DP83916 Datasheet, PDF (54/96 Pages) National Semiconductor (TI) – SONICTM-16 Systems-Oriented Network Interface Controller

5 0 Bus Interface (Continued)
FIGURE 5-6 Bus Request Timing BMODEe1
TL F 11722 – 28
5 4 2 Block Transfers
The SONIC-16 performs block operations during all bus ac-
tions thereby providing efficient transfers to memory The
block cycle consists of three parts The first part is the bus
acquisition phase as discussed above in which the SONIC-
16 gains access to the bus Once it has access of the bus
the SONIC-16 enters the second phase by transferring data
to from its internal FIFOs or registers from to memory The
SONIC-16 transfers data from its FIFOs in either EXACT
BLOCK mode or EMPTY FILL
EXACT BLOCK mode In this mode the number of words
(or long words) transferred during a block transfer is deter-
mined by either the Transmit or Receive FIFO thresholds
programmed in the Data Configuration Register
EMPTY FILL mode In this mode the DMA completely fills
the Transmit FIFO during transmission or completely emp-
ties the Receive FIFO during reception This allows for
greater bus latency
When the SONIC-16 accesses the Descriptor Areas (i e
RRA RDA CDA and TDA) it transfers data between its
registers and memory All fields which need to be used are
accessed in one block operation Thus the SONIC-16 per-
forms 4 accesses in the RRA (see Section 3 4 4 2) 7 ac-
cesses in the RDA (see Section 3 4 6 1) 2 3 or 6 accesses
in the TDA (see Section 3 5 4) and 4 accesses in the CDA
5 4 3 Bus Status
The SONIC-16 presents three bits of status information on
pins S2 – S0 which indicate the type of bus operation the
SONIC-16 is currently performing (Table 5-2) Bus status is
valid when at the falling edge of AS or the rising edge of
ADS
TABLE 5-2 Bus Status
S2 S1 S0
Status
1 1 1 The bus is idle The SONIC-16 is not
performing any transfers on the bus
1 0 1 The Transmit Descriptor Area (TDA) is
currently being accessed
0 0 1 The Transmit Buffer Area (TBA) is
currently being read
0 1 1 The Receive Buffer Area (RBA) is
currently being written to Only data is
being written though not a Source or
Destination address
0 1 0 The Receive Buffer Area (RBA) is
currently being written to Only the
Source or Destination address is being
written though
1 1 0 The Receive Resource Area (RRA) is
currently being read
1 0 0 The Receive Descriptor Area (RDA) is
currently being accessed
0 0 0 The CAM Descriptor Area (CDA) is
currently being accessed
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