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PDSP16515AB0AC Datasheet(PDF) 5 Page - Zarlink Semiconductor Inc |
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PDSP16515AB0AC Datasheet(HTML) 5 Page - Zarlink Semiconductor Inc |
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5 / 25 page  PDSP16515A 5 generated internally, and applied to the incoming real or complex data with no time penalty. No external ROM is needed to support these windows. The Blackman-Harris window gives improved dynamic range over the Hamming window when two closely spaced frequencies are to be detected, and one is of smaller magnitude than the other. It does, however, reduce the actual frequency resolution, and the Hamming window may then be preferable. Data in and out of the device is represented by 16 bit real and imaginary components, with 16 bit sine and cosine values contained in an internal ROM. Conditional scaling, coupled with word growth through the butterfly data path, gives increased dynamic range. Transforms can be computed with sample sizes of either 256 or 1024 data points. The 256 point option can alternatively be used to simultaneously execute either four 64 point transforms, or sixteen 16 point transforms. The 16 point mode can only be used with a rectangular window, and no overlapping of data blocks is possible. The device can be configured, either, to perform continuous transforms in a real time application, or as slave processor to a more general purpose signal processing system. In the continuous mode, with transform sizes of 256 points or less, it contains three internal control units which simultaneously allow new data to be loaded, present data to be transformed, and previous results to be dumped. Additional, external, input/ output buffering is not needed. The internal input buffer also allows data blocks to be overlapped by either 50% or 75%, apart from the mode with no overlaps. When 1024 point transforms are to be calculated, without loss of incoming data during the transform time, it is necessary to use an input buffer. This requirementcan be satisfied by an external buffer memory. In any of the real or complex modes it is possible to obtain higher performance by connecting devices in parallel. It is then possible to increase the sampling rate to that of the system clock used for internal operations. The mode of operation of the device is controlled by 16 bits in a control register. These are loaded through the AUX15:0 port when a control signal DEF is active low. This port is also used to provide the imaginary component of complex input data, and, if complex transforms are to be performed, an external tristate buffer will be needed to isolate the control information. This should only be enabled when DEF is active. DEF is also used to initialise the internal circuitry, and can be a simple power on reset if control parameters need not be subsequently changed. Data Precision During each pass of a radix-4 fast Fourier transform it is possible for either component of a particular result to grow by a factor of up to four in the first pass, and 5.242 in subsequent passes. This is between two and three bits in each pass and the data path must allow for this word growth to avoid any possibility of overflow. At the end of the data path the word is preserved at 18 bits and stored in the internal RAM. Any un- necessary word growth to prevent overflow thus results in loss of arithmetic precision, and has a detrimental effect on the dynamic range achievable. In practice these large word growths only occur when bipolar complex square waves are transformed, and even then will not occur on every pass. The PDSP16515A compromises by allowing a 2 bit word growth during the butterfly calculation in the first pass. This is equivalent to ignoring the most significant bit of the 19 bit final result, which is assumed to be an extra sign bit, and then selecting the next 18 bits for storage. In S S 29 14 13 0 -- MULTIPLIER "1" FIRST ADDER 19Bit Result 18 1 0 - REGISTER FILE SECOND ADDER 19Bit Result 18 1 0 - REGISTER FILE THIRD ADDER 19Bit Result 18 - 1 18 18 Shift left until largest point has one sign bit. 18 SIN / COS ROM 16 RAM INPUT 17 - 0 SELECT CR BIT3 SELECT Figure. 3 One of Four Data Paths |
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