CHAPTER 4 INTERFACES
PRELIMINARY
NEC confidential and Proprietary
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(2) Status polling (Multi-PHY operation with 1 TXCLAV)
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 TXCLAV0 is used. TXCLAV1-3 are not used, and these signals are fixed
to low.
The ATM device can send a cell to the
µ
PD98413 only when the port has indicated to the ATM device that it is
ready to receive at least one complete cell. The
µ
PD98413 sends transmit cell buffer available information for
that port to the ATM device when that port is polled, using the TXCLAV0. Once the TXCLAV0 response for a
particular port indicates buffer availability, responses to subsequent polls of that port continues to indicate buffer
availability until after the second cycle of the transfer of a cell to that port. The FIFO availability information
indicated by TXCLAV0 is programmable by the FTHT register.
The ATM device polls by presenting the port address on TXADDR[1:0]. The
µ
PD98413 responds two clock
cycles later by driving TXCLAV0 high if the port can accept one or more complete ATM cells; TXCLAV0 is driven
low otherwise. The TXCLAV0 is not applicable on the first cycle after TXSOC is asserted, on this cycle, the
µ
PD98413 keeps the status before the transfer is started.
TXADDR[1:0] during the clock cycle before asserting the TXENB_B signal will select the port which will receive
the next cell. The
µ
PD98413 will decode this signal and the specified port will be ready to receive cell data from
the ATM device at the next clock cycle.
The following figure shows an example of the transmit timing in the status polling.
Figure 4-7. Transmit Timing (Status Polling)
TXCLK
TXENB_B
TXSOC
TXDATA[31:0]
P5
P6
P4
P7
P8
P10
P11
H1
P1
X : Invalid
TXCLAV0
P3
P2
P3
P4
TXPRTY
TXADDR[1:0]
PORT0
PORT1
P12
X
X
PORT1
PORT1
X
PORT2
PORT3
PORT0
PORT2
PORT3
P9
Back-to-back transfer of cells is possible when two cells have to be sent the same port and this port indicates
that it can receive the second cell. In the case of back-to-back transfer the ATM device implicitly reselects the
µ
PD98413 port by leaving the TXENB_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 TXSOC is asserted to indicate the start of
cell. This example is illustrated below.
Содержание UPD98413
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