CD2458 Tutorial

Program Example (Automatic Gain Control)

This example gives an AGC function to keep the output power constant for an input varying -43dB to 0dB. If you do not have enough knowledge about the feed back loop, you would probably think "Let's make something and determine the parameters with experiment.". This example is made for such engineers. (Fig.1)

Let's assume the input signal amplitude changes from 0 to +/- 0.25 in average, and the peak input value is smaller than +/- 0.9. This is just for your easy understanding of this example.

There is unknown signal level that should be amplified to keep the best dynamic range at post process. First, you need to know long term average power of the input signal, then gradually change the gain of the amplifier so that the output from the amplifier keeps necessary average power. The output signal level from the AGC may be set to high level with that the post process will have easier data handling.

How do you get the average power of the input signal? If you have big memory for AGC, you might store every input signal sample squared in the memory, and calculate the average power by adding N pieces of stored data. The oldest sample(s) will be discarded for new sample(s) input after the calculation. If you take large N, the time constant of the AGC may be big, whereas short time constant is expected with small N. This method needs ridiculously big resource consumption: N word memory and N times addition for every computation. You may need to compromise in this part for better cost performance.

Flow Chart

As usual, let's assume a simple "interrupt driven routine" for AGC. The AGC program works only at an interrupt, and the DSP gets into an idle state to wait for next interrupt once the interrupt service routine is executed. During the interrupt service, one new input sample is read and one output datum goes out at the beginning, then the "GAIN control" routine will be executed for next output (input x GAIN) calculation.

The Fig.2 shows a flow chart for the AGC part of interrupt service routine. If you are working on an Audio frequency application, you might have only 22.6uS between interrupts. Assuming 20MHz operation, there is only about 450 cycles of instruction execution time. Although you do not need to run the AGC service routine at every interrupt as a compromise, we'll assume the AGC routine runs at every interrupt just for easier programming.

The GAIN increment/decrement parameter determines the time constant of the AGC in this example program. You can find out your best time constant with an experiment. You should try impulse noise, continuous wave, 0dB -> -50dB, -50dB ->0dB, signals periodically changing its amplitude in various frequency, etc.. Do you see unstable conditions at any of those input forms?

Program List

                    #
                    #
                    #
                    #  AGC  Examples For CD2458
                    #      (c)2002 Clarkspur Design, Inc.
                    #
                    Initialize:
0000 08C0 00FF      LDI SP,'hff
0002 0844 0001 0001 LDLI ST,'h010001
0005 0810 2000      LDI ah,'h2000
0007 5021           OUTD 'h2,ah          # Enable 44.1KHz interrupt
0008 0838 1E00      LDI r3,'h1E00
000A 0938 0100      LDI r7,'h100
000C 54B0           MODF ie3
                    stayhere:
                    #call @INT2service
000D 4C00 000D      BRA @stayhere
                    
                    
                    INT2service:
000F EE4A           TGL st,'ha
0010 4800 0048      CALL  @IO
0012 0021           LD x,ah
0013 0601           LD a,001
0014 0031           LD y,ah
0015 0017           LD ah,ph
0016 4602           LD 002,A
0017 4800 001D      CALL @GAIN
0019 0602           LD A,002
001A 3077           SHA 'h37  #SHA -9
001B 54B0           MODF ie3
001C 015C           RET
                    
                    
                    
                    GAIN:
001D 3046           SHA 6
001E 9050           MOD abs       #Compromized from squared to abs.
                    
001F 9170           SWAP
0020 0600           LD a,000
0021 304F           SHA 'hF
0022 00A1           LD tl,ah
0023 9170           SWAP
                    
0024 4313           LD (r7),ah
0025 16FB           MODR r7+,r3-
0026 4C01 002C      BRA @SK1,nrz
                    
0028 0838 1E00      LDI r3,'h1E00
002A 0938 0100      LDI r7,'h100
                    
                    SK1:
002C 8600           ADD a,000
002D 9020           MOD sat
002E 200A           SUB a,tl
002F 4600           LD 000,a
0030 6801 1000      CMPI a,'h1000,,,fl
                    
0032 4C0F 0043      BRA @SK2,neg
                    
0034 0601           LD a,001
0035 2800 0030      SUBI a,'h0030
                    
                    SK4:
0037 4C07 003B      BRA @SK6,pos
0039 0810 0010      LDI ah,10
                    SK6:
                    
003B 6801 4000      CMPI a,'h4000,,,fl
003D 4C0F 0041      BRA @SK3,neg
                    
                    #LDI ah,'h4000,,,fl
003F 0811           DW 'h0811
0040 4000           DW 'h4000
                    
                    SK3:
0041 4601           LD 001,a
0042 015C           RET
                    
                    SK2:
0043 0601           LD a,001
0044 8800 0030      ADDI a,'h0030
0046 4C00 0037      BRA @SK4
                    
                    
                    IO:
0048 3048           SHA 'h8      #IO codec on LSB 16 bits
0049 5001           OUTD 0,ah
004A 1010           INPD ah,0
004B 3078           SHA 'h38     #SHA -8  IO codec on LSB 16 bits
004C 015C           RET
                    
     FFFC           org 'hfffc
FFFC 0000           dw @Initialize
FFFD 000F           dw @INT2service
FFFE 000F           dw @INT2service
FFFF 000F           dw @INT2service