ST20-C1 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ST20-C1' PDF : 205 Pages View PDF
5.6 Scaling
internal format accumulator to a user-visible accumulator.
ShortMode and biquad
15
47
38
0
User-visible accumulator
0
1
Internal accumulator
LongMode
47
Implied binary point
31
38
Bit initially set for rounding
0
User-visible accumulator
0
1
Internal 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.
mac_scale
0
1
2
3
coefficient shift
0
left 4
left 8
left 9
Table 5.5 mac_scale values
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
®
Share Link: 
