ST20-C1 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ST20-C1' PDF : 205 Pages View PDF
5 Multiply accumulate
Minimum
Maximum
Hex Value
FFFF8000
00007FFF
Decimal Value
-1
+0.9999
Table 5.10 Q15 limiting values
Memory access
A Q15 stored in memory may be loaded to Areg with a lsxinc instruction which will
automatically sign-extend the value. Similarly a Q15 may be written to memory with a
ssinc instruction (which will discard the top 16 bits).
Saturation
A well-formed Q15 value is sign extended to the width of the field, and so bits 15 to 31
inclusive will all be identical. Conversely, if the bits from 15 to 31 inclusive are not all
identical, then the value is not well-formed. It has either overflowed (if b31=0, so
positive) or underflo wed (if bit31=1, so negative).
A Q15 value in Areg may be saturated with the sequence:
ldc 00007FFF;
order;
ldc FFFF8000;
order;
rev;
5.7.6 Q31 format
Q31 format is a 32-bit signed fractional value in the range -1 ≤ n < 1. The value is
stored in two’s-complement form with a sign bit (bit 31), an implied binary point
between bits 31 and 30, and 31 significant fractional bits (bit 30 to bit 0).
A Q31 value in a 32-bit field is organiz ed as shown in Figure 5.9.
Most significant
Bit position 31 30 29 28 27 26
Least significant
210
-(20) 2-1 2-2 2-3 2-4 2-5
2-29 2-30 2-31
binary point
sign bit
Figure 5.9 Q31 data format
The minimum and maximum representable values are as shown in Table 5.11.
67/205
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