What is the catch with IDMA on MPC860?

[Geir Frode Raanes]

On Fri, 17 Mar 2000, Richard Hendricks wrote:

>
>
> Geir Frode Raanes wrote:
> >
> > Revision C or later of the '860 will even perform
> > a single-address _burst_ transfer on IDMA channel 1.
> > This I would love to do from our in-house designed
> > frame grabber to main memory. Then I could avoid
> > disabling all interrupts while bursting. Meaning
> > I could still catch run-away situations on a
> > time-out basis. Today things simply lock up...

> The biggest complaint I hear is the performance stinks.
> Which it does, since it is really a software based DMA
> algorithm running on the CPM.

Possibely. But where does the manual say that the
performance stinks? It should be documented somewhere.

> I *think* you mean Single-buffer burst flyby mode.

That is correct. I just quoted the subtitle of figure 20-15.
But the chapter is named Single-buffer burst flyby mode.
The following figure numbers are collected from the current
version of the MPC860UM user manual in PDF form @ motorola.

Figure 14-5:  Single-Beat Cycle Basic Timing Zero Wait States
              Two clock cycles per bus cycle.

Figure 14-12: Burst-Read Cycle with Zero Wait state.
              Five clock cycles per four words burst cycle.

Figure 20-15: Single Buffer/Address IDMA1 Burst Timing.
              /TA alternating resulting in one waitstate per
              word and thus nine clock cycles per four words
              burst cycle.

Figure 14-14: Burst-Read Cycle with Wait States between Beats.
              Would have been identical to figure 20-15
              with identical /TA pattern.


Now, the question is - what controls the /TA line?
Will I get full speed if actively driving the /TA line low for
the duration of the IDMA1 burst cycle? Or for that matter in the
Single-address/Single-cycle Fly-by transfer of figure 20-10?
The subtitles states that /TA is externally generated. What then
stops me from running the IDMA bus cycle with normal PIO mode
timing as per figure 14-5?


Figure 20-10: /SDACK timing diagram
              Single-Address Peripheral write
              __EXTERNALLY GENERATED /TA__


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