82801CA Datasheet, PDF (99/521 Pages) Intel Corporation – I/O Controller Hub 3-S (ICH3-S)

Functional Description
5.5.8 Asserting DMA Requests
Peripherals that need DMA service encode their requested channel number on the LDRQ# signal.
To simplify the protocol, each peripheral on the LPC I/F has its own dedicated LDRQ# signal (they
may not be shared between two separate peripherals). The ICH3 has two LDRQ# inputs, allowing
at least two devices to support DMA or bus mastering.
LDRQ# is synchronous with LCLK (PCI clock). As shown in Figure 5-11, the peripheral uses the
following serial encoding sequence:
• Peripheral starts the sequence by asserting LDRQ# low (start bit). LDRQ# is high during idle
conditions.
• The next three bits contain the encoded DMA channel number (MSB first).
• The next bit (ACT) indicates whether the request for the indicated DMA channel is active or
inactive. The ACT bit will be a 1 (high) to indicate if it is active and 0 (low) if it is inactive.
The case where ACT is low will be rare, and is only used to indicate that a previous request for
that channel is being abandoned.
• After the active/inactive indication, the LDRQ# signal must go high for at least 1 clock. After
that one clock, the LDRQ# signal can be brought low to the next encoding sequence.
If another DMA channel also needs to request a transfer, another sequence can be sent on LDRQ#.
For example, if an encoded request is sent for channel 2 and then channel 3 needs a transfer before
the cycle for channel 2 is run on the interface, the peripheral can send the encoded request for
channel 3. This allows multiple DMA agents behind an I/O device to request use of the LPC
interface, and the I/O device does not need to self-arbitrate before sending the message.
Figure 5-11. DMA Request Assertion Through LDRQ#
LCLK
LDRQ#
Start
MSB
LSB
ACT
Start
5.5.9
Abandoning DMA Requests
DMA Requests can be deasserted in two fashions: on error conditions by sending an LDRQ#
message with the “ACT” bit set to “0,” or normally through a SYNC field during the DMA
transfer. This section describes boundary conditions where the DMA request needs to be removed
prior to a data transfer.
There may be some special cases where the peripheral desires to abandon a DMA transfer. The
most likely case of this occurring is due to a floppy disk controller which has overrun or underrun
its FIFO, or software stopping a device prematurely.
In these cases, the peripheral wishes to stop further DMA activity. It may do so by sending an
LDRQ# message with the ACT bit as “0.” However, since the DMA request was seen by the ICH3,
there is no guarantee that the cycle has not been granted and will shortly run on LPC. Therefore,
peripherals must take into account that a DMA cycle may still occur. The peripheral can choose not
to respond to this cycle, in which case the host will abort it, or it can choose to complete the cycle
normally with any random data.
This method of DMA deassertion should be prevented whenever possible, to limit boundary
conditions both on the ICH3 and the peripheral.
Intel^® 82801CA ICH3-S Datasheet
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