AM79C972 datasheet(47/130 Pages) AMD

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Am79C972

Table 5.

Descriptor Write Sequence

FIFO DMA Transfers

Am79C972 microcode will determine when a FIFO

DMA transfer is required. This transfer mode will be

used for transfers of data to and from the Am79C972

FIFOs. Once the Am79C972 BIU has been granted bus

mastership, it will perform a series of consecutive

transfer cycles before relinquishing the bus. All trans-

fers within the master cycle will be either read or write

cycles, and all transfers will be to contiguous, ascend-

ing addresses. Both non-burst and burst cycles are

used, with burst mode being the preferred mode when

the device is used in a PCI bus application.

Non-Burst FIFO DMA Transfers

In the default mode, the Am79C972 controller uses

non-burst transfers to read and write data when ac-

cessing the FIFOs. Each non-burst transfer will be per-

formed sequentially with the issue of an address and

the transfer of the corresponding data with appropriate

output signals to indicate selection of the active data

bytes during the transfer.

FRAME will be deasserted after every address phase.

Several factors will affect the length of the bus master-

ship period. The possibilities are as follows:

Bus cycles will continue until the transmit FIFO is filled

to its high threshold (read transfers) or the receive FIFO

is emptied to its low threshold (write transfers). The

exact number of total transfer cycles in the bus master-

ship period is dependent on all of the following vari-

ables: the settings of the FIFO watermarks, the

conditions of the FIFOs, the latency of the system bus

to the Am79C972 controller’s bus request, the speed of

bus operation and bus preemption events. The TRDY

response time of the memory device will also affect the

number of transfers, since the speed of the accesses

will affect the state of the FIFO. During accesses, the

FIFO may be filling or emptying on the network end. For

example, on a receive operation, a slower TRDY re-

sponse will allow additional data to accumulate inside

of the FIFO. If the accesses are slow enough, a com-

plete DWord may become available before the end of

the bus mastership period and, thereby, increase the

number of transfers in that period. The general rule is

that the longer the Bus Grant latency, the slower the

bus transfer operations; the slower the clock speed, the

higher the transmit watermark; or the higher the re-

ceive watermark, the longer the bus mastership period

will be.

Note: The PCI Latency Timer is not significant during

non-burst transfers.

SWSTYLE

BCR20[7:0]

BWRITE

BCR18[5]

AD Bus Sequence

Address = XXXX XX04h

Data = MD2[15:0],

MD1[15:0]

Idle

Address = XXXX XX00h

Data = MD1[31:24]

Address = XXXX XX08h

Data = MD2[31:0]

Idle

Address = XXXX XX04h

Data = MD1[31:16]

Address = XXXX XX00h

Data = MD2[31:0]

Idle

Address = XXXX XX04h

Data = MD1[31:16]

Address = XXXX XX00h

Data = MD2[31:0]

Data = MD1[31:16]

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