82443MX Datasheet, PDF (111/173 Pages) Intel Corporation – PCIset

82443MX PCIset
PCICLK
REQ#
Start CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7
GNT#
Start Bit0 Bit1 Bit2
dma_s_ch
Figure 15. DMA Serial Channel Passing Protocol
The requesting device must encode the channel request information as shown above, where CH0–CH7 are
one clock active high states representing DMA channel requests 0–7.
The 440MX encodes the granted channel on the GNT# line as shown above, where the bits have the same
meaning as shown in Figure 15. For example, the sequence [start, bit 0, bit 1, bit 2]=[0,1,0,0] grants DMA
channel 1 to the requesting device, and the sequence [start, bit 0, bit 1, bit 2]=[0,0,1,1] grants DMA channel 6
to the requesting device.
All PCI DMA expansion agents must use the Channel Passing protocol as described, and must function
according to one of the following three cases:
1. If a PCI DMA expansion agent has more than one request active, it must resend the request serial
protocol after one of the requests has been granted the bus and it has completed its transfer. The
expansion device should drive its REQ# inactive for two clocks and then transmit the serial channel
passing protocol again, even if there are no new requests from the PCI expansion agent to the 440MX.
For example: If a PCI expansion agent has active requests for DMA channel 1 and channel 5, it passes
this information to the 440MX through the expansion channel passing protocol. If after receiving GNT#
(assume for CH5) and having the device finish its transfer (device stops driving request to PCI expansion
agent) it then must re-transmit the expansion channel passing protocol to inform the 440MX that DMA
Channel 1 is still requesting the bus, even if that is the only request the expansion device has pending.
2. If a PCI DMA expansion agent has a request go inactive before the 440MX asserts GNT#, it must resend
the expansion channel passing protocol to update the 440MX with this new request information. For
example: If a PCI expansion agent has DMA Channel 1 and 2 requests pending it sends them serially to
the 440MX using the expansion channel passing protocol. If, however, DMA channel 1 goes inactive into
the expansion agent before the expansion agent receives a GNT# from the 440MX, the expansion agent
MUST pull its REQ# line high for one clock and resend the expansion channel passing information with
only DMA Channel 2 active. Note that the 440MX does not do anything special in this case because
DREQ# going inactive before DACK# is received is not allowed in the ISA DMA protocol and, therefore,
does not need to work properly in this protocol either. This requirement is needed to support Plug-n-Play
ISA devices that toggle DREQ# lines to determine if those lines are free in the system.
3. If a PCI expansion agent has sent its serial request information and receives a new DMA request before receiving
GNT# the agent must resend the serial request with the new request active. For example: If a PCI expansion agent
has already passed requests for DMA Channels 1 and 2 and sees DMA Channel 3 active before GNT# is received,
the device must pull its REQ# line high for one clock and resend the expansion channel passing information with
all three channels active.
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