Datasheet 139
Functional Description
5.6.6 DMA Request Deassertion
An end of transfer is communicated to the PCH through a special SYNC field
transmitted by the peripheral. An LPC device must not attempt to signal the end of a
transfer by deasserting LDREQ#. If a DMA transfer is several bytes (such as, a transfer
from a demand mode device) the PCH needs to know when to deassert the DMA
request based on the data currently being transferred.
The DMA agent uses a SYNC encoding on each byte of data being transferred, which
indicates to the PCH whether this is the last byte of transfer or if more bytes are
requested. To indicate the last byte of transfer, the peripheral uses a SYNC value of
0000b (ready with no error), or 1010b (ready with error). These encodings tell the PCH
that this is the last piece of data transferred on a DMA read (PCH to peripheral), or the
byte that follows is the last piece of data transferred on a DMA write (peripheral to the
PCH).
When the PCH sees one of these two encodings, it ends the DMA transfer after this byte
and deasserts the DMA request to the 8237. Therefore, if the PCH indicated a 16-bit
transfer, the peripheral can end the transfer after one byte by indicating a SYNC value
of 0000b or 1010b. The PCH does not attempt to transfer the second byte, and
deasserts the DMA request internally.
If the peripheral indicates a 0000b or 1010b SYNC pattern on the last byte of the
indicated size, then the PCH only deasserts the DMA request to the 8237 since it does
not need to end the transfer.
If the peripheral wishes to keep the DMA request active, then it uses a SYNC value of
1001b (ready plus more data). This tells the 8237 that more data bytes are requested
after the current byte has been transferred, so the PCH keeps the DMA request active
to the 8237. Therefore, on an 8-bit transfer size, if the peripheral indicates a SYNC
value of 1001b to the PCH, the data will be transferred and the DMA request will remain
active to the 8237. At a later time, the PCH will then come back with another START–
CYCTYPE–CHANNEL–SIZE etc. combination to initiate another transfer to the
peripheral.
The peripheral must not assume that the next START indication from the PCH 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 assured that they will receive the next START indication from the
PCH.
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 stops the transfer on the LPC bus as indicated, fills the upper byte with
random data on DMA writes (peripheral to memory), and indicates to the 8237 that the
DMA transfer occurred, incrementing the 8237’s address and decrementing its byte
count.
5.6.7 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, that 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.
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