Motorola DSP Developer's Kit

decimate-fir-c.asm

Y = mot###_decimate( X, r, nfilt, 'fir' )

Description

Function decimate resamples data at a lower rate after lowpass FIR filtering. Input vector X is a complex vector

Input/Output

Input Parameters: complex Vector X, int r, int nfilt

Output Parameters: complex Vector Y

Algorithm

The MEX function decimate_fir.m calculates vector b, vector list, int nout, and int nbeg,. The CMEX function loads these parameters into dsp memory for asm function use.

nd = length(idata);
m = size(idata,1);
nout = ceil(nd/r);
b = fir1(nfilt,1/r);
gd = grpdelay(b,1,8);
list = round(gd(1)+1.25):r:nd;
lod = length(list);
nlen = nout-lod;
nbeg = r-(nd-list(length(list)));
Assembly function decimate-fir-c.asm, and then follow these steps 
to calculate decimated vector Y:
nfilt = nfilt+1
itemp = 2*idata(1)-idata((nfilt+1):-1:2)
[odata,zf] = filter(b,1,idata,zi)
odata = odata(list)

Memory & Register

Memory allocation:

• Input vector X is located in X memory
• Input r is loaded in register N3
• Input nfilt is loaded in register N5
• Output vector Y is located in Y memory

In X memory:

• X:(#INA) stores the vector idata

(In the idata area, the first half stores the real part, the second half stores the imaginary part)

• X:(#INA+(length of idata)) stores vector b

(In the b area, the first half stores the real part, and the second half stores the imaginary part)

• X:(#INA+(length of idata)+(length of b)) stores the vector zi/zf

(In the zi/zf area, the first half stores the real part, and the second half stores the imaginary part)

• X:(#INA+(length of idata)+(length of b)+(length of zi)) stores nout
• X:(#INA+(length of idata)+(length of b)+(length of zi)+1) stores the length of list
• X:(#INA+(length of idata)+(length of b)+(length of zi)+2) stores the vector list
• X:(#INA-1) stores the length of idata
• X:(#INA-2) stores r
• X:(#INA-3) stores nfilt
• X:(#INA-4) stores the length of vector b, nfilt+1
• X:(#INA-5) stores the length of decimated output vector, nout
• X:(#INA-6) stores the length of list, lod
• X:(#INA-7) stores nbeg
• X:(#INA-8) stores nlen = nout-lod

In Y memory:

• Y:(#INB) stores the vector odata

(In the odata area, the first half stores the real part, and the second half stores the imaginary part)

• Y:(#INB+2*(length of odata)) stores the vector itemp

length of itemp is 2*2*(nfilt+1)

Input data length:

Length of idata is 2*

Length of odata is 2*

Length of b is nfilt+1

Length of zi, zf is nfilt+1

Length of list is lod

Length of itemp is 2*2*(nfilt+1)

Register usage:

• N2 stores the length of idata (n)
• N3 stores r
• N5 stores nfilt
• R2 stores the length of vector b (nfilt+1)

Status Register

No status registers or bits are set explicitly during the function execution.

Data Size Limit

The input vector X length must longer than r*(nfilt+1). The length of vector X can't be larger than the continuous available data memory size.

Data Range Limit

The input vector X range is [-1.0, +1.0].

Precision

In the case of DSP563, precision is 21 bits.

In the case of DSP566, precision is 12 bits.

decimate-fir-r.asm

decimate-iir-r.asm