AM79C970A datasheet(62/219 Pages) AMD

62 / 219 page

AMD

P R E L I M I N A R Y

Am79C970A

19436A-32

GNT

REQ

DEVSEL

TRDY

PAR

FRAME

CLK

PAR

IRDY

DEVSEL is sampled

DATA

0110

0000

0011

MD2

PAR

DATA

PAR

Figure 29. Descriptor Ring Write In Burst Mode

FIFO DMA Transfers

PCnet-PCI II controller microcode will determine when a

FIFO DMA transfer is required. This transfer mode will

be used for transfers of data to and from the

PCnet-PCI II controller FIFOs. Once the PCnet-PCI II

controller BIU has been granted bus mastership, it will

perform a series of consecutive transfer cycles before

relinquishing the bus. All transfers within the master cy-

cle will be either read or write cycles, and all transfers

will be to contiguous, ascending 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 PCnet-PCI II controller uses

non-burst transfers to read and write data when

accessing the FIFOs. Each non-burst transfer will be

performed sequentially, with the issue of an address,

and the transfer of the corresponding data with appropri-

ate output signals to indicate selection of the active data

bytes during the transfer.

FRAME will be deasserted

after every address phase. The number of data transfer

cycles contained within a single bus mastership period

is in general dependent on the programming of the

DMAPLUS option (CSR4, bit 14). Several other factors

will also affect the length of the bus mastership period.

The possibilities are as follows:

If DMAPLUS is cleared to ZERO, a maximum of 16

transfers will be performed by default. This default value

may be changed by writing to the DMA Transfer Counter

(CSR80). Note that DMAPLUS = 0 merely sets a

maximum value. The minimum number of transfers in

the bus mastership period will be determined by all of

the following variables: the settings of the FIFO water-

marks (CSR80), the conditions of the FIFOs, the value

of the DMA Transfer Counter (CSR80), the value of the

DMA Bus Timer (CSR82), and any occurrence of

preemption

that

takes

place

during

the

bus

mastership period.

If DMAPLUS is set to ONE, 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

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