XRT4500 Datasheet, PDF (55/99 Pages) Exar Corporation – MULTIPROTOCOL SERIAL NETWORK INTERFACE IC

XRT4500
MULTIPROTOCOL SERIAL NETWORK INTERFACE IC
REV. 1.0.7
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FIGURE 23. THE BEHAVIOR OF THE TXC AND TXD SIGNALS AT THE DCE AND DTE SCCS, (DATA RATE =
1.0MBPS, “DCE-TO-DTE” PROPAGATION DELAY = 160NS, “DTE-TO-DCE” PROPAGATION DELAY = 160NS)
TXC (at DCE)
TXC (at DTE)
TXD (at DTE)
TXD (at DCE)
1us
500ns
180ns
Figure 23 indicates that the TXC (Transmit Clock) sig-
nal will originate at the DCE SCC terminal. However,
because of the “DCE-to-DTE” propagation delay, the
TXC signal will arrive at the DTE SCC 160ns later.
Per the various “Communication Interface Standards”
(e.g., EIA-530A, etc.), the DTE must update the data
on the “TXD” line upon detection of the rising edge of
the “incoming” TXC clock signal. Hence, Figure 1.10
illustrates the DTE SCC toggling the TXD line coinci-
dent with the rising edge of TXC. Finally, because of
the “DTE to DCE” propagation delay, the TXD signal
will arrive at the DCE SCC 160 ns later.
Recall that the DCE SCC is using the TXC clock sig-
nal to sample the data on the “incoming” TXD line.
The scenario depicted in Figure 1.10 indicates that if
the Data Rate (between the DCE and DTE) is
1.0Mbps; and that if the “DCE to DTE” and “DTE to
DCE” propagation delays are each 160ns, then the
DCE SCC will be provided with 180ns of set-up time,
(in the TXD line) prior to sampling the data. For most
digital IC’s, this amount of set-up time is sufficient
long and should not result in any bit errors.
Case 2 - “2 Clock DCE/DTE” Operation at 1.544
Mbps
Now let's consider the case where the DCE and DTE
Terminals are now exchanging data at a rate of
1.544Mbps (e.g., the DS1 rate). Further, let's consid-
er that the “DCE-to-DTE” and “DTE-to-DCE” propa-
gation delays are each 160ns (as in the prior case).
Given these conditions, Figure 24 illustrates the re-
sulting clock and signal wave-forms for the TXC and
TXD at both the DCE and DTE SCCs.
FIGURE 24. THE BEHAVIOR OF THE TXC AND TXD SIGNALS AT THE DCE AND DTE SCCS (DATA RATE =
1.544MBPS, DCE-TO-DTE PROPAGATION DELAY = 160NS, DTE-TO-DCE PROPAGATION DELAY = 160NS)
TXC (at DCE)
TXC (at DTE)
TXD (at DTE)
TXD (at DCE)
324 ns
648ns
4ns
The scenario depicted in Figure 24 indicates that if
the Data Rate (between the DCE and the DTE) is
1.544Mbps and that if the “DCE-to-DTE” and the
“DTE-to-DCE” propagation delays are each 160ns,
then the DCE SCC will be provided with 4ns of set-up
time (in the TXD line) prior to sample the data. For
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