66
Am79C973/Am79C975
P R E L I M I N A R Y
Figure 31. 16-Bit Software Model
Note:
The value of CSR2, bits 15-8, is used as the
upper 8-bits for all memory addresses during bus mas-
ter transfers.
Figure 32 illustrates the relationship between the initial-
ization base address, the initialization block, the re-
ceive and transmit descriptor ring base addresses, the
receive and transmit descriptors, and the receive and
transmit data buffers, when SSIZE32 is set to 1.
Polling
If there is no network channel activity and there is no
pre- or post-receive or pre- or post-transmit activity
being performed by the Am79C973/Am79C975 con-
troller, then the Am79C973/Am79C975 controller will
periodically poll the current receive and transmit de-
scriptor entries in order to ascertain their ownership. If
the DPOLL bit in CSR4 is set, then the transmit polling
function is disabled.
A typical polling operation consists of the following se-
quence. The Am79C973/Am79C975 controller will use
the current receive descriptor address stored internally
to vector to the appropriate Receive Descriptor Table
Entry (RDTE). It will then use the current transmit de-
scriptor address (stored internally) to vector to the ap-
propriate Transmit Descriptor Table Entry (TDTE). The
accesses will be made in the following order: RMD1,
then RMD0 of the current RDTE during one bus arbitra-
tion, and after that, TMD1, then TMD0 of the current
TDTE during a second bus arbitration. All information
collected during polling activity will be stored internally
in the appropriate CSRs, if the OWN bit is set (i.e.,
CSR18, CSR19, CSR20, CSR21, CSR40, CSR42,
CSR50, CSR52).
A typical receive poll is the product of the following con-
ditions:
1. Am79C973/Am79C975 controller does not own the
current RDTE
and
the poll time has elapsed
and
RXON = 1 (CSR0, bit 5),
or
2. Am79C973/Am79C975 controller does not own the
next RDTE
and
there is more than one receive de-
scriptor in the ring
and
the poll time has elapsed
and
RXON = 1.
Initialization
Block
MOD
PADR[15:0]
PADR[31:16]
PADR[47:32]
LADRF[15:0]
LADRF[31:16]
LADRF[47:32]
LADRF[63:48]
RDRA[15:0]
RES
IADR[15:0]
IADR[31:16]
CSR1
CSR2
TDRA[15:0]
RES
RLE
RDRA[23:16]
TLE
TDRA[23:16]
Rcv
Buffers
RMD
RMD
RMD
RMD
Rcv Descriptor
Ring
N
N
N
N
1st desc.
start
2nd
desc.
RMD0
Xmt
Buffers
TMD
TMD
TMD
TMD
Xmt Descriptor
Ring
M
M
M
M
1st desc.
start
2nd
desc.
TMD
Data
Buffer
N
Data
Buffer
1
Data
Buffer
2
Data
Buffer
M
Data
Buffer
1
Data
Buffer
2
21510B21510D-36