Xilinx V2.1 - Manual (Page 17)

The Nature of Signals in the Xilinx Blockset 17
Xilinx Blockset Overview
As an example, the figures shown below depict the Xilinx Negate block parameters
dialog box with full and user defined precision. Note in the latter case the additional
options for selecting quantization and overflow behavior.
Figure 2-2: User-Defined Precision Options (available if selected instead of full
precision)
Valid and Invalid Data
In the Xilinx Blockset portion of a Simulink model, every data sample is accompanied
by a handshake validation signal. In the corresponding hardware, every data-carrying
bus has a companion net that carries a valid or invalid status indicator. This is a
commonly used handshaking mechanism. There are different circumstances under
which the status indicator may be set to invalid. For example, invalid data might
mean that a pipeline has not yet filled up, or it may denote bursty outputs, as with an
FFT. Blocks in the Xilinx Blockset can use this valid bit signal to determine what to do
with the input data. Some of the Xilinx blocks, for example, the storage blocks and the
FFT, use the valid bit to determine when to store input data.
Port Data Types
Selecting the Port Data Types option (under the Format menu in the Simulink
window) shows the data type and precision of a signal. An example port data type
string is Fix_11_9, which indicates that the signal is a signed 11-bit number with the
binary point 9 bits from the right side. Similarly, an unsigned signal is indicated by the
UFix_ prefix.