Cirrus Logic CS5378 - Manual (Page 45)

CS5378
DS639F3 45
13.3 On-Chip FIR Coefficients
Two sets of on-chip coefficients, linear phase and
minimum phase, are available for FIR1 and FIR2.
Performance of the on-chip coefficient sets is very
good, with excellent ripple and stop band charac-
teristics as described in Figure 26 and Table 14.
Which on-chip coefficient set to use is selected by
a data word following the ‘Write ROM Coeffi-
cients’ configuration command. See “Filter Coef-
ficient Selection” on page 38 for information about
selecting on-chip coefficient sets.
13.4 Programmable FIR Coefficients
A maximum of 255 + 255 coefficients can be pro-
grammed into FIR1 and FIR2 to create a custom
filter response. The total number of coefficients for
the FIR filter is fundamentally limited by the avail-
able computation cycles in the digital filter, which
itself is determined by the digital filter clock rate.
Custom filter sets should normalize the maximum
coefficient value to 24-bit two’s complement full
scale, 0x7FFFFF, and scale all other coefficients
accordingly. To maintain maximum internal dy-
namic range, the CS5378 FIR filter performs dou-
ble precision calculations with an automatic gain
correction to scale the final output.
Custom FIR coefficients are uploaded using the
‘Write FIR Coefficients’ configuration command.
See “EEPROM Configuration Commands” on
page 27 or “Microcontroller Configuration Com-
mands” on page 33 for information about writing
custom FIR coefficients.
13.5 FIR Filter Synchronization
The FIR1 and FIR2 filters are synchronized to the
external system by the MSYNC signal, which is
generated from the SYNC input. The MSYNC sig-
nal sets a reference time (time 0) for all filter oper-
ations, and the FIR filters are restarted to phase
align with this reference time.