Altera Floating-Point - Manual (Page 46)

Category Parameter Values Description
Accumulat
or Size
MSBA — The weight of the MSB of the accumulator. For
example, in a financial simulation, if the value of
a stock cannot exceed 100,000 dollars, use a
value of ceil(log
2
(100000))=17.
In a circuit simulation where the circuit adds
numbers in the 0 to 1 range, for one year, at 400
MHz, use a value of
ceil(log
2
(365 x 60 x 60 x 24 x 400 x 10 6 ))=54.
The
ao
output signal goes high when the MSB of
the accumulated value has a weight larger than
MSBA. The result of the accumulation is then
invalid. Altera recommends adding a few guard
bits to avoid possible accumulator overflow. A
few guard bits have little impact on the
accumulator size.
The default value is 20.
LSBA — The weight of the LSB of the accumulator and
the accuracy of the accumulator. Because an N
term accumulation can invalidate the log
2
(N)
LSBs of the accumulator, you must consider the
length of the accumulation and the range of the
inputs when setting this parameter.
For example, if a 2
-30 accuracy is required over
an accumulation of 1024 numbers, then set the
LSBA to:
(-30 - log
2
(1024)) = -40.
Any input 2 e ×1.F, where F is the mantissa and e
is less than the LSBA will be shifted out of the
accumulator. The
au
output signal goes high to
indicate this situation.
The default value is -26.
Required
Perform‐
ance
Target frequency Any positive
integer value.
Choose the frequency in MHz at which this core
is expected to run. This together with the target
device family will determine the amount of
pipelining in the core.
The default value is 200 MHz.
Optional Generate an enable
port
— Choose if the accumulator should have an
enable signal.
This parameter is disabled by default.
UG-01058
2014.12.19
ALTERA_FP_ACC_CUSTOM Parameters 4-5
ALTERA_FP_ACC_CUSTOM IP Core Altera Corporation
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