Datasheet

SCEA019 - January 2001
Texas Instruments GTLP Frequently Asked Questions 7–111
28 What do I do with unused or undriven GTLP A- or B-port I/Os?
As with any other logic device, it is important that unused or undriven inputs or inputs/outputs
(I/Os) not be left floating. This prevents high current flow through the device if the input should
reach the threshold level. There is no need to prevent the transistor outputs from floating as
long as its input is tied to V
CC
or GND. The following is recommended and summarized in
Table 8:
• A port (LVTTL side of the device)
– I/Os on all devices with bus hold
Unused or undriven - No action required, bus hold will maintain last known logic state
– Inputs on all devices without bus hold with inverted signals (currently, SN74GTLP817
only)
Unused or undriven - Tie pin to V
CC
with a 220-Ω up to 1-kΩ pullup resistor to
maintain a logic high.
– Inputs on all devices without bus hold with selectable true/complementary feature
(currently, SN74GTLP1394 only)
Unused or undriven - Tie pin to V
CC
or GND with a 220-Ω up to 1-kΩ pullup resistor to
maintain a logic high or low, depending on whether the device is in true or inverted
mode of operation, and if signals are unidirectional or bidirectional, to keep the B port
in its tied condition.
• B port (GTLP side of device)
– I/Os on all devices
Unused - Tie pin to ground with a 220-Ω up to 1-kΩ pulldown resistor to maintain a
logic low.
Undriven - No action required because the pin is pulled high to V
TT
by the termination
resistors.
Table 8. Control of Undriven or Unused Pins
A I/O
Bus Hold No Bus Hold
B I/O
True Inverted True Inverted True
Inverted
Undriven No action required
Unused
No action required
Pull up to V
CC
Pull down to GND.
Pull down to GND.
Precharge - Each B-port I/O pin has its own precharge circuitry if the device is equipped with a
BIAS V
CC
pin. Because the precharge current is very low (<10 uA), if the pin is connected to
GND, it does not reach 1-V precharge. However, other B-port I/O pins that are not connected
to GND are properly precharged to 1 V.