I am programming on an embedded microcontroller (TMS320F28069) a 32-bit floating point MCU. I was going over some of the example projects, and one of them implements a simple FIR filter on ADC sampled data.
Block diagram here
Let's say the ADC buffer has 10 elements. And let's say the filter has length 3 (FILTER_LEN=3
). The filter implementation is pretty straightforward, it starts at the end of the delay chain and moves its way to the beginning.
float32 ssfir(float32 *x, float32 *a, Uint16 n)
{
Uint16 i; // general purpose
float32 y; // result
float32 *xold; // delay line pointer
/*** Setup the pointers ***/
a = a + (n-1); // a points to last coefficient
x = x + (n-1); // x points to last buffer element
xold = x; // xold points to last buffer element
/*** Last tap has no delay line update ***/
y = (*a--)*(*x--);
/*** Do the other taps from end to beginning ***/
for(i=0; i<n-1; i++)
{
y = y + (*a--)*(*x); // filter tap
*xold-- = *x--; // delay line update
}
/*** Finish up ***/
return(y);
}
Now, here is how the ADC circular buffer is implemented.
//---------------------------------------------------------------------
interrupt void ADCINT1_ISR(void) // PIE1.1 @ 0x000D40 ADCINT1
{
static float32 *AdcBufPtr = AdcBuf; // Pointer to ADC data buffer
static float32 *AdcBufFilteredPtr = AdcBufFiltered; // Pointer to ADC filtered data buffer
PieCtrlRegs.PIEACK.all = PIEACK_GROUP1; // Must acknowledge the PIE group
//--- Manage the ADC registers
AdcRegs.ADCINTFLGCLR.bit.ADCINT1 = 1; // Clear ADCINT1 flag
//--- Read the ADC result:
*AdcBufPtr = ADC_FS_VOLTAGE*(float32)AdcResult.ADCRESULT0;
//--- Call the filter function
xDelay[0] = *AdcBufPtr++; // Add the new entry to the delay chain
*AdcBufFilteredPtr++ = ssfir(xDelay, coeffs, FILTER_LEN);
//--- Brute-force the circular buffer
if( AdcBufPtr == (AdcBuf + ADC_BUF_LEN) )
{
AdcBufPtr = AdcBuf; // Rewind the pointer to the beginning
AdcBufFilteredPtr = AdcBufFiltered; // Rewind the pointer to the beginning
}
}
xDelay
is a float32 array of length FILTER_LEN
initialized with 0s and coeffs
is a float32 array of length FILTER_LEN
initialized with the filter coefficients.
My question is, what is going on here:
//--- Call the filter function
xDelay[0] = *AdcBufPtr++; // Add the new entry to the delay chain
*AdcBufFilteredPtr++ = ssfir(xDelay, coeffs, FILTER_LEN);
How do values ever get stored in xDelay[1]
or xDelay[2]
?
Their implementation seems to work fine, but I'm not following how old data gets pushed back in the xDelay array.