9

IDT77V106L25

In the transmit direction, the PHY first asserts TXCLAV (transmit cell available) to indicate that it has room in its transmit FIFO to accept at least one 53-byte

ATM cell. When the ATM layer device is ready to begin passing the cell, it asserts

TXEN (transmit enable) and TXSOC (start of cell), coincident with the first

byte of the cell on TXDATA.

TXEN can remain asserted for the duration of the cell transfer, or the ATM device may deassert TXEN at any time once the cell

transfer has begun; data is transferred only when

TXEN is asserted.

In the receive direction,

RXEN indicates when the ATM device is prepared to receive data. As with transmit, it may be asserted or deasserted at any time.

Note that this Utopia interface can be operated in either cell-mode or in byte-mode as determined by bit 1 in the Master Control Register. In cell-mode,

which is the default, the 77V106L25 does not assert TXCLAV until it has enough room in it’s transmit FIFO to accept a complete cell, and doesn’t assert RXCLAV

until it has a complete cell in the receive FIFO. It will not deassert TXCLAV or RXCLAV until at or near the end of the transfer of a cell.

In byte-mode, the phy can assert TXCLAV before it has room for a complete cell. It will modulate TXCLAV to prevent the FIFO from overflowing. Likewise,

it may assert RXCLAV before a complete cell has been received, and will modulate RXCLAV to prevent the FIFO from underflowing. There is generally little

advantage to the byte-mode, so most users will leave the 77V106L25 in the default cell-mode.

In both transmit and receive, TXSOC and RXSOC (start of cell) is asserted for one clock, coincident with the first byte of each cell. Odd parity is utilized

across each 8-bit data field, which means that for an all-zero pattern. the corresponding parity bit is one.

The following figures show examples of the Utopia Level 1 handshake.

TXCLK

TXSOC

TXCLAV

TXEN

TXDATA[7:0],

TXPARITY

X

H1

H2

P44

P45

P46

P47

P48

X

77v106 drw 16

.

TXCLK

TXSOC

TXCLAV

TXEN

TXDATA[7:0],

TXPARITY

P46

P47

P48

H1

H2

H3

H4

X

H5

77v106 drw 17

77v106 drw

Figure 4. Utopia Transmit Handshake - Single Cell

Figure 5. Utopia Transmit Handshake - Back to Back Cells and