ST20-C1 STMICROELECTRONICS [STMicroelectronics], ST20-C1 Datasheet - Page 60

• Manufacturer Part Number: ST20-C1
• Description: Instruction Set Reference Manual
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

5.6 Scaling
internal format accumulator to a user-visible accumulator.
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
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
60/205
ShortMode and biquad
LongMode
Coefficient scaling
47
47
15
38
38
31
Implied binary point
Figure 5.2 Accumulator scaling
Table 5.5 mac_scale values
mac_scale
0
1
2
3
0
1
User-visible accumulator
coefficient shift
Bit initially set for rounding
left 4
left 8
left 9
0
0
1
User-visible accumulator
0
0
Internal accumulator
Internal accumulator