TAS5548DCAR datasheet(32/116 Pages) TI1

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2−23 Bit

S_xxxx.xxxx_xxxx_xxxx_xxxx_xxxx_xxx

2−4 Bit

2−1 Bit

20 Bit

Sign Bit

23 Bit

M0007-01

10110111

(-73)

-73

+ 11001101

(-51)

-51

10000100 (-124)

-124

+ 11010011

(-45)

-45

01010111

(57)

-169

+ 00111011

(59)

10010010 (-110)

-110

8-Bit ALU Operation

(Without Saturation)

Rollover

TAS5548

SLES270A – NOVEMBER 2012 – REVISED APRIL 2015

www.ti.com

To maximize the linear range of the 76-bit ALU, saturation logic is not used. In MAC computations, intermediate

overflows are permitted, and it is assumed that subsequent terms in the computation flow will correct the

overflow condition. The biquad filter structure used in the TAS5548 is the “direct form I” structure and has only

one accumulation node (for an example, see Biquad Filters). With this type of structure, intermediate overflow

are allowable as long as the designer of the filters has assured that the final output will bounded and not

overflow. Figure 27 is an example, using 8-bit arithmetic for ease of illustration, of a bounded computation that

experiences intermediate overflow condition.

The DAP memory banks include a dual port data RAM for storing intermediate results, a coefficient RAM, and a

fixed program ROM. Only the coefficient RAM, assessable via the I2C bus, is available to the user.

Figure 27. DAP ALU Operation With Intermediate Overflow

7.4.10.2 28-Bit 5.23 Number Format

All mixer gain coefficients are 28-bit coefficients using a 5.23 number format. Numbers formatted as 5.23

numbers have 5 bits to the left of the binary point and 23 bits to the right of the binary point. This is shown in

Figure 28.

Figure 28. 5.23 Format

The decimal value of a 5.23 format number can be found by following the weighting shown in Figure 29. If the

most significant bit is logic 0, the number is a positive number, and the weighting shown yields the correct

number. If the most significant bit is a logic 1, then the number is a negative number. In this case, every bit must

be inverted, a 1 added to the result, and then the weighting shown in Figure 29 applied to obtain the magnitude

of the negative number.

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