XRT72L56 Datasheet, PDF (482/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
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PRELIMINARY
HDLC Controller block will begin to sample the data
on the TxHDLCDat_n[7:0] input pins, upon the rising
edge of the TxHDLCClk_n clock output signal. Each
byte of data that is sampled and latched into the
Transmit HDLC Controller block will be encapsulated
into an outbound HDLC frame.
After the last byte of data has been latched into the
Transmit HDLC Controller block, then the user must
pull the Snd_FCS_n input pin "High" for either two or
four TxHDLCClk_n clock periods.
If the user has configured the Transmit HDLC Control-
ler block to append a 16-bit CRC value (from here on,
referred to as CRC-16); then the user must pull and
hold the Snd_FCS_n input pin "High" for two (2)
TxHDLCClk_n clock periods. Conversely, if the user
has configured the Transmit HDLC Controller block to
append a 32 bit CRC value (from here on, referred to
as CRC-32); then the user must pull and hold the
Snd_FCS_n input pin "High" for (4) TxHDLCClk_n
periods. Pulling the Snd_FCS_n input pin "High"
configures the Transmit HDLC Controller to begin its
insertion of either the CRC-16 or CRC-32 value into
the back-end of the outbound HDLC Controller.
Selecting either CRC-16 or CRC-32
The user can configure the Transmit HDLC Controller
block (within Channel n) to append either a CRC-16
or CRC-32 value to the user-data within the outbound
HDLC frame, by writing the appropriate value into Bit
5 (CRC-32 Select). Setting this bit-field "High" config-
ures the Transmit HDLC Controller block to append a
CRC-32 value to the user data, within the outbound
HDLC frame. Conversely, setting this bit-field "Low"
configures the Transmit HDLC Controller block to ap-
pend a CRC-16 value to the user data, within the out-
bound HDLC frame.
A simple illustration of the resulting outbound HDLC
Frame (when CRC-32 Select, is invoked) is presented
below in Figure 214 .
FIGURE 214. A SIMPLE ILLUSTRATION OF AN OUTBOUND HDLC FRAME, AS ASSEMBLED BY THE TRANSMIT HDLC
CONTROLLER, WHEN CRC-32 IS SELECTED.
CRC-32
Trailer
User Supplied Data
HDLC Frame
Similarly, a simple illustration of the resulting out-
bound HDLC Frame (when CRC-16 is selected) is
presented below in Figure 215 .
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