DS2155 datasheet(174/262 Pages) DALLAS

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Figure 24-5 shows an example of direct status for the transmit direction. Signals UT-CLAV[3:0] are

associated to PHY port addresses #4, #3, #2, and #1. There is no need for a unique null device, thus “X =

don’t care” represents any address between 0 and 31 on the address lines UT-ADDRx or any data on the

data bus. The polling of PHY ports starts while no cell transfer takes place. The ATM layer has pending

cells for all four PHY ports (one individual queue for each PHY port) but all four PHY ports cannot

accept a cell. With rising clock edge #2, PHY port #1 indicates that it can accept a complete cell

(UT-CLAV0 asserted). The ATM layer detects this at clock edge #3. It selects that PHY port by placing

address #1 on the address lines with rising clock edge #3. PHY port #1 detects this at clock edge #4. At

clock edge #5, PHY port #1 detects UT-ENB asserted, therefore cell transfer for PHY port #1 starts with

rising clock edge #5 (byte H1).

Figure 24-5. Example of Direct Status Indication, Transmit Direction

At clock edge #5, the ATM layer detects a cell available at PHY port #3 (UT-CLAV2 asserted). With

rising clock edge #52, PHY port #1 indicates that it cannot accept an additional cell by deasserting UT-

CLAV0. Thus, at clock edge #57, the ATM layer detects only UT-CLAV2 asserted (UT-CLAV1 and UT-

CLAV3 remain deasserted). The ATM layer deselects PHY port #1 and selects PHY port #3 for cell

transfer with rising clock edge #57 by placing address #3 on the address lines and deasserting UT-ENB.

PHY port #1 and PHY port #3 detect this at clock edge #58. At clock edge #59, PHY port #3 detects

UT-ENB asserted, therefore cell transfer for PHY port #3 starts with rising clock edge #59 (byte H1). For

additional examples, refer to [3].

N-4

UT-CLK

UT-ADDRx

UT-CLAV(1)

UT-ENB

UT-SOC

P45 P46

P48

P47

UT-DATAx

X = Don't Care

UT-CLAV(2)

UT-CLAV(3)

UT-CLAV(4)

PORT #1

PORT #2

PORT #3

PORT #4

P44

Port #1 Transfer

Port #3

DS2155 Datasheet(PDF) 174 Page - Dallas Semiconductor

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