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CHAPTER 3 FUNCTIONAL DESCRIPTION
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The
μ
PD98405 provides an interface between a host system and a PMD layer device. It is controlled by the
host system whenever the host system accesses the internal registers of the
μ
PD98405 via the bus interface.
Data is directly transmitted or received by the internal DMA controller of the
μ
PD98405, to or from system
memory under the management of the host system. An indication is written to system memory for each packet
by means of a DMA status that indicates the end of transmission or reception. The host system must,
therefore, allocate the following three areas in system memory to enable data to be transmitted or received
using the
μ
PD98405.
(a) Transmit buffer area:
Saves transmit data.
(b) Receive buffer area:
Saves receive data.
(c) Mailbox area:
Saves transmit/receive indication.
The
μ
PD98405 uses dedicated external memory as its control memory to execute reception processing.
The control memory may have a capacity of up to 2M bytes. It is divided into the four areas described below.
The size of the control memory is determined mainly by the number of channels through which the
μ
PD98405
simultaneously transmits or receives data. The boundaries between the four areas are set, by the host system,
into a register of the
μ
PD98405.
<1>
Receive lookup table area
<2>
Receive free buffer pool pointer area
<3>
ABR lookup table area
<4>
Free block pool area
For details of the control memory, see
Section 5.2
.
Figure 3-2 shows an outline of the
μ
PD98405 system.