Echelon FT 3150 Smart Transceiver - Manual (Page 44)

Chapter 3 - Input/Output Interfaces
38 FT 3120 / FT 3150 Smart Transceiver Data Book
• The FT Smart Transceiver hardware
The contribution of the scheduler to the overall timing characteristic is approximately uniform across all 34 I/O
function blocks since its contribution to the overall I/O timing is at a relatively high functional level.
The contribution of firmware and hardware varies from one I/O object to another (e.g., Bit I/O versus Neurowire I/O),
with one area generally being the dominant factor.
Scheduler-Related I/O Timing Information
As part of the FT Smart Transceiver firmware, the scheduler provides an orderly and predictable means to facilitate
the evaluation of user-defined events. The when clause, provided by the Neuron C language, is used to specify such
events. For more information on the operation of the scheduler, refer to the Neuron C Programmer’s Guide.
There is a finite latency associated with the operation of the scheduler. The time required for the scheduler to evaluate
the same when clause in a particular user application code is, to a large extent, a function of the size of the user code,
the total number of when clauses, and the state of the events associated with those when clauses. Therefore, it is
impossible to specify a nominal value for this latency, as each application will have its own distinct behavior under
different circumstances.
The best case latency can be viewed in several ways, each exposing a different aspect of the scheduler operation. A
simple example consists of having an application program consisting of two when clauses, both of which always
evaluate to TRUE, as shown below.
IO_0 output bit testbit;
when (TRUE) {
io_out(testbit, 1);
}
when (TRUE) {
io_out (testbit, 0);
}
Processing of when clauses is done in a round-robin fashion; therefore, the Neuron C code above performs alternating
activation of the IO0 pin in order to isolate and extract the timing parameters associated with the scheduler. The
waveform seen on pin IO0 of the FT Smart Transceiver, as a result of the above code, is shown in Figure 3.4.