Cirrus Logic CS8900A - Manual (Page 80)

80 DS271F5
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
memory via host DMA. This section describes
receive frame pre-processing and Memory
and I/O space receive operation. Section 5.3
on page 90 through Section 5.4 on page 94
describe DMA operation.
5.2.1 Terminology: Packet, Frame, and
Transfer
The terms Packet, Frame, and Transfer are
used extensively in the following sections.
They are defined below for clarity:
5.2.1.1 Packet
The term "packet" refers to the entire serial
string of bits transmitted over an Ethernet net-
work. This includes the preamble, Start-of-
Frame Delimiter (SFD), Destination Address
(DA), Source Address (SA), Length field, Data
field, pad bits (if necessary), and Frame Check
Sequence (FCS, also called CRC). Figure 9
shows the format of a packet.
5.2.1.2 Frame
The term "frame" refers to the portion of a
packet from the DA to the FCS. This includes
the Destination Address (DA), Source Address
(SA), Length field, Data field, pad bits (if nec-
essary), and Frame Check Sequence (FCS,
also called CRC). Figure 9 shows the format of
a frame. The term "frame data" refers to all the
data from the DA to the FCS that is to be trans-
mitted, or that has been received.
5.2.1.3 Transfer
The term "transfer" refers to moving data
across the ISA bus, to and from the CS8900A.
During receive operations, only frame data are
transferred from the CS8900A to the host (the
preamble and SFD are stripped off by the
CS8900A's MAC engine). The FCS may or
may not be transferred, depending on the con-
figuration. All transfers to and from the
CS8900A are counted in bytes, but may be
padded for double word alignment.
5.2.2 Receive Configuration
After each reset, the CS8900A must be config-
ured for receive operation. This can be done
automatically using an attached EEPROM or
by writing configuration commands to the
CS8900A's internal registers (see Section 3.4
on page 21). The items that must be config-
ured include:
• which physical interface to use;
• which types of frames to accept;
• which receive events cause interrupts;
and,
• how received frames are transferred.
Yes No Use
DMA?
Frame Held
On Chip
Frame DMAed
to Host Memory
Host Reads
Frame from
Host Memory
Frame Pre-
Processed
Frame
Temporarily
Buffered
Packet Received
Preamble and
Start-of-Frame
Delimiter Removed
Host Reads
Frame from
CS8900A Memory
Figure 20. Frame Reception