dp83901a National Semiconductor Corporation, dp83901a Datasheet - Page 9

• Manufacturer Part Number: dp83901a
• Description: Serial Network Interface Controller
• Manufacturer: National Semiconductor Corporation

6 0 Direct Memory Access Control (DMA)
The DMA capabilities of the SNIC greatly simplify the use of
the DP83901A in typical configurations The local DMA
channel transfers data between the FIFO and memory On
transmission the packet is DMA’d from memory to the FIFO
in bursts Should a collision occur (up to 15 times) the pack-
et is retransmitted with no processor intervention On recep-
tion packets are DMAed from the FIFO to the receive buffer
ring (as explained below)
A remote DMA channel is also provided on the SNIC to
accomplish transfers between a buffer memory and system
memory The two DMA channels can alternatively be com-
bined to form a single 32-bit address with 8- or 16-bit data
DUAL DMA CONFIGURATION
An example configuration using both the local and remote
DMA channels is shown below Network activity is isolated
Dual Bus System
9
on a local bus where the SNIC’s local DMA channel per-
forms burst transfers between the buffer memory and the
SNIC’s FIFO The Remote DMA transfers data between the
buffer memory and the host memory via a bidirectional I O
port The Remote DMA provides local addressing capability
and is used as a slave DMA by the host Host side address-
ing must be provided by a host DMA or the CPU The SNIC
allows Local and Remote DMA operations to be interleaved
SINGLE CHANNEL DMA OPERATION
If desirable the two DMA channels can be combined to
provide a 32-bit DMA address The upper 16 bits of the
32-bit address are static and are used to point to a 64 kbyte
(or 32k word) page of memory where packets are to be
received and transmitted
TL F 10469 – 6