Xhold Timing When The External Host Starts A Transfer To Dsp Instead Of; Granting The Dsp Access To The Expansion Bus(Internal Bus Arbiter Disabled); Possible Expansion Bus Arbitration Scenarios (Internal Bus Arbiter Disabled) - Texas Instruments TMS320C6201 Reference Manual

Expansion Bus Arbitration
Figure 8–27. XHOLD Timing When the External Host Starts a Transfer to DSP Instead of

Granting the DSP Access to the Expansion Bus(Internal Bus Arbiter Disabled)

XHOLD (output)
XHOLDA (input)
XAS (input)
XCS (input)
XBLAST (input)
Table 8–19. Possible Expansion Bus Arbitration Scenarios
(Internal Bus Arbiter Disabled)
XARB = '0'
Current
External Host
XBOFF
Activity
asserted
Host transfer
to the
N/A
expansion bus
in progress
8-46
Table 8–19 shows possible scenarios that can happen when the internal bus
arbiter is disabled (XARB =0).
Current DSP state
DMA request to ex-
pansion bus IO port
pending
DMA request to ex-
pansion bus IO port,
and aux. DMA re-
quests are pending
Aux. DMA request
pending
Actions
If the DMA request comes before or at the same time
when the host started the transfer, the DSP asserts
the XHOLD and keep it asserted during the host
transfer.
If the DMA request came after the host started the
transfer, the DSP waits for the host transfer to com-
plete and then asserts XHOLD.
After the DSP gets the expansion bus the pending aux-
iliary DMA request is executed first (since for the ex-
pansion bus, the aux. DMA channel always has priority
over the other DMA channels). After the auxiliary DMA
transfer is completed, the DSP starts the DMA transfer
and does not drop the XHOLD between these two
transfers.
If the auxiliary DMA request comes prior to the host
starting the transfer, the DSP asserts the XHOLD and
keeps it asserted until the host starts the transfer.
Once the host starts the transfer, the DSP drops the
request (see Figure 8–24). The DSP re-asserts the
XHOLD after the host completes the transfer.
If the auxiliary DMA request comes after the host is
started the transfer, the DSP waits for the host trans-
fer to complete and asserts the XHOLD.