FIR filter in 8051 micro

Hi all,
           I have a set of samples for which I want to apply FIR
filtering (assuming that filtering will be good). I know that digital
filtering will give near precise filtering but I have little knowledge
in FIR/IIR and I do not know where to apply which filter(yes, I
googled ) . I need a small code segment that will do FIR filtering for
my samples . I am using an 8051 microcontroller .
Thanks in advance.

romeshkulasekhara@gmail.com wrote:
> Hi all,
>            I have a set of samples for which I want to apply FIR
> filtering (assuming that filtering will be good). I know that digital
> filtering will give near precise filtering but I have little knowledge
> in FIR/IIR and I do not know where to apply which filter(yes, I
> googled ) . I need a small code segment that will do FIR filtering for
> my samples . I am using an 8051 microcontroller .
> Thanks in advance.

You need to learn how to do triple or quadruple precision math in the 
8051. You can read the book at http://www.dspguide.com/ and the online 
courses at http://www.bores.com/courses/intro/index.htm do begin 
addressing your needs.

The architecture of the 8051 makes processing filters on it very slow. 
There are some modern variants that have facilities that can speed it up 
a bit, but it would not be my processor of choice. Even if some of the 
variants have a multiplication op code, it will be single precision and 
that's not enough for most filters.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

>I need a small code segment that will do FIR filtering for
>my samples . I am using an 8051 microcontroller .

Hello,

the link below is an online filter calculator that produces C code for the
designed filter. I doubt that you want to compile it for the 8051, but it
produces some working code to start with.

Cheers

Markus

http://www-users.cs.york.ac.uk/~fisher/mkfilter/

>I need a small code segment that will do FIR filtering for
>my samples . I am using an 8051 microcontroller .

Hello,

the link below is an online filter calculator that produces C code for the
designed filter. I doubt that you want to compile it for the 8051, but it
produces some working code to start with.

Cheers

Markus

http://www-users.cs.york.ac.uk/~fisher/mkfilter/

mnentwig wrote:
>> I need a small code segment that will do FIR filtering for
>> my samples . I am using an 8051 microcontroller .
> 
> Hello,
> 
> the link below is an online filter calculator that produces C code for the
> designed filter. I doubt that you want to compile it for the 8051, but it
> produces some working code to start with.
> 
> Cheers
> 
> Markus
> 
> http://www-users.cs.york.ac.uk/~fisher/mkfilter/
> 
He said he wants an FIR. The only FIRs that page can generate are 
Hilbert transforms.

Steve

Well, you're right, and raised cosine.
There's probably plenty of FIR example code out there anyway.

-Markus

mnentwig wrote:
> Well, you're right, and raised cosine.
> There's probably plenty of FIR example code out there anyway.

I told him it probably won't fit, either in the time he has (which 
wasn't stated) or in the available space. The crippled machine has one 
data pointer. There's an increment instruction for it, but not a 
decrement. The Harvard architecture makes it difficult to fetch 
coefficients from the program ROM, so one way or another coefficients 
need to be stored in the RAM segment. Circular buffers can be kludged 
up. I remember building a pair for interrupt-driven serial 
communication. (There's a bug in the UART timing, so if 9 bits are 
needed, you have to do the timing "by hand". For 8 bits, you tell it 
you're sending 9.)

The only reason to use that machine is price. If you're too cheap to 
spring for the difference between it and, say, a 68HC11 or -12, you get 
what you deserve. If I needed to do any low-octane DSP, I'd at least use 
a 68HC16. Nowadays, there's better.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

romeshkulasekhara@gmail.com wrote:

> I have a set of samples for which I want to apply FIR
> filtering (assuming that filtering will be good). I know that digital
> filtering will give near precise filtering but I have little knowledge
> in FIR/IIR and I do not know where to apply which filter(yes, I
> googled ) . I need a small code segment that will do FIR filtering for
> my samples . I am using an 8051 microcontroller .


It is hard to imagine more unsuitable architecture for the FIR filter 
then x51. I doubt if there is any application for which this filter 
could be useful.

Perhaps, you should look at CIC or moving average filters.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Jerry Avins wrote:

> I told him it probably won't fit, either in the time he has (which 
> wasn't stated) or in the available space. The crippled machine has one 
> data pointer. There's an increment instruction for it, but not a 
> decrement. The Harvard architecture makes it difficult to fetch 
> coefficients from the program ROM, so one way or another coefficients 
> need to be stored in the RAM segment. Circular buffers can be kludged 
> up.

Oh, come on :)

movc A, @A + DPTR
mov  B, A
mov  A, @R0
mul AB
....
inc DPTR
inc R0
mov A, R0
anl A, #buffer
mov R0, A
....


> I remember building a pair for interrupt-driven serial 
> communication. (There's a bug in the UART timing, so if 9 bits are 
> needed, you have to do the timing "by hand". For 8 bits, you tell it 
> you're sending 9.)
> 
> The only reason to use that machine is price. If you're too cheap to 
> spring for the difference between it and, say, a 68HC11 or -12, you get 
> what you deserve. If I needed to do any low-octane DSP, I'd at least use 
> a 68HC16. Nowadays, there's better.
> 
> Jerry

First, filtering using an embedded 8-bit micro is possible. Ignore people
who are talking double precision, etc, etc....they aren't embedded people
with experience in this area. I have invented an algorithm that works well
on a PIC (8-bit also). I call it the "tri-band filter". I had a situation
where I had to generate a measure of the power spectrum in 3 bands in an
audio spectrum of a piano (about 4.5KHz). So, I took 32 samples at a time
(I was limted by memory) and ran a FIR filter on the data. I basically
chose a FIR filter of the form ax0+bx1+cx2 and centered it at the midpoint
of my frequency range. My sample rate was twice the highest frequency (by
Nyquist).
Now, I used some online filter coefficient generator. What I noticed is
the LPF coefficients were the same as the HPF coefficients just swapping
signs here and there, so as long as I calculated ax0, bx1 and cx2, I could
use the same calculations for both filters. The second trick I used was to
round any floating point values to the nearest powers of two. For example,
one term was 0.3664, and I rounded it to 0.375 (3/8*x), which can be
calculated as ((x << 2) + x) >> 3, which involves no multiplies (my PIC
didn't have a multiply) and only involves integer calculations. Note that
the frequency response of the filter (if you choose your coefficients
carefully) was almost identical to the non-altered one and my results were
very accurate (much better than my older state variable filter that I used
to use). If you combine the touching of coords to be combinations of power
of two so you can do your calculations using shifts and adds and reuse
coefficients to give you both High and Low pass results simultaneously. The
band pass was obtained by subtracting the LPF and HPF values from the
original data. You will need to use a 16 bit accumulator (assuming you are
using 8-bit sampled data) and remember to shift left and add BEFORE
shifting right as you'll get more accurate results that way.
This algorithm works very well for 8-bit processors.
Enjoy...