ST20-C1 datasheet(58/205 Pages) STMICROELECTRONICS

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5.4 Biquad IIR filter

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5.3.3

Scaling

Scaling of the input data in the X vector is controlled by the mac_scale field of the

status register, as described in section 5.6.

5.3.4

Accumulator format mode

smacloop supports two data formats for the initial and final accum ulator value. If

mac_mode is 0 then Q15 (sign extended to 32 bits) is used, and the mode is said to

ShortMode.If mac_mode is 1 then Q31 is used, and the mode is said to be

LongMode.

5.4

Biquad IIR filter

The

biquad instruction performs a fixed sequence of 5 multiply-accumulates. Figure

5.1 shows an example using Q14 format. The parameters to the instruction are

pointers to three vectors of 16-bit values:

• an X input data vector,

• a Y results vector and

• a coefficient vector C. This vector must be word-aligned.

biquad calculates the next item in the Y vector according to the following formula, and

writes this to memory, incrementing the X and Y pointers by two bytes:

Y[2] =

X[0].C[0] + X[1].C[1] + X[2].C[2] + Y[0].C[3] + Y[1].C[4]

The X and Y vectors must be either both word-aligned or neither word-aligned.

The C pointer is left unchanged. This allows successive

biquad instructions to be

executed back-to-back to generate a set of filter outputs with no additional overhead.

biquad scales the X input data according to the mac_scale field of the status register,

as described in section 5.6. The mac_scale field ma y be set using

smacinit,as

described in section 5.3.

mac_buffer

Buffer size

(data items)

Buffer size

(bytes)

infinite

316

432

128

256

512

Table 5.4 mac_buffer coding

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