XRT4500CV datasheet(53/99 Pages) EXAR

53 / 99 page

XRT4500

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MULTIPROTOCOL SERIAL NETWORK INTERFACE IC

REV. 1.0.7

The important things to note about Figure 20 are as

follows.

1. The DCE Terminal is the ultimate source of all

clock signals.

2. The DCE Serial Communications Controller (SCC)

will transmit the TXC clock signal to the DTE node.

3. The DTE SCC will update the state on the TXD

line, upon the rising edge of the “incoming” TXC clock

signal when ‘Clock Invert’ is not activated.

4. The DTE SCC will generate the rising edge of the

SCTE clock signal, upon receipt of the rising edge of

the “incoming” TXC clock signal when clock invert is

not activated.

5. The DCE SCC will use the falling edge of the

SCTE clock signal in order to sample the “incoming”

TXD signal.

6. Because the DTE provides the SCTE clock signals

and since the falling edge of this clock signal will oc-

cur at the middle of the bit-period (for the signal on

the TXD line); the “3-Clock DTE/DCE Interface” is

largely immune to the affects of propagation delay

(via the DCE SCC to DTE SCC” link and the “DTE

SCC to DCE SCC” link), and will operate properly

over a very wide range of data rates.

Figure 21 presents an illustration of the wave-forms of

the signals that are transported across a “3-Clock

DTE/DCE” Interface. Further, this figure indicates that

a “3-Clock DTE/DCE” Interface provides the DCE

SCC with a TXD to TXC set-up time of “one-half” of

the bit-period (0.5 * tb). Hence, a “3-Clock DTE/DCE”

Interface can support very wide range of data rates,

and still insure that the DCE SCC will be provided a

sufficient “TXD to TXC” set-up time.

The “2-Clock” DTE/DCE Interface

Although the Data Communications standards rec-

ommends the use of these three clock signals; in

practice, some Data Communications Equipment

manufacturers will build equipment that only supports

the transmission of “2-Clock” signals. The reason for

this can be due to cost, or due to the fact that the Da-

ta Communications Equipment manufacturer is using

an SCC that only handles 2-clock signals. When Data

Communications Equipment Manufacturers design

their DCE or DTE equipment to only support the

transmission of two clocks over the DTE/DCE Inter-

face; these two clocks signals are typically the “TXC”

(Transmit Clock) and the “RXC” (Receive Clock) sig-

nals. Figure 22 presents an illustration of a DTE and

DCE exchanging data over a “2-Clock DCE/DTE” In-

terface.

NOTE: In the “2-Clock DTE/DCE” Interface, the DTE Termi-

nal does not supply the SCTE clock signal back to the DCE.

FIGURE 21. ILLUSTRATION OF THE WAVE-FORMS OF THE SIGNALS THAT ARE TRANSPORTED ACROSS A “3-CLOCK

DTE/DCE” INTERFACE

TXC (at DCE)

TXC (at DTE)

TXD (at DTE)

TXD (at DCE)

0.5*tb

SCTE (at DTE)

SCTE (at DCE)

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