A useful feature of the black box is its ability to support dynamic output port types and rates. For example, it is often necessary to set an output port width based on the width of an input port. SysgenPortDescriptor provides member variables that allow you to determine the configuration of a port. You can set the type or rate of an output port by examining these member variables on the block's input ports.
For example, you can obtain the width and rate of a port (in this case din) as follows:
input_width = this_block.port('din').width;
input_rate = this_block.port('din').rate;
The SysgenBlockDescriptor object provides Boolean member variables inputTypesKnown and inputRatesKnown that tell
whether the port types and rates have been propagated to the
block. If you are setting dynamic output port types or rates
based on input port configurations, the configuration calls
should be nested inside conditional statements that check that
values of inputTypesKnown and
inputRatesKnown.
The following code shows how to set the width of a dynamic output port dout to have the same width
as input port din:
if (this_block.inputTypesKnown)
dout.setWidth(this_block.port('din').width);
end
Setting dynamic rates works in a similar manner. The code below sets the sample
rate of output port dout to be
twice as slow as the sample rate of input port din:
if (this_block.inputRatesKnown)
dout.setRate(this_block.port('din').rate*2);
end