NXP Semiconductors MPC5644A, Reference Manual, Page: 1226 / 1735

Decimation Filter
MPC5644A Microcontroller Reference Manual, Rev. 6
1226 Freescale Semiconductor
Comparing Equation 26-3 with Equation 26-4 , we obtain the relationship between the calculated values of
coefficients and the values to be loaded in the DECFILTER_COEFn registers. See Table 26-33 below to
obtain the coefficients. A scale factor of eight is used, being the smallest divider factor to have all
coefficient values in the range (–1 £ Coef < +1). Also note that the signal of the An coefficients has signals
inverted.
26.8.2 Input data calculation
The 24-bit words of input data are samples of a sum of two tones: one tone of 30 kHz and another tone of
250 kHz. The samples were calculated at the rate of 800k samples per second. The tones have the same
amplitude and it is assured that the resultant amplitude is smaller than 1 so as to obtain samples in the range
(–1

sample < +1). It is supposed the input data are signed values in the two’s complement format in the
range (–1

sample < +1).
Table 26-32. Coefficient values given by SPW digital filter design tool
Coefficient Decimal value Coefficient Decimal value
B0 0.0221455 A0 1.0
B1 0.00445582948893748 A1 -2.69772868375858
B2 0.0318517846509088 A2 3.234056294853
B3 0.00445582948893748 A3 -1.92028561712454
B4 0.0221455 A4 0.47939080709495
Table 26-33. Coefficient values for decimation filter
SCALE S = 1 S = 8
COEFn Decimal value Decimal value
Hexadecimal values
(24 bits)
Coef0 = B0/S 0.0221455 0.00276815891266 0x005AB5
Coef1 = B1/S 0.00445582948893748 0.00055694580078 0x001240
Coef2 = B2/S 0.0318517846509088 0.00398147106171 0x008277
Coef3 = B3/S 0.00445582948893748 0.00055694580078 0x001240
Coef4 = B4/S 0.0221455 0.00276815891266 0x005AB5
Coef5 = –A1/S 2.69772868375858 0.33721613883972 0x2B29E6
Coef6 = –A2/S –3.234056294853 –0.40425717830658 0xCC414E
Coef7 = –A3/S 1.92028561712454 0.24003565311432 0x1EB97D
Coef8 = –A4/S –0.47939080709495 –0.05992400646210 0xF8546A