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IDT82P2521
21(+1) CHANNEL HIGH-DENSITY E1 LINE INTERFACE UNIT
Functional Description
29
December 7, 2005
3
FUNCTIONAL DESCRIPTION
3.1
RECEIVE PATH
3.1.1
RX TERMINATION
The receive line interface supports Receive Differential mode and
Receive Single Ended mode, as selected by the R_SING bit (b3,
RCF0,...). In Receive Differential mode, both RTIPn and RRINGn are
used to receive signal from the line side. In Receive Single Ended mode,
only RTIPn is used to receive signal.
In Receive Differential mode, the line interface can be connected
with E1 120
twisted pair cable or E1 75 coaxial cable. In Receiver
Single Ended mode, the line interface can only be connected with 75
coaxial cable.
The receive impedance matching is realized by using internal imped-
ance matching or external impedance matching for each channel in
different applications.
3.1.1.1 Receive Differential Mode
In Receive Differential mode, three kinds of impedance matching are
supported: Fully Internal Impedance Matching, Partially Internal Imped-
ance Matching and External Impedance Matching.
Figure-7 shows an
overview of how these Impedance Matching modes are switched.
Fully Internal Impedance Matching circuit uses an internal program-
mable resistor (IM) only and does not use an external resistor. This
configuration saves external components and supports 1:1 Hitless
Protection Switching (HPS) applications without relays. Refer to
Partially Internal Impedance Matching circuit consists of an internal
programmable resistor (IM) and a value-fixed 120
external resistor
(Rr). Compared with Fully Internal Impedance Matching, this configura-
tion provides considerable savings in power dissipation of the device.
For example, In E1 120
PRBS mode, the power savings would be
0.57 W. For power savings in other modes, please refer to
Chapter 8External Impedance Matching circuit uses an external resistor (Rr)
only.
Figure-7 Switch between Impedance Matching Modes
To support some particular applications, such as hot-swap or Hitless
Protection Switch (HPS) hot-switchover, RTIPn/RRINGn must be forced
to enter high impedance state (i.e., External Impedance Matching). For
hot-swap, RTIPn/RRINGn must be always held in high impedance state
during /after power up; for HPS hot-switchover, RTIPn/RRINGn must
enter high impedance state immediately after switchover. Though each
channel can be individually configured to External Impedance Matching
through register access, it is too slow for hitless switch. Therefore, a
hardware pin - RIM - is provided to globally control the high impedance
for all 22 receivers.
When RIM is low, only External Impedance Matching is supported for
all 22 receivers and the per-channel impedance matching configuration
bits - the R_TERM[2:0] bits (b2~0,
RCF0,...) and the R120IN bit (b4,
When RIM is high, impedance matching is configured on a per-
channel basis. Three kinds of impedance matching are all supported
and selected by the R_TERM[2:0] bits (b2~0,
RCF0,...) and the R120IN
match internal or external impedance. If the R_TERM[2] bit (b2,
RCF0,...) is ‘0’, internal impedance matching is enabled. The R120IN bit
(b4,
RCF0,...) should be set to select Partially Internal Impedance
Matching or Fully Internal Impedance Matching. The internal program-
mable resistor (IM) is determined by the R_TERM[1:0] bits (b1~0,
matching is enabled. The configuration of the R120IN bit (b4,
RCF0,...)and the R_TERM[1:0] bits (b1~0,
RCF0,...) is ignored.
A twisted pair cable can be connected with a 1:1 transformer or
without a transformer (transformer-less), while a coaxial cable must be
connected with a 1:1 transformer.
Table 1 lists the recommended imped-
show the connection for one channel.
IM
RTIP
RRING
RIN
R_TERM2
1 0
0 1
Receive
path
R_TERM[1:0]
RIM
R120IN
Rr = 120