AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 208

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

LAPP Software Requirements
Software needs to set up a receive ring with descriptors
formed into groups of 3. The first descriptor of each
group should have OWN = 1 and STP = 1, the second
descriptor of each group should have OWN = 1 and
STP = 0. The third descriptor of each group should have
OWN = 0 and STP = 0. The size of the first buffer (as
D-4
N2: EOM
AMD
N0: Packet preamble, SFD
Ethernet
activity:
and destination address
are arriving.
Wire
{
N1: 64th byte of packet
C9: Controller writes descriptor #3.
C8: Controller is performing intermittent
C7: Controller writes descriptor #2.
C6: "Last chance" lookahead to
C5: Controller is performing intermittent
C4: Lookahead to descriptor #3 (OWN).
C3: SRP interrupt
C2: Controller writes descriptor #1.
C1: Controller is performing intermittent
C0: Lookahead to descriptor #2.
data arrives.
bursts of DMA to fill data buffer #3.
descriptor #3 (OWN).
bursts of DMA to fill data buffer #2.
is generated.
bursts of DMA to fill data buffer #1.
}
Controller
Ethernet
activity:
Figure D1. LAPP Timeline
Am79C970A
Buffer
Buffer
Buffer
#3
#2
#1
indicated in the first descriptor), should be at least equal
to the largest expected header size; however, for maxi-
mum efficiency of CPU utilization, the first buffer size
should be larger than the header size. It should be equal
to the expected number of message bytes, minus the
time needed for Interrupt latency and minus the
S10: Driver sets up TX descriptor.
S9: Application processes packet, generates TX packet.
S7: Driver polls descriptor of buffer #3.
S6: Driver copies data from buffer #2 to the application buffer.
S5: Driver polls descriptor #2.
S4: Driver copies data from buffer #1 to the application buffer.
S1: Interrupt latency.
S0: Driver is idle.
Software
activity:
} {
{
S8: Driver calls application
S3: Driver writes modified application
S2: Driver call to application to
to tell application that
packet has arrived.
pointer to descriptor #3.
get application buffer pointer.
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