6

ICS1887

This circuit provides the PECL to CMOS conversion for less

than $0.80 in single unit quantities. Note that the LM393 has

two amplifiers, so the unused one is tied inactive.

A running production change will be made to the ICS1887 to

change the CD input to PECL. Therefore, boards should be

laid out with a direct normal PECL termination connection

stuffing option. This allows either version of the part to be

used by stuffing one of two sets of external components. A

version of this circuit is shown in the diagram on the previous

page.

With ICS1887 devices that have a TTL-compatible CD input,

the “Differential PECL to CMOS Conversion Circuit” com-

ponents need to be placed on the PCB and the “Normal PECL

Transceiver Termination” resistors (82

Ω and 130Ω) as well as

the option select jumper should NOT be placed.

When the final ICS1887 device with the PECL CD input is

used, none of the components in the “Differential PECL to

CMOS Conversion Circuit” or the “Unused amp connection”

circuits should be used. Only the four termination resistors

(87

Ω and 130Ω) and the option select jumper are needed.

Note that these resistors should be located near the ends of the

transmission lines.

Clocking

Parallel data that is to be serialized for transmission must be

presented to the data transmitter device with a certain amount

of setup and hold time to a given clock.

The PDT chip expects data to setup relative to the 25 MHz

Local Symbol Clock (LSCLK). This clock is an input to the

device.

The ICS1887 expects data to be setup relative to the 25 MHz

Reference In Clock (REF_IN). This clock is an input to the

ICS1887 device. Note that the REF_IN pin of the ICS1887 is

a CMOS input with a switching point of 50% of VDD. If this

pin is driven by a TTL output, a pull-up resistor to VDD must

be used. The ICS1887 device also provides a Transmit Clock

(TXC) output, which is a 50% duty cycle (nominal) copy of

the REF_IN input. The ICS1887 is designed to provide a very

low skew between the REF_IN and the TCLK.

Loopback

The AMD PDR & PDT chips have an external loopback con-

nection between the two chips. The ICS1887 also has a

loopback function, but it is totally internal to the device.

Optical Transmitter Off Control

The PDT chip has an input (FOTOFF) which can force an

optical transceiver to be off. The ICS1887 performs the same

behavior with the TXOFF~ pin.

Test Mode

Both the AMD PDR & PDT have a test mode that allows auto-

mated testers to test internal logic without the PLL clock

multiplier. The ICS1887 does not have a similar test mode.

Transmit Current Selection

The ICS1887 allows the PECL transmit current level to be set

externally. An 887

Ω resistor to the VDD supply is recom-

mended.