82801DB Datasheet, PDF (102/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
The peripheral must not assume that the next START indication from the ICH4 is another grant to
the peripheral if it had indicated a SYNC value of 1001b. On a single mode DMA device, the 8237
will re-arbitrate after every transfer. Only demand mode DMA devices can be guaranteed that they
will receive the next START indication from the ICH4.
Note: Indicating a 0000b or 1010b encoding on the SYNC field of an odd byte of a 16-bit channel (first
byte of a 16-bit transfer) is an error condition.
Note: The host will stop the transfer on the LPC bus as indicated, fill the upper byte with random data on
DMA writes (peripheral to memory), and indicate to the 8237 that the DMA transfer occurred,
incrementing the 8237’s address and decrementing its byte count.
5.5.14
SYNC Field / LDRQ# Rules
Since DMA transfers on LPC are requested through an LDRQ# assertion message, and are ended
through a SYNC field during the DMA transfer, the peripheral must obey the following rule when
initiating back-to-back transfers from a DMA channel.
The peripheral must not assert another message for eight LCLKs after a deassertion is indicated
through the SYNC field. This is needed to allow the 8237, which typically runs off a much slower
internal clock, to see a message deasserted before it is re-asserted so that it can arbitrate to the next
agent.
Under default operation, the host will only perform 8-bit transfers on 8-bit channels and 16-bit
transfers on 16-bit channels.
The method by which this communication between host and peripheral through system BIOS is
performed is beyond the scope of this specification. Since the LPC host and LPC peripheral are
motherboard devices, no “plug-n-play” registry is required.
The peripheral must not assume that the host will be able to perform transfer sizes that are larger
than the size allowed for the DMA channel, and be willing to accept a SIZE field that is smaller
than what it may currently have buffered.
To that end, it is recommended that future devices which may appear on the LPC bus, which
require higher bandwidth than 8-bit or 16-bit DMA allow, do so with a bus mastering interface and
not rely on the 8237.
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Intel^® 82801DB ICH4 Datasheet