82801FB Datasheet, PDF (126/786 Pages) Intel Corporation – Intel I/O Controller Hub 6 (ICH6) Family

Functional Description
5.7.4
5.7.5
5.7.6
Terminal Count
Terminal count is communicated through LAD[3] on the same clock that DMA channel is
communicated on LAD[2:0]. This field is the CHANNEL field. Terminal count indicates the last
byte of transfer, based upon the size of the transfer.
For example, on an 8-bit transfer size (SIZE field is 00b), if the TC bit is set, then this is the last
byte. On a 16-bit transfer (SIZE field is 01b), if the TC bit is set, then the second byte is the last
byte. The peripheral, therefore, must internalize the TC bit when the CHANNEL field is
communicated, and only signal TC when the last byte of that transfer size has been transferred.
Verify Mode
Verify mode is supported on the LPC interface. A verify transfer to the peripheral is similar to a
DMA write, where the peripheral is transferring data to main memory. The indication from the host
is the same as a DMA write, so the peripheral will be driving data onto the LPC interface.
However, the host will not transfer this data into main memory.
DMA Request De-assertion
An end of transfer is communicated to the ICH6 through a special SYNC field transmitted by the
peripheral. An LPC device must not attempt to signal the end of a transfer by de-asserting
LDREQ#. If a DMA transfer is several bytes (e.g., a transfer from a demand mode device) the
ICH6 needs to know when to de-assert 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 ICH6 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 ICH6 that this is the last piece of data transferred on a
DMA read (ICH6 to peripheral), or the byte that follows is the last piece of data transferred on a
DMA write (peripheral to ICH6).
When the ICH6 sees one of these two encodings, it ends the DMA transfer after this byte and de-
asserts the DMA request to the 8237. Therefore, if the ICH6 indicated a 16-bit transfer, the
peripheral can end the transfer after one byte by indicating a SYNC value of 0000b or 1010b. The
ICH6 does not attempt to transfer the second byte, and de-asserts the DMA request internally.
If the peripheral indicates a 0000b or 1010b SYNC pattern on the last byte of the indicated size,
then the ICH6 only de-asserts 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 ICH6 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 ICH6, the data will be
transferred and the DMA request will remain active to the 8237. At a later time, the ICH6 will then
come back with another START–CYCTYPE–CHANNEL–SIZE etc. combination to initiate
another transfer to the peripheral.
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Intel^® I/O Controller Hub 6 (ICH6) Family Datasheet