ST20-C1 datasheet(60/205 Pages) STMICROELECTRONICS

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5.6 Scaling

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internal format accumulator to a user-visible accumulator.

Figure 5.2 Accumulator scaling

The accumulator is left-shifted 8 bits so that the assumed binary point is moved from

below bit 30 to below bit 38. The accumulator value is saturated from bit 38 upwards.

At the end of the multiply-accumulate sequence the accumulator value is shifted down

(right) by an extra 8 bits to compensate for the left shift of 8 on coefficient inputs.

5.6.2

Coefficient scaling

The standard data format for data and coefficients is 1.15 (Q15). The product of two

1.15 numbers is 2.30. The coefficient value for each multiply-accumulate operation is

pre-scaled before being fed into the multiplier. The shift distances are controlled by the

mac_scale field: of the status register, as shown in Table 5.5.

The standard behavior for 1.15 (Q15) values is to shift the coefficients b y 8 places,

using mac_scale set to 2, which exactly compensates the extra right shift of 8 on the

mac_scale

coefficient shift

left 4

left 8

left 9

Table 5.5 mac_scale values

ShortMode and biquad

LongMode

User-visible accumulator

Internal accumulator

User-visible accumulator

Implied binary point

Bit initially set for rounding

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