TAS5504 Datasheet, PDF (21/96 Pages) Texas Instruments – 4 Channel Digital Audio PWM Processor

Introduction
The decimal value of a 5.23 format number can be found by following the weighting shown in Figure 1−6. 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 1−6 applied to obtain the
magnitude of the negative number.
2³ Bit
2² Bit
2⁰ Bit
2^−1 Bit
2^−4 Bit
2^−23 Bit
(1 or 0) x 2³ + (1 or 0) x 2² + … + (1 or 0) x 2⁰ + (1 or 0) x 2^−1 + … + (1 or 0) x 2^−4 + … + (1 or 0) x 2^−23
Figure 1−6. Conversion Weighting Factors—5.23 Format to Floating Point
Gain coefficients, entered via the I²C bus, must be entered as 32-bit binary numbers. The format of the 32-bit
number (4-byte or 8-digit hexadecimal number) is shown in Figure 1−7.
Sign
Bit
Integer
Digit 1
Fraction
Digit 1
Fraction
Digit 2
Fraction
Digit 3
Fraction
Digit 4
Fraction
Digit 5
Fraction
Digit 6
0
u u u u S x x x x. x x x x x x x x x x x x x x x x x x x x x x x
Coefficient
Digit 8
Coefficient
Digit 7
Coefficient
Digit 6
Coefficient
Digit 5
Coefficient
Digit 4
Coefficient
Digit 3
Coefficient
Digit 2
Coefficient
Digit 1
u = unused or don’t care bits
Digit = hexadecimal digit
Figure 1−7. Alignment of 5.23 Coefficient in 32-Bit I²C Word
As Figure 1−7 shows, the hex value of the integer part of the gain coefficient cannot be concatenated with the
hex value of the fractional part of the gain coefficient to form the 32-bit I²C coefficient. The reason is that the
28-bit coefficient contains 5 bits of integer, and thus the integer part of the coefficient occupies all of one hex
digit and the most significant bit of the second hex digit. In the same way, the fractional part occupies the lower
3 bits of the second hex digit, and then occupies the other five hex digits (with the eighth digit being the
zero-valued most significant hex digit).
1.4.2.2 48-Bit 25.23 Number Format
All level adjustment and threshold coefficients are 48-bit coefficients using a 25.23 number format. Numbers
formatted as 25.23 numbers means that there are 25 bits to the left of the decimal point and 23 bits to the right
of the decimal point. This is shown in Figure 1−8.
SLES123 − October 2004
TAS5504
13