ICS1887
Substituting the ICS1887
for the AMD PDR & PDT
CD PECL Input: Board Layout Options
This note describes the issues involved in re-
placing the AMD PDR & PDT with the
ICS1887.
There are a number of implementation differ-
ences between AMD’s PDR & PDT and the
ICS1887.
This note describes the differences
and how they affect an application.
Option 1
Differential PECL to CMOS Conversion Circuit
Signal Detect
Many twisted pair and fiber optic transceivers
provide a signal detect indication that becomes
active when the amount of energy being re-
ceived reaches a threshold that makes it appear
to be data and not ambient noise.
The AMD PDR device has a single ended
PECL input (SDI) and provides a TTL level
output (SDO) that tracks the input. The input
controls the source that the PLL locks to. When
signal detect is asserted, the PLL locks to the
incoming receive data. When signal detect is
deasserted, the PLL locks to the LSCLK input
to prevent locking to an off center frequency.
The current
ICS1887
device provides a single
TTL-compatible input, carrier detect (CD~).
When carrier detect is asserted, the
ICS1887
locks to the incoming receive data. When car-
rier detect is deasserted, or if carrier detect is
asserted and no data is present on the receive
inputs, the PLL will free run and continue to
provide RXCLK at the nominal 25 MHz
frequency. This allows the carrier detect input
to always be tied to an asserted level (ground).
If a true signal detect is required by a chip that
connects to the
ICS1887,
a simple, low cost
PECL to CMOS converter can be used. The
following circuit implements this function:
Option 2
Single-Ended PECL to CMOS Conversion Circuit
5
ICS [ INTEGRATED CIRCUIT SYSTEMS ]
