User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 7: DMA Page
159
ASIC M
ODE
T
RANSFERS
The ethernet and serial interface receive DMA engines have a special "ASIC mode" that enables received
packets to be passed through an ASIC on the HyperTransport fabric or PCI bus. This is in addition to just
sending the packet by address to an I/O destination. It allows connection of a simple ASIC that is logically
inserted into the path from the interface to memory.
The ASIC mode is intended for use in configurations where there is an assist ASIC on the HyperTransport or
PCI through which some or all of a packet must be passed. Examples include header classification engines
and encryption/decryption devices.
shows an example packet flow.
Figure 29: Packet Reception Flow using DMA ASIC Mode
ASIC mode is enabled by setting the asic_xfr_en bit in the
dma_config1
register. The normal DMA descriptors
are fetched for each packet. Rather than sending the packet header to the first buffer in the descriptor, the
packet is directed to the address range set in the
dma_asic_addr
register. The asicxfr_size is set to the
number of cache lines of the packet that should be sent to the ASIC; if there is an offset specified this is one
less than the maximum number of cache lines that should be sent to the ASIC address range (i.e. if the
asicxfr_size is 0 and asci_xfr_en is set then one cache line will be sent to the ASIC). The size sent to the ASIC
must match the size set for the first descriptor or behavior of the engine is UNDEFINED. If the packet length
is shorter than the size specified by the asicxft_size field then the entire packet is sent to the ASIC, otherwise
after the configured size has been sent to the ASIC the DMA controller advances to the second buffer in the
descriptor and will transfer to memory in the normal way. If the packet spans to subsequent descriptors they
will be used to send the data to memory; data is only directed to the ASIC following a start of packet.
MEM
Controller
HT
ASIC
DMA
Network
BCM1250
PHY
DDR
SDRAM
BCM1125/H