Datasheet

SCEA019 - January 2001
7–112 Texas Instruments GTLP Frequently Asked Questions
29 Can I connect the SN74GTLPH16612GR unused I/O pins of the A ports
and B ports directly to GND?
I have seen FSC’s reference design and the unused pins were connected directly to
ground. Per the GTLP FAQ, we cannot do that because there should be a resistor for
safety purposes.
The A ports and B ports can be connected directly to GND on unused pins. Historically, there
has always been a resistor used when tying the pin to V
CC
or GND because it is required for
bipolar inputs. It is not required for CMOS inputs (like those used on all GTLP devices) and
the inputs can be tied directly to GND or V
CC
. FSC’s proposal to tie unused pins on both the A
ports and B ports of the GTLPH16612 to GND is acceptable. Our GTLPH16612, is identical to
the FSC GTLP16612, but with better B-port edge-rate slew control.
30 How does an extender card affect signal integrity?
I am designing a new card basket using GTLP and need to provide for an extender
card. Should I leave it unterminated and hope it works OK, or terminate it lightly? How
about terminating with a resistor in series with a capacitor? The dc pullup to 1.5 V will
be taken care of on the backplane.
Leave it unterminated. There will be signal integrity problems if the extender card is used on
the driver card, but the signal integrity will be OK if used on a receiver card. There will be
about 20 pF extra loading and some additional line delay caused by the longer stub trace, but
the receiver signal integrity should be close to what it was without the extender card, and
other receiver cards should be mostly unaffected.