摘要：
中断服务程序将左声道的每个数据送入滤波器，然后从DAC电路输出滤波后的结果。
源：http://blog.sina.com.cn/s/blog_493520900102uy26.html
内容来自于上篇博文，第七章，FIR滤波器
http://blog.sina.com.cn/s/blog_493520900102uxzu.html
使用FDATool工具，构建一个Kaiser窗低通滤波器，采样频率8kHz，生成滤波器系数：
1）例程1
文件 fir_fltcoeff.h，以浮点形式表示的量化后的滤波器系数
//define the FIR filter length
#define N_FIR 40
float h[N_FIR] = { ‐ 0.004314029307,‐ 0.013091321622,‐0.016515087727,
‐0.006430584433, 0.009817876267, 0.010801880238,
‐0.006567413713,‐ 0.016804829623, 0.000653253913,
0.022471280087, 0.010147131468,‐0.025657740989,
‐0.026558960619, 0.023048392854, 0.050385290390,
‐0.009291203588,‐ 0.087918503442,‐0.033770330014,
0.187334796517, 0.401505729848, 0.401505729848,
0.187334796517,‐ 0.033770330014,‐0.087918503442,
‐0.009291203588, 0.050385290390, 0.023048392854,
‐0.026558960619,‐ 0.025657740989, 0.010147131468,
0.022471280087, 0.000653253913,‐0.016804829623,
‐0.006567413713, 0.010801880238, 0.009817876267,
‐0.006430584433,‐ 0.016515087727,‐0.013091321622,
‐0.004314029307 };
实际电路用AIC3106，每次采样中断来临后读入一组数据，分别为左右声道的数据，各16位。中断服务程序将左声道的每个数据送入滤波器，然后从DAC电路输出滤波后的结果。右声道数据不变。
文件 ISRs_Plus.c ，中断服务程序：
// Welch, Wright, & Morrow,
// Real-time Digital Signal Processing, 2011
// Modified by Mark Wickert February 2012 to include GPIO ISR start/stop postings
///////////////////////////////////////////////////////////////////////
// Filename: ISRs_fir_float.c
//
// Synopsis: Interrupt service routine for codec data transmit/receive
// floating point FIR filtering with coefficients in *.h file
//
///////////////////////////////////////////////////////////////////////
#include "DSP_Config.h"
#include "fir_fltcoeff.h" //coefficients in float format
// Function Prototypes
long int rand_int(void);
// Data is received as 2 16-bit words (left/right) packed into one
// 32-bit word. The union allows the data to be accessed as a single
// entity when transferring to and from the serial port, but still be
// able to manipulate the left and right channels independently.
#define LEFT 0
#define RIGHT 1
volatile union {
Uint32 UINT;
Int16 Channel[2];
} CodecDataIn, CodecDataOut;
float x_buffer[N_FIR]; //buffer for delay samples
interrupt void Codec_ISR()
///////////////////////////////////////////////////////////////////////
// Purpose: Codec interface interrupt service routine
//
// Input: None
//
// Returns: Nothing
//
// Calls: CheckForOverrun, ReadCodecData, WriteCodecData
//
// Notes: None
///////////////////////////////////////////////////////////////////////
{
WriteDigitalOutputs(1); // Write to GPIO J15, pin 6; begin ISR timing pulse
int i;
float result = 0; //initialize the accumulator
if(CheckForOverrun()) // overrun error occurred (i.e. halted DSP)
return; // so serial port is reset to recover
CodecDataIn.UINT = ReadCodecData(); // get input data samples
//Work with Left ADC sample
//x_buffer[0] = 0.25 * CodecDataIn.Channel[ LEFT];
//Use the next line to noise test the filter
x_buffer[0] = 0.125*((short) rand_int());//scale input by 1/8
//Filtering using a 32-bit accumulator
for(i=0; i< N_FIR; i++)
{
result += x_buffer[i] * h[i];
}
//Update filter history
for(i=N_FIR-1; i>0; i--)
{
x_buffer[i] = x_buffer[i-1];
}
//Return 16-bit sample to DAC
CodecDataOut.Channel[ LEFT] = (short) result;
// Copy Right input directly to Right output with no filtering
CodecDataOut.Channel[RIGHT] = CodecDataIn.Channel[ RIGHT];
WriteCodecData(CodecDataOut.UINT); // send output data to port
WriteDigitalOutputs(0); // Write to GPIO J15, pin 6; end ISR timing pulse
}
//White noise generator for filter noise testing
long int rand_int(void)
{
static long int a = 100001;
a = (a*125) % 2796203;
return a;
}
可以看出，中断服务程序是逐点响应的，每次来一个数据，都要做一次滤波器运算，并将原有的数据前移一位，将新数据追加在缓冲区末尾。
float x_buffer[N_FIR]; //用于保存现在的新值和过去的数值，长度与滤波器系数相同
x_buffer[0] // 用于保存当前值
x_buffer[1] // 保存过去前一点的值，以此类推
h[0...39] // 滤波器系数
result += x_buffer[i] * h[i]; // 当前点滤波器输出结果，循环从0到39