, I compress the original information.
>
> For example, if intermediate computation result is less than certain
> threshold, I will output 5 bits, but if it is greater than the
> threshold, I only need to output 3 bits.
>
> How could I handle the definition of the output signal?
> I thougth about using 2 'Z's to concatenate with 3 bits. It didn't work,
> turned my output to "ZX". Sad
>
> Any suggestions or hints? TIA
>

It kind of depends on how you want to output transmitted. If you are
transmitting the results one at a time and they are self delimitting,
set the other bits to zero or one. Otherwise transmit an additional
signal of 3 or 4 bits indicating how long the result is in bits.

If you want to pack the results into a fixed size packet (eg byte) then
use a shift register to accumulate the results. When then is more than
a byte worth, you shift a byte out. This is kind of like a fifo.

Prasanth Kumar

Thx, guys,
So, I need to either use two buses with an extra flag bit. Or just add
padding '0' or '1'.
I will take the first choice since I need to have pure compressed code.

systolic