Datasheet

SCEA019 - January 2001
7–110 Texas Instruments GTLP Frequently Asked Questions
25 Tell me more about bus hold.
All GTLPH devices have bus hold on the A port and are described in the data sheet as
“Use of pullup or pulldown resistors with the bus-hold circuitry is not recommended.”
However, a note in the data sheet states “All unused inputs of the devices must be held
at V
CC
or GND to ensure proper operation.” Don’t these statements conflict?
No, A-port data inputs with bus-hold cells should not use a pullup or pulldown resistor, but, if
required, a properly sized resistor that overrides the bus-hold cell can be used. The note
refers to the control inputs, not the data inputs. The control inputs always must be tied low or
high if unused or undriven and never allowed to float because the device would not operate
properly. There is no bus-hold cell on any control input on any TI device.
26 Tell me more about power-up 3-State (PU3S).
Your GTLP data sheet states, “When /OEAB is low, the outputs are active.” However, in
the next paragraph about PU3S, the data sheet states: “To ensure the high-impedance
state during power up or down, /OE should be tied to V
CC
through pullup resistor,”
which means that /OE is high. This contradicts the previous statement. Should /OEAB
be high or low?
The /OE should be pulled high to keep the device in the high-impedance state, even though it
is featured with PU3S, which keeps the device in the high-impedance state, when V
CC
is zero
to 1.5 V, to prevent bus contention. The first statement assumes you are going to overdrive
the pullup resistor when you want the outputs to be active and pull /OE to GND.
27 Should I tie GTLPH16612 control inputs to 3.3 V or to 5 V?
I need to tie the SN74GTLPH16612GR LEAB (pin 2) high for transparent operation and
am not sure what voltage it should be tied to.
See the "V
I
except B Port" specification in the data sheet, which is 5.5 V maximum. Confusion
might arise because the GTLPH16612 has both a 3.3-V and 5-V V
CC
. Other GTLP devices
are 3.3-V V
CC
only, and they list the nominal V
I
as V
CC
and maximum V
I
as 5.5 V to show that
the V
I
normally is connected to 3.3 V, but you can go up to 5.5 V. In either case, you can use
3.3 V or 5 V, depending on which voltage is most convenient, but you would normally use
3.3 V for any GTLP device.