Version: **
Question:
What happens if the internal TCXpire signal is not used in the decision point of the FX2LP GPIF state machine? Also, how do you run GPIF continuously?
Answer:
In certain applications, you do not know how many GPIF transactions may occur. The completion of GPIF may depend only on RDY signals not involving TCXpire (such as FV and LV). This KBA discusses such a scenario.
In the Block Diagram tab of the GPIF Designer utility, if you right-click RDY, the ‘Internal Status’ and ‘External Inputs’ sections are displayed, as shown in the following figure
In the Internal Status section, if the ‘Subst TC for RDY5’ option is unchecked:
- TCXpire does not appear in the drop-down section of the Specify Decision Point window in the state machine. This indicates that the internal TCXpire signal cannot be used at the decision point.
- The state transition occurs based on the logic resulting from other RDY signals (in the above diagram they are EF#, FF#, FIFOFlag, and IntReady) selected from the decision point drop-down list.
If the ‘Subst TC for RDY 5’ option is checked:
- TCXpire appears in the drop-down section of Decision Point in the state machine. This indicates that the internal TCXpire signal can be used (optional) at the decision point. If the internal TCXpire signal is used, then the GPIF stops when the number of transactions mentioned in the GPIFTCB are complete.
- If the internal TCXpire signal is not selected from the decision point drop-down list, GPIF stops based on the logic resulting from other RDY signals.
Note that if the internal TCXpire signal is not used at decision point, then the value loaded into the GPIFTCB does not matter. It is also acceptable to not initialize the GPIFTCB in the firmware; however, do not load zero into the GPIFTCB.