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VP16256 Datasheet(PDF) 8 Page - Zarlink Semiconductor Inc |
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VP16256 Datasheet(HTML) 8 Page - Zarlink Semiconductor Inc |
8 / 19 page 8 VP16256 FILTER ACCURACY Input data and coefficients are both represented by 16-bit two’s complement numbers. The coefficients are converted to twelve bits by rounding towards zero. This is achieved as follows. If the coefficient is positive then the least significant 4 bits are discarded. If the coefficient is negative then the logical ‘OR’ of the least significant 4 bits are added to the remainder of the word. Twelve bit coefficients can be used directly provided the least significant four bits are set to zero. The FIR filter results are calculated using a multiplier accumulator structure as shown in Fig. 9. The truncation and word growth allowed for in the data path are explained in Fig. 10. The 16-bit data and 12-bit coefficient inputs (each with one sign bit before the binary point), are presented to the multiplier. This produces a 28-bit result with two bits before the binary point. Producing the full 28-bit result ensures that if both the data and coefficients are set to logic 1 a valid result is generated. Prior to entering the accumulator the least significant 4 bits of the multiplier result are truncated and the resulting 24 bits sign extended to 32 bits. The final accumulator result is 32 bits with 10 bits before the binary point. Thus 9 bits of word growth are allowed within the accumulator. All accumulator bits are made available on the output pins. In cascade mode the middle 16 bits from the network A accumulator are fed round to the network B data inputs, see Fig. 10. COEFFICIENT ADDER INPUT DATA ACCUMULATOR RESULT S 8 7 6 5 4 3 2 1 0 S S S S S S S S S 0 S 0 -26 S S ACCUMULATOR RESULT These bits are passed to filter network B during cascade mode -25 -24 -23 -22 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -22 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -22 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -15 ACCUMULATOR RESULT MULTIPLIER RESULT COEFFICIENT INPUT DATA Sign extended to 32 bits, least significant 4 bits truncated Multiplication producing a 28-bit result Fig. 10 Filter accuracy Fig. 9 Multiplier Accumulator |
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