TAS5707
SLOS556–NOVEMBER 2008........................................................................................................................................................................................... www.ti.com
The decimal value of a 3.23 format number can be found by following the weighting shown in Figure 33. 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 34 applied to obtain the magnitude
of the negative number.
21 Bit
20 Bit
2–1 Bit
2–4 Bit
2–23 Bit
(1 or 0) ´ 21 + (1 or 0) ´ 20 + (1 or 0) ´ 2–1 + ....... (1 or 0) ´ 2–4 + ....... (1 or 0) ´ 2–23
M0126-01
Figure 34. Conversion Weighting Factors—3.23 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 35
Fraction
Digit 6
Sign
Bit
Fraction
Digit 1
Fraction
Digit 2
Fraction
Digit 3
Fraction
Digit 4
Fraction
Digit 5
Integer
Digit 1
u
u
u
u
u
u
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
0
S
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
M0127-01
Figure 35. Alignment of 3.23 Coefficient in 32-Bit I2C Word
Sample calculation for 3.23 format
db
0
Linear
1
Decimal
8388608
14917288
4717260
Hex (3.23 Format)
800000
5
1.77
00E39EA8
-5
X
0.56
L = 10(X/20)
0047FACC
D = 8388608 × L H = dec2hex (D, 8)
Sample calculation for 9.17 format
db
0
Linear
1
Decimal
131072
Hex (9.17 Format)
20000
5
1.77
231997
38A3D
-5
X
0.56
L = 10(X/20)
73400
11EB8
D = 131072 × L
H = dec2hex (D, 8)
30
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