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target termination



Hello everyone, here's a question on industry support (or should I say tolerance) -- but first a little spec. background.

In the PCI Spec. Rev. 2.1, section 3.3.3.2.1 "Target Termination Signalling Rules":

Under the verbiage for Rule 1:

When both FRAME# and IRDY# are asserted, the master has committed to complete two data phases. The master is unable to deassert FRAME# until the current data phase completes because IRDY# is asserted. Because a data phase is allowed to complete when STOP# and IRDY# are asserted, the master is allowed to start the final data phase by deasserting FRAME# and keeping IRDY# asserted. The master must deassert IRDY# the clock after the completion of the last data phase.

Under the verbiage for Rule 3:

When both STOP# and TRDY# are asserted in the same data phase, the target will transfer data in that data phase. In this case, TRDY# must be deasserted when the data phase completes. As before, STOP# must remain asserted until the transaction ends whereupon it is deasserted.

Now, consider the following waveforms (please infer STS and high-Z where appropriate):

                            Figure 1:
           |  0  |  1  |  2  |  3  |  4  |  5  |  6  |

CLK     ___/--\__/--\__/--\__/--\__/--\__/--\__/--\__

FRAME#  -----\_______________________/---------------
IRDY#   -----------\_______________________/---------
TRDY#   -----------\_______________________/---------
DEVSEL# -----------\_______________________/---------
STOP#   ------------------------\__________/---------

                            Figure 2:
           |  0  |  1  |  2  |  3  |  4  |  5  |  6  |

CLK     ___/--\__/--\__/--\__/--\__/--\__/--\__/--\__

FRAME#  -----\_____________________________/---------
IRDY#   -----------\_____________________________/---
TRDY#   -----------\_______________________/---------
DEVSEL# -----------\_____________________________/---
STOP#   ------------------------------\__________/---
 

With respect to Fig. 1, the first snippit of verbiage indicates that this could be a legal cycle on PCI from the master's point-of-view (during cycle 3 the master has committed to two more transfers). However, the second snippit of verbiage indicates that the target *must* deassert TRDY# in cycle 4 and not in cycle 5 as shown. According to the spec. a target that limits the transfer to four words would exhibit the behaviour shown in Fig. 2.

In reality, the waveform of Fig. 1 is just as simple to support as that of Fig. 2 (comparable cost), and has better performance because it occupies the PCI bus for one less cycle.

Has anyone else considered this case before? Personally, I think this should have been permitted from the beginning since it can be easily supported.
 

So, here are my questions.....

Q1.    Are there any targets out there that already support/exhibit the behaviour in Fig. 1?
Q2.    Are there any initiators (or other bus trackers) out there that *cannot* support/tolerate the behaviour in Fig. 1? (ie. either they break, or ignore the IRDY# / TRDY# assertions in cycle 5)
Q3.    Would anyone else like to see support for the mode of Fig. 1 supported by the spec.?

Clearly there are compatibility issues to consider before such an item could even hope to be sanctioned by the spec. However, existing designs with an eye to simplicity would probably tolerate the operation of Figure 1. For future designs, I would suggest at least that the operation of Fig 1. not be precluded.

For instance, the initiator might ignore STOP# in determining whether data was transferred in a given clock-cycle. In general I would expect that this is generally true since it is a simpler decision, and the initiator would then be able to tolerate the signalling of Fig. 1. In particular, WRT Figure 1, had STOP# not even been asserted, the cycle would be perfectly valid. The only contentious point I see here is that the master state-machine would have to be in a state (after first seeing FRAME#, IRDY# and STOP# asserted) in which the subsequent assertion of IRDY# and TRDY# would effect a transfer.

Additionally, for bus trackers, once STOP# has been seen they only need to be concerned with the state of IRDY#. In particular the termination of a data phase can be indicated by IRDY and [TRDY or STOP] (in positive logic). In this type of implementation, the simultaneous assertion of TRDY# and STOP# in Fig 1. would be irrelavent -- and thus Fig. 1 would be tolerated by such bus trackers.

Does anyone else have any other thoughts on this?

Michael Tresidder
V3 Semiconductor