CHAPTER 4 INTERFACES
PRELIMINARY
NEC confidential and Proprietary
124
The
µ
PD98413 supports back-to-back transfer of cells. In the case of back-to-back transfer the ATM device
implicitly reselects the
µ
PD98413 port and RXENB_B asserted during the next to the last cycle of the cell transfer.
The second cell is transferred immediately after the previous one and the RXSOC is asserted to indicate the start
of cell. This example is illustrated below.
Figure 4-10. Back-to-back Cell Reception (Direct Status Indication)
RXCLK
RXENB_B
RXSOC
RXDATA[31:0]
P12
X
P11
H1
P8
P9
P10
P1
X : Invalid
RXCLAV0
RXCLAV1
RXCLAV2
RXCLAV3
P10
P2
P3
P4
RXPRTY
RXADDR[1:0]
X
PORT0
X
P11
P12
X
X
H1
(2) Status polling (Multi-PHY operation with 1 RXCLAV)
In the status-polling mode, the ATM device can receive the
µ
PD98413 port FIFO status information through the
polling mechanism. In this mode, only RXCLAV0 is used. RXCLAV1-3 are not used, and these signals are fixed
to low.
The ATM device controls the flow of data from the
µ
PD98413 on a per cell basis. The ATM device can explicitly
select the
µ
PD98413 port for transfer of a cell only when the port has indicated to the ATM device that it has at
least one cell available, using the RXCLAV0.
The ATM device polls by presenting the port address on RXADDR[1:0]. The
µ
PD98413 responds two clock
cycles later by driving RXCLAV0 high if the port is ready to send one or more complete ATM cells to the ATM
device; RXCLAV0 is driven low otherwise. Once the RXCLAV0 response for a particular port indicates cell
availability, responses to subsequent polls of that port continues to indicate cell availability until the RXSOC is
asserted for that port.
RXADDR[1:0] during the clock cycle before asserting the RXENB_B signal will select the port which will transfer
the next cell across the ATM interface. The
µ
PD98413 will decode this signal and the specified port will be ready
to transfer cell data two clock cycles before the end of the cell transfer, unless a back-to-back transfer is intended.
The decode-response timing between the RXENB_B and the RXDATA[31:0] is therefore two clock cycles
.
The following figure shows an example of the transmit timing in the status polling.
Summary of Contents for NEASCOT-P65
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