NXP Semiconductors MPC5644A, Reference Manual, Page: 1225 / 1735

Decimation Filter
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor 1225
eQADC can send either conversion data or timestamp data. The conversion data is filtered by the
decimation filter and the timestamp is bypassed and sent back to the eQADC.
In the eQADC application, the TAG field is used to address the appropriate RFIFO in the eQADC block.
In this case, only addresses 0–5 are used since there are only six RFIFOS available in the eQADC block.
Figure 26-24. Decimation filter/eQADC interface
26.8 Filter example simulation
The decimation filter block operation was checked in a Verilog simulation using calculated filter
coefficient values and noisy input data. The expected output values and the RMS error were then
calculated
.
26.8.1 Coefficients calculation
T
he coefficients were calculated using a digital filter design tool. We have supplied some hypothetical filter
parameters to the tool and obtained the filter coefficients. The input parameters are:
• Filter characteristics: elliptic/low pass
• Filter type: 4th order IIR
• Input sample rate: 800k sample/s
• Passband edge: 100 kHz
• Stopband edge: 150 kHz
• Passband attenuation: £ 1 dB
The software tool gives the IIR filter coefficients in the Z-transform format expressed by Equation 26-4 :
Eqn. 26-4
The coefficient results in fixed point decimal representation are shown in Table 26-32 .
ADC
eQADC
(Analog)
Buffer
CAL
FIFO
Control
PSI
(Master)
PSI
(Slave)
CFIFO
RFIFO
Control
Coefficients
Filter
Decimation Filter
Serial
Slave-Bus
Y s
 
X s
 
-----------
B
0
B
1
s B
2
s
2
B
3
s
3
B
4
s
4
+ + + +
A
0
A
1
s – A
2
s
2
– A
3
s
3
– A
4
s
4
–
--------------------------------------------------------------------------- =