26 Aug
2014
26 Aug
'14
3:27 p.m.
I wrote:
Some moron at TI has revised the
SN5404/SN54LS04/SN7404/SN74LS04
datasheet to claim that all unused inputs must be held at Vcc or GND
On Tue, Aug 26, 2014 at 2:46 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Is it? I thought that for plain TTL and (I think) LS
it was good practice
to pull unused inputs up via a resistor and not link them straight to
Vcc.
For "plain" TTL yes. One of the purported advantages of LS was not
needing to do that. I forgot about that detail when commenting on the
TI datasheet lossage.
Something about supply line spikes causing breakdown
of the input
transistor.
I've heard that explanation and I suppose it sounds plausible but I would like
to find an authoritative statement from TI (i.e., in a TI data sheet
or application
note).
Fairchild AN-363 "Designing with TTL" dated June 1984 mentions
standard TTL, LS, S, ALS, AS, L, and F (but omits H), and states
without reference to a specific family:
A good design rule is to tie unused inputs to a solid logic level.
Inputs are usually tied to V CC through a 1 k? to 5 k? resistor,
since tying them to ground means supplying the I IL current
instead of the I IH current. I IL is several orders of magnitude
greater than I IH . The resistor is recommended to protect the
input against V CC voltage surges and to protect the system
against the possibility of the input shorting directly to ground.
A single 1k resistor can handle up to 10 inputs.
The other thing commonly done was to tie unused "don't care" inputs
to some other logic output, provided that the fanout wasn't
exceeded. Even when the input wasn't a "don't care", sometimes
a spare gate was used, e.g., one inverter from a 7404 with the input
tied to ground, to drive multiple other TTL inputs that needed to
be high.