Finite Impulse Response Filters - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 172

mulate in A8. One can save the content of AMR at the end of processing one buffer
and restore it before using it again with a pair of MVC instructions: MVC AMR,Bx
and MVC Bx,AMR using a B register.
Build and run this project as FIRcirc . Verify an FIR bandpass filter centered
at 1750 Hz. Halt, and Restart the program.
Place a breakpoint within the ASM function FIRcircfunc.asm at the branch
instruction to return to the calling C function ( B B3 ). View memory at the address
delays and verify that this buffer of size 256 is initialized to zero. Right-click on
the memory window to toggle “Float in Main Window” (for a better display). Run
the program. Execution stops at the breakpoint. Verify that the newest sample (16
bits) is stored at the end (higher address) of the buffer (at 0x3FE and 0x3FF ).
Memory location 0x400 (in A9) contains the address 0x301 , where the subsequent
new sample is to be stored. This address represents the starting address of the buffer.
View the core registers and verify that A7 contains this address.
Run the project again and observe the new sample stored at the beginning of the
buffer. This 16-bit data sample is stored at 0x300 and 0x301 . Animate now and
observe where each new sample is being stored in memory. Note that A7 is incre-
mented to 0x303 , 0x305 ,...The circular method of updating the delays is more
efficient. It is important that the buffer is aligned on a boundary with a power of 2.
While a buffer may be “naturally aligned,” one must make sure that it is (an address
with LSBs as zeros) if such buffer is to be used as circular.
Example 4.14: FIR Implementation Using C Calling an ASM Function
Using a Circular Buffer in External Memory ( FIRcirc_ext )
This example implements an FIR filter using a circular buffer in external memory.
The same C source program FIRcirc.c and ASM function FIRcircfunc.asm
as in the previous example are used, but with a modified linker command file.
This linker command file FIRcirc_ext.cmd is listed in Figure 4.37. The section
circdata designates the memory section buffer_ext , which starts in external
memory at 0x80000000 .
Build this project as FIRcirc_ext . Load the executable file and view the
memory at the address delays . This should display the external memory section
that starts at 0x80000000 . Verify that the circular buffer is in external memory,
where all the delay samples are initialized to zero. Place a breakpoint as in Example
4.13, run the program up to the breakpoint, and verify that the newest input sample
is stored at the end of the circular buffer at 0x800000FE and 0x800000FF . Reg-
ister A9 contains the last address, and register A7 contains the address where the
subsequent 16-bit input sample is to be stored ( 0x80000001 ). Run the program
again (to the set breakpoint) and verify that the subsequent acquired sample is
stored at the beginning of the buffer at the address 0x80000001 . Remove the
breakpoint, Restart/run, and verify that the output is the same FIR bandpass filter
centered at 1750 Hz, as in Example 4.13.